Trude Storelvmo

• Smith, William McFarlane; Visioni, Daniele; Kravitz, Ben; Dijk, Evelien Van; Storelvmo, Trude & Hunt, Hugh (2026). Parallel and diverging responses across four earth system models in response to cirrus cloud thinning climate intervention experiment. Environmental Research: Climate. 5(3). doi: 10.1088/2752-5295/ae8c7a. Full text in Research Archive
• Alterskjær, Kari; Steensen, Birthe Marie Rødssæteren; Stjern, Camilla Weum; Storelvmo, Trude; Fischer, Erich & Myhre, Gunnar (2026). Cleaning Air Pollution Over Central Europe Enhances Extreme Precipitation. Geophysical Research Letters. ISSN 0094-8276. 53(14). doi: 10.1029/2025GL117968. Full text in Research Archive Show summary

  Extreme precipitation is becoming increasingly severe, with growing societal and environmental impacts. This study investigates how anthropogenic aerosol emissions have influenced extreme precipitation over Central Europe. Using the Weather Research and Forecasting Model, we compare two 20-year simulations with aerosol concentrations representative of 1980 (polluted) and 2020 (clean) conditions. The results reveal a pronounced intensification of extreme precipitation under cleaner air. This intensification stems from shorter event durations coupled with unchanged precipitation amounts. Specifically, hourly precipitation amounts above the 99th, 99.9th and 99.97th percentiles increase by 2.5%, 6.4% and 7.9%, respectively. The most extreme events show the largest changes, with disproportionate increases in total amount, intensity, and frequency. With aerosol concentrations projected to further decline across many densely populated regions, the risks from severe and frequent extreme precipitation are likely to grow. These findings underscore the urgent need for targeted adaptation strategies.

• Im, Ulas; Samset, Bjørn Hallvard; Nenes, Athanasios; Thomas, Jennie L.; Kokkola, Harri & Dubovik, Oleg [Show all 41 contributors for this article] (2026). Aerosol-Cloud Interactions: Overcoming a Barrier to Projecting Near-Term Climate Evolution and Risk. AGU Advances. 7(1). doi: 10.1029/2025AV001872. Full text in Research Archive Show summary

  Aerosol-cloud interactions (ACI) are a major source of uncertainty in climate science, critically affecting our ability to project near-term climate evolution and assess societal risks. These interactions influence effective radiative forcing, cloud dynamics, and precipitation patterns, yet remain insufficiently constrained due to limitations in observations, modeling, and process understanding. This uncertainty hampers robust policy advice across multiple domains—from estimating remaining carbon budgets and climate sensitivity, to anticipating regional extreme events and evaluating climate interventions such as solar radiation modification. In many cases, the influence of ACI is either underappreciated or excluded from decision-making frameworks due to its complexity and lack of quantification. This perspective outlines a path forward to overcome these barriers by leveraging emerging opportunities in satellite remote sensing, ground-based and airborne observations, high-resolution climate modeling, and machine learning. We identify key areas where rapid progress is feasible, including improved retrievals of cloud microphysical properties, better representation of natural aerosols in a warming world, and enhanced integration of observational and modeling communities. Even as anthropogenic aerosol and its impacts on clouds is reducing owing to emissions controls, addressing ACI uncertainties remains essential for refining climate projections, supporting effective mitigation and adaptation strategies, and delivering actionable science to policymakers in a rapidly changing climate system.

• Bruno, Olimpia; Shaw, Jonah K.; Storelvmo, Trude & Hoose, Corinna (2026). Evaluation of Global Climate and Storm-Resolving Model Representations of Mixed-Phase Clouds and Their Hemispheric Contrasts. Atmosphere. 17(2). doi: 10.3390/atmos17020156. Full text in Research Archive
• Bjordal, Jenny; Smith, Anthony A.; Cornec, Henri & Storelvmo, Trude (2026). NorESM2–DIAM: a coupled model for investigating global and regional climate-economy interactions. Geoscientific Model Development. ISSN 1991-959X. 19(3), p. 1337–1365. doi: 10.5194/gmd-19-1337-2026. Full text in Research Archive
• Mostue, Idunn Aamnes; Venegas, Guillermo Andrés Valenzuela; Baños, David; Storelvmo, Trude & Zeyringer, Marianne (2026). Widespread increase in frequency and duration of European wind droughts based on CMIP6 projections. iScience. 29(3). doi: 10.1016/j.isci.2026.115075. Full text in Research Archive Show summary

  To combat climate change, wind power represents an important energy source. However, the availability and reliability of wind resources are subject to weather conditions and the potential impact of climate change. This study utilizes five CMIP6 model projections under the high-emission SSP5-8.5 scenario to analyze future wind speeds and extreme wind events across Europe for the periods 2040–2059 and 2080–2099. We find a widespread increase in the annual frequency of weak wind events up to 10%–20% in some regions, most significantly over western Europe. While winter frequencies for weak winds may decrease in some regions, the duration of these events is projected to rise in northern and eastern Europe. Conversely, and with less model agreement, strong wind events could increase by up to 70% in the southern North Sea and offshore northern Spain. These results underscore the critical need for climate adaptive and climate-resilient energy systems planning.

• Lippe, Filip von der; Carlsen, Tim; Storelvmo, Trude & David, Robert Oscar (2026). Shortening of the Arctic cold air outbreak season detected by a phenomenological machine learning approach. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 26(2), p. 1565–1585. doi: 10.5194/acp-26-1565-2026. Full text in Research Archive Show summary

  Abstract. Marine cold air outbreaks (CAOs) frequently occur in the Arctic when cold air moves over the relatively warm ocean, resulting in large turbulent fluxes, instability and cloud formation. Given the high frequency of CAOs during the Arctic winter, the associated clouds have a large impact on the region's radiative balance. Due to Arctic warming, the prevalence of CAOs and their clouds may change, impacting the Arctic radiative balance and potentially amplifying or mitigating local and global warming. To better understand how CAO clouds respond to Arctic warming, this study has developed a phenomenological CAO cloud classification tool that utilizes machine learning methods to identify closed and open cell clouds in CAOs from MODIS satellite imagery. This new approach achieves better performance in identifying CAO clouds compared to the marine cold air outbreak index calculated using MERRA-2 reanalysis, with accuracies of 85.4 % and 78.0 %, respectively. The new approach has revealed frequent CAO cloud formation in regions of high sea surface temperatures, with occurrence maxima along the Norwegian coast and the Northern Atlantic region south of Iceland. Furthermore, the approach reveals trends in CAO cloud cover that suggest a shortening of the CAO season, characterized by an approximate 10 %, increase in cloud coverage during winter and a nearly 20 % decrease during the shoulder months over the past 25 years. These trends suggest a positive radiative feedback during winter in response to climate change, underscoring the importance of further investigating these clouds to understand the trajectory of future Arctic climate.

• Wall, Casey J.; Paynter, David; Qin, Yi; Debolskiy, Matvey; Duffy, Margaret L. & Michibata, Takuro [Show all 12 contributors for this article] (2025). Decomposing Cloud Radiative Feedbacks by Cloud-Top Phase. Journal of Climate. ISSN 0894-8755. 38(16), p. 4023–4043. doi: 10.1175/JCLI-D-24-0538.1. Full text in Research Archive Show summary

  Changes in cloud scattering properties and emissivity that arise from atmospheric warming cause substantial radiative feedbacks in model projections of anthropogenic climate change, and the relative importance of the underlying mechanisms is poorly understood. One leading hypothesis is that ice-to-liquid conversions cause clouds to optically thicken, producing a major negative feedback. We test this hypothesis by developing a method to decompose cloud radiative feedbacks by cloud-top phase. The method is applied to an ensemble of six state-of-the-art global climate models run with prescribed sea surface temperature. In these simulations, the global mean of the net cloud scattering and emissivity feedback from cloud-phase conversions ranges from −0.17 to −0.01 W m−2 K−1, while the overall net cloud feedback ranges from 0.02 to 0.91 W m−2 K−1. The multimodel mean of the cloud scattering and emissivity feedback from cloud-phase conversions is approximately 19% of the magnitude of the multimodel mean of the overall cloud feedback (−0.10 vs 0.52 W m−2 K−1). These results indicate that cloud-phase conversions cause a robust negative feedback by changing cloud scattering and emissivity, but this mechanism makes a modest contribution to the overall cloud feedback at the global scale. Significance Statement Climate warming changes Earth’s cloud properties, which then change the temperature further. One cloud-climate feedback mechanism involves the conversion of cloud ice particles to liquid droplets with warming. This makes clouds more opaque, causing them to reflect more solar radiation back to space. It is widely accepted that this mechanism dampens climate warming, but its importance relative to other feedback mechanisms has been unclear. This study develops a method to estimate the cloud opacity feedback from ice-to-liquid conversions in climate-model simulations. On average, cloud-opacity changes from phase conversions explain approximately 19% of the overall cloud–climate feedback in an ensemble of six climate models. This finding clarifies the importance of cloud-phase conversions in projections of anthropogenic climate change.

• Haugvaldstad, Ove Westermoen; Oliviè, Dirk Jan Leo; Storelvmo, Trude & Schulz, Michael (2025). Dust radiative forcing in CMIP6 Earth System models: insights from the AerChemMIP piClim-2xdust experiment. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 25, p. 13199–13219. doi: 10.5194/acp-25-13199-2025. Full text in Research Archive Show summary

  Mineral dust significantly affects the downwelling and upwelling shortwave (SW) and longwave (LW) radiative fluxes, and changes in dust can therefore alter the Earth's energy balance. This study analyses the dust effective radiative forcing (DuERF) in nine CMIP6 Earth System Models (ESMs) using the piClim-2xdust experiment from AerChemMIP. The piClim-2xdust experiment uses a global dust emission tuning factor to double the emission flux. The DuERF is decomposed into contributions from dust-radiation (direct DuERF) and dust-cloud (cloud DuERF) interactions. The net direct DuERF ranges from −0.56 to 0.05 W m−2. Models with lower (higher) dust absorption and a smaller (larger) fraction of coarse dust show the most negative (positive) direct DuERF. The cloud DuERF is positive in most models, ranging from −0.02 to 0.2 W m−2, however, they differ in their LW and SW flux contributions. Specifically, NorESM2-LM shows a positive LW cloud DuERF attributable to the effect of dust on cirrus clouds. The dust forcing efficiency varies tenfold among models, indicating that uncertainty in DuERF is likely underestimated in AerChemMIP. There is a consistent fast precipitation response associated with dust decreasing atmospheric radiative cooling (ARC). Models with strongly absorbing dust show reduced precipitation, explainable by decreased clear-sky ARC (up to 3.2 mm yr−1). In NorESM2-LM, this decrease is associated with a cloudy sky ARC due to an increase in cirrus clouds (up to 5.6 mm yr−1). Taken together, these findings suggest that the fast precipitation response induced by dust alone may be significant and comparable to that caused by anthropogenic black carbon.

• Graff, Lise Seland; Tjiputra, Jerry; Gjermundsen, Ada; Born, Andreas; Debernard, Jens Boldingh & Goelzer, Heiko [Show all 20 contributors for this article] (2025). Sensitivity of winter Arctic amplification in NorESM2. Earth System Dynamics (ESD). ISSN 2190-4979. 16(5), p. 1671–1698. doi: 10.5194/esd-16-1671-2025. Full text in Research Archive Show summary

  While Arctic amplification is a robust feature of both observed and projected climate change, projections of Arctic climate change are characterized by substantial uncertainty. To better understand the drivers of this uncertainty, we performed a coordinated set of fully coupled experiments with the second version of the Norwegian Earth System Model (NorESM2), in which selected processes of key importance for the Arctic climate have been modified. They include improved representation of (1) mixed-phase clouds, (2) eddy processes in the upper ocean, (3) Greenland ice sheet coupling, (4) snow on sea ice processes, and (5) ozone chemistry. For each modification, we carried out sensitivity experiments following the protocols for the historical simulation of the sixth phase of the Coupled Model Intercomparison Project (CMIP6) and a future high-emissions scenario (ssp585). This results in an ensemble of modified historical and ssp585 experiments. The sensitivity experiments all demonstrate enhanced future Arctic warming compared to the unmodified historical and ssp585 experiments. Moreover, the amplitude of the additional warming varies considerably, with the difference between the experiment with the strongest and weakest Arctic mean warming reaching ∼ 9 K during the winter season by the end of the 21st century. The warming signal is dominated by a relatively uniform Arctic warming, which, according to the CMIP6 ssp585 long-term extension, starts to equilibrate during the 22nd century. Surface temperature decomposition shows that winter warming is primarily driven by an enhanced greenhouse effect due to increased cloud cover, near-surface humidity, and the resulting increase in downwelling longwave radiation. The temperature response is most pronounced in the sea ice retreat regions, with the greatest variability between experiments occurring on the Atlantic side. We also identify an emergent constraint, linking changes in Arctic surface temperatures to changes in ocean heat fluxes and sea ice area. This highlights the importance of correctly representing (contemporary) Northern Hemisphere (NH) sea ice when assessing future projected Arctic warming.

• Zoëga, Tómas; Storelvmo, Trude & Krüger, Kirstin (2025). Climate State Dependent Response to Very Large Icelandic Effusive Volcanic Eruptions. Geophysical Research Letters. ISSN 0094-8276. 52(16). doi: 10.1029/2025gl114638. Full text in Research Archive Show summary

  Effusive volcanic eruptions have been common in Iceland throughout the Holocene with the largest ones happening prior to the industrial revolution. Such eruptions can affect climate through the formation of sulfate aerosols and subsequent impacts on clouds. As different atmospheric conditions modulate the cloud and climate responses to aerosol perturbations, a pre‐industrial effusive eruption might have different climate impacts were it to happen today or in the future. Here we use an Earth system model to simulate the surface climate response to Icelandic effusive volcanic eruptions under pre‐industrial, present day, and end of the 21st century climate conditions. For the last case, high anthropogenic greenhouse gas and aerosol emissions are assumed. We find that the climate state significantly modulates the climate response, especially in the Arctic where we model amplified surface warming during winter under pre‐industrial conditions and stronger surface cooling during summer in warmer climates.

• Dammann, Stian Leer-Salvesen; Schäfer, Britta; David, Robert Oscar & Storelvmo, Trude (2025). Observations and Model Simulations of Phase Heterogeneity in Arctic Clouds. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 130(13). doi: 10.1029/2024JD042714. Full text in Research Archive Show summary

  Mixed-phase clouds (MPCs) play a key role in Earth's radiation budget particularly in the Arctic where they are ubiquitous year-round. An important characteristic of MPCs is how the cloud phases are mixed, which affects interactions between ice and liquid. Observations show that phase tends to be nonuniform with ice and liquid forming spatially separated single-phased “pockets” not accounted for in climate models. These pockets may vary in size from the micron-scale to several hundred kilometers making them notoriously difficult to study, and the factors influencing cloud-phase heterogeneity remain uncertain. We quantify size distributions of phase pockets in an observed and modeled Arctic MPC occurring 12 November 2019 in Ny-Ålesund, Svalbard. The case is simulated with the Weather Research and Forecasting model constrained with representative aerosol concentrations following measurements from the Ny-Ålesund Aerosol Cloud Experiment. We find that phase pockets exhibit broad size distributions with the smallest pockets occurring most frequently. Observations reveal mean pocket lengths of 2 km, whereas the simulated pockets are about 5 times longer on average. Simulated pocket size distributions are highly sensitive to prescribed aerosols. Moreover, we observe a pronounced increase of 6.5 km in the mean length of mixed-phase pockets when secondary ice production is enhanced in simulations. These results shed light on the link between cloud microphysics and the in-cloud distribution of phase and provide a potential framework for representation of sub-grid scale phase variability in climate models.

• Jadav, Arti; Waman, Deepak; Pant, Chandra Shekhar; Patade, Sachin; Gautam, Martanda & Phillips, Vaughan [Show all 9 contributors for this article] (2025). An Improved Convection Parameterization with Detailed Aerosol–Cloud Microphysics for a Global Model. Journal of the Atmospheric Sciences. ISSN 0022-4928. 82(1), p. 197–231. doi: 10.1175/JAS-D-23-0175.1. Full text in Research Archive Show summary

  A new microphysical treatment that includes aerosol–cloud interactions and secondary ice production (SIP) mechanisms is implemented in the convection scheme of the Community Atmosphere Model, version 6 (CAM6). The approach is to embed a 1D Lagrangian parcel model in the bulk convective plume of the existing deep convection parameterization. Aerosol activation, growth processes including collision/coalescence, and three processes of SIP mechanisms, two of which are normally overlooked in atmospheric models, are represented in this embedded parcel model. These microphysical processes are treated with a hybrid bin/bulk scheme and a high spatial and temporal resolution for the integration of the embedded parcel in 1D, allowing vertical velocity to determine the microphysical evolution following the in-cloud motion during ascent. Simulations of an observed case (Midlatitude Continental Convective Clouds Experiment) of a mesoscale convective system in Oklahoma, United States, with a single-column model (SCAM) version of CAM, are compared with aircraft in situ and ground-based observations of microphysical properties from the convection and precipitation. Results from the validation show the new microphysical scheme has a good representation of the ice initiation in the bulk convective plume, including the known and empirically quantified pathways of primary and secondary initiation, with benefits for the accuracy of properties of its supercooled cloud liquid. The sensitivity simulations and use of tagging tracers for the validated simulation confirm that the newly included SIP mechanisms are of paramount importance for convective microphysics and can be successfully treated in the global model.

• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2025). Modelled surface climate response to effusive Icelandic volcanic eruptions: sensitivity to season and size. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 25(5), p. 2989–3010. doi: 10.5194/acp-25-2989-2025. Full text in Research Archive Show summary

  Effusive, long-lasting volcanic eruptions impact climate through the emission of gases and the subsequent production of aerosols. Previous studies, both modelling and observational, have made efforts to quantify these impacts and untangle them from natural variability. However, due to the scarcity of large and well-observed effusive volcanic eruptions, our understanding remains patchy. Here, we use an Earth system model to systematically investigate the climate response to high-latitude, effusive volcanic eruptions, similar to the 2014–2015 Holuhraun eruption in Iceland, as a function of eruption season and size. The results show that the climate response is regional and strongly modulated by different seasons, exhibiting midlatitude cooling during summer and Arctic warming during winter. Furthermore, as eruptions increase in size in terms of sulfur dioxide emissions, the climate response becomes increasingly insensitive to variations in emission strength, levelling off for eruptions between 20 and 30 times the size of the 2014–2015 Holuhraun eruption. Volcanic eruptions are generally considered to lead to surface cooling, but our results indicate that this is an oversimplification, especially in the Arctic, where warming is found to be the dominant response during autumn and winter.

• Hellmuth, Franziska; Carlsen, Tim; Daloz, Anne Sophie; David, Robert Oscar; Che, Haochi & Storelvmo, Trude (2025). Evaluation of biases in mid-to-high-latitude surface snowfall and cloud phase in ERA5 and CMIP6 using satellite observations. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 25(2), p. 1353–1383. doi: 10.5194/acp-25-1353-2025. Full text in Research Archive Show summary

  Supercooled liquid-containing clouds (sLCCs) play a significant role in Earth's radiative budget and the hydrological cycle, especially through surface snowfall production. Evaluating state-of-the-art climate models with respect to their ability to simulate the frequency of occurrence of sLCCs and the frequency with which they produce snow is, therefore, critically important. Here, we compare these quantities as derived from satellite observations, reanalysis datasets, and Earth system models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) and find significant discrepancies between the datasets for mid- and high latitudes in both hemispheres. Specifically, we find that the ERA5 reanalysis and 10 CMIP6 models consistently overestimate the frequency of sLCCs and snowfall frequencies from sLCCs compared to CloudSat–CALIPSO satellite observations. The biases are very similar for ERA5 and the CMIP6 models, which indicates that the discrepancies in cloud phase and snowfall stem from differences in the representation of cloud microphysics rather than the representation of meteorological conditions. This, in turn, highlights the need for refinements in the models’ parameterizations of cloud microphysics in order for them to represent cloud phase and snowfall accurately. The thermodynamic phase of clouds and precipitation has a strong influence on simulated climate feedbacks and, thus, projections of future climate. Understanding the origin(s) of the biases identified here is, therefore, crucial for improving the overall reliability of climate models.

• Gjelsvik, Astrid Bragstad; David, Robert Oscar; Carlsen, Tim; Hellmuth, Franziska; Hofer, Stefan & McGraw, Zachary [Show all 8 contributors for this article] (2025). Using a region-specific ice-nucleating particle parameterization improves the representation of Arctic clouds in a global climate model. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 25(3), p. 1617–1637. doi: 10.5194/acp-25-1617-2025. Full text in Research Archive Show summary

  Projections of global climate change and Arctic amplification are sensitive to the representation of low-level cloud phase in climate models. Ice-nucleating particles (INPs) are necessary for primary cloud ice formation at temperatures above approximately −38 °C and thus significantly affect cloud phase and cloud radiative effect (CRE). Due to their complex and insufficiently understood variability, INPs constitute an important modelling challenge, especially in remote regions with few observations, such as the Arctic. In this study, INP observations were carried out at Andenes, Norway, in March 2021. These observations were used as a basis for an Arctic-specific and purely temperature-dependent INP parameterization, which was implemented into the Norwegian Earth System Model (NorESM). This implementation results in an annual average increase in cloud liquid water path (CLWP) of 70 % for the Arctic and improves the representation of cloud phase compared to satellite observations. The change in CLWP in boreal autumn and winter is found to likely be the dominant contributor to the annual average increase in net surface CRE of 2 W m−2. This large surface flux increase brings the simulation into better agreement with Arctic ground-based measurements. Despite the fact that the model cannot respond fully to the INP parameterization change due to fixed sea surface temperatures, Arctic surface air temperature increases by 0.7 °C in boreal autumn. These findings indicate that INPs could have a significant impact on Arctic climate and that a region-specific INP parameterization can be a useful tool to improve cloud representation in the Arctic region.

• Rauer, Heike; Aerts, Conny; Cabrera, Juan; Deleuil, Magali; et, al., including: & Erikson, Anders [Show all 847 contributors for this article] (2025). The PLATO mission. Experimental astronomy (Print). ISSN 0922-6435. 59. doi: 10.1007/s10686-025-09985-9. Full text in Research Archive Show summary

  PLATO (PLAnetary Transits and Oscillations of stars) is ESA’s M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2R) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5%, 10%, 10% for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO‘s target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile towards the end of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

• Storelvmo, Trude; Yuan, Menghan; Leirvik, Thomas; Alterskjær, Kari; Phillips, Peter C. B. & Smith, Chris (2025). Assessing the robustness and implications of econometric estimates of climate sensitivity. Environmental Research Letters. 20(2). doi: 10.1088/1748-9326/adabfc. Full text in Research Archive Show summary

  Earth's transient climate response (TCR) quantifies the global mean surface air temperature change due to a doubling of atmospheric CO2 concentration after 70 years of a compounding 1% per year increase. TCR is highly correlated with near-term climate projections, and thus of relevance for climate policy, but remains poorly constrained in part due to uncertainties in the representation of key physical processes in Earth System Models (ESMs). Within state-of-the-art ESMs participating in the Coupled Model Intercomparison Project (CMIP6), the TCR range (1.1 ºC–2.9 ºC) is too wide to offer useful guidance to policymakers. Similarly, the sixth report of the Intergovernmental Panel on Climate Change, while not solely reliant on ESMs for its TCR assessment, produced a very likely range of 1.2 ºC–2.4 ºC. To complement earlier, ESM-based, estimates, we here present a new TCR estimate of 2.17 (1.72–2.77) ºC (95% confidence interval), derived based on a statistical relationship between surface air temperature and observational proxies for its main drivers, i.e. changes in atmospheric greenhouse gases and aerosols. We show that, within uncertainty, this method correctly diagnoses TCR from 20 CMIP6 ESMs if the same input variables are taken from the ESMs that are available from observations. This increases confidence in the new observation-based central estimate and range, which is respectively higher and narrower than the mean and spread of the estimates from the entire ensemble of CMIP6. Many ESM-based estimates tend to produce TCRs lower than the observational range reported here. Our findings suggest that a misrepresentation of the aerosol cooling effect could be the cause of this discrepancy. Further, the revised TCR estimate suggests a downward revision of the remaining carbon budgets aligned with the overarching goal of the Paris agreement.

• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2025). Arctic warming from a high-latitude effusive volcanic eruption. Scientific Reports. 15(1). doi: 10.1038/s41598-025-98811-5. Full text in Research Archive Show summary

  The effusive Holuhraun eruption in Iceland emitted large quantities of sulfur into the troposphere during the fall and winter of 2014–15. Previous studies have shown that the resulting volcanic aerosols led to reduced insolation, and thus surface cooling, through increased cloud shortwave reflectance, mostly over the North Atlantic and Europe. Less attention has been paid to the Arctic, which at the time of the eruption received limited sunlight. Based on evidence from observations and model simulations, here we argue that increased cloud liquid water path and cloud cover following the 2014–15 Holuhraun eruption led to surface warming in the Arctic through trapping of longwave radiation. Our results show that sulfur emissions from the eruption led to extended lifetime of low and middle level clouds, reducing the longwave radiative cooling of the surface. This is the first time, to our knowledge, that an effusive volcanic eruption is shown to have this effect. Given the high level of volcanic activity in Iceland, these findings demonstrate the need to further investigate the climate impacts of high-latitude effusive volcanic eruptions. Moreover, marine cloud brightening through cloud seeding has been suggested as one way to combat anthropogenic climate change but, as our results suggest, such actions might have counteractive regional consequences.

• Haywood, Jim M.; Boucher, Olivier; Lennard, Chris; Storelvmo, Trude; Tilmes, Simone & Visioni, Daniele (2025). World Climate Research Programme lighthouse activity: an assessment of major research gaps in solar radiation modification research. Frontiers in Climate. 7. doi: 10.3389/fclim.2025.1507479. Full text in Research Archive Show summary

  It is increasingly evident that maintaining global warming at levels below those agreed in the legally binding international treaty on climate change. i.e., the Paris Agreement, is going to be extremely challenging using conventional mitigation techniques. While future scenarios of climate change frequently include extensive use of terrestrial and marine carbon dioxide removal in the second part of the 21st century, it is unproven that these techniques can be scaled-up to reach the scale required to significantly reduce concentrations of atmospheric carbon dioxide and significant uncertainties and detrimental side-effects exist. These issues have led to increasing interest in so-called “Solar Radiation Modification” whereby the global mean temperature of the Earth is reduced by either blocking a small fraction of sunlight from reaching it or by increasing the Earth’s albedo to reflect a small proportion of incident sunlight back out to space. Here we systematically identify key research gaps associated with the two most prominent Solar Radiation Modification techniques, i.e., Stratospheric Aerosol Injection (SAI) and Marine Cloud Brightening (MCB). We provide an assessment of the research gaps associated with other less prominent SRM techniques. We assert that transparency and inclusivity in SRM research is essential in providing objective and impartial research findings to each and every stakeholder in an equitable way.

• Schäfer, Britta; David, Robert Oscar; Hodnebrog, Øivind & Storelvmo, Trude (2025). Response of an Arctic Mixed-Phase Cloud to Ice-Nucleating Particle Perturbations and Warming. Geophysical Research Letters. ISSN 0094-8276. 52(10). doi: 10.1029/2024GL114467. Full text in Research Archive Show summary

  The Arctic is warming faster than any other region on Earth. This warming affects Arctic clouds, both directly and through changes in aerosol concentrations, triggering feedbacks that may further amplify the warming. Here we present simulations of a wintertime cloud case from Ny-Ålesund with parameterizations optimized for representing secondary ice production (SIP). We compare cloud phase and its impact on radiation in present-day conditions with simulations where we perturb (a) ice-nucleating particle concentrations (INPC) and (b) atmospheric and surface temperatures using a pseudo-global-warming approach. Increasing the INPC leads to cloud thinning and reduced downward longwave radiation at the surface (SDLR). Intriguingly, with warming we find an increase in cloud ice due to increased rime splintering (RS) and subsequent SIP, which also leads to reduced SDLR. This can be explained by an upward shift in the temperature region where RS is active to higher altitudes where more liquid water is present.

• Hofer, Stefan; Hahn, Lily; Shaw, Jonah; McGraw, Zachary; Bruno, Olimpia & Hellmuth, Franziska [Show all 11 contributors for this article] (2024). Realistic representation of mixed-phase clouds increases projected climate warming. Communications Earth & Environment. 5, p. 1–12. doi: 10.1038/s43247-024-01524-2. Full text in Research Archive Show summary

  Clouds are the main source of uncertainties when projecting climate change. Mixed-phase clouds (MPCs) that contain ice and supercooled-liquid particles are especially hard to constrain, and climate models neither agree on their phase nor their spatial extent. This is problematic, as models that underestimate contemporary supercooled-liquid in MPCs will underestimate future warming. Furthermore, it has recently been shown that supercooled-liquid water in MPCs is not homogeneously-mixed, neither vertically nor horizontally. However, while there have been attempts at observationally constraining MPCs to constrain uncertainties in future warming, all studies only use the phase of the interior of MPCs. Using novel satellite observations, and contrary to current knowledge, we show that MPCs are more liquid at the cloud top globally. We use these observations to constrain, for the first time, the cloud top phase in addition to the interior of MPCs in a global climate model, leading to +1C more 21st century warming i

• Schäfer, Britta; David, Robert Oscar; Georgakaki, Paraskevi; Pasquier, Julie Thérèse; Sotiropoulou, Georgia & Storelvmo, Trude (2024). Simulations of primary and secondary ice production during an Arctic mixed-phase cloud case from the Ny-Ålesund Aerosol Cloud Experiment (NASCENT) campaign. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 24(12), p. 7179–7202. doi: 10.5194/acp-24-7179-2024. Full text in Research Archive Show summary

  The representation of Arctic clouds and their phase distributions, i.e., the amount of ice and supercooled water, influences predictions of future Arctic warming. Therefore, it is essential that cloud phase is correctly captured by models in order to accurately predict the future Arctic climate. Ice crystal formation in clouds happens through ice nucleation (primary ice production) and ice multiplication (secondary ice production). In common weather and climate models, rime splintering is the only secondary ice production process included. In addition, prescribed number concentrations of cloud condensation nuclei or cloud droplets and ice-nucleating particles are often overestimated in Arctic environments by standard model configurations. This can lead to a misrepresentation of the phase distribution and precipitation formation in Arctic mixed-phase clouds, with important implications for the Arctic surface energy budget. During the Ny-Ålesund Aerosol Cloud Experiment (NASCENT), a holographic probe mounted on a tethered balloon took in situ measurements of number and mass concentrations of ice crystals and cloud droplets in Svalbard, Norway, during fall 2019 and spring 2020. In this study, we choose one case study from this campaign that shows evidence of strong secondary ice production and use the Weather Research and Forecasting (WRF) model to simulate it at a high vertical and spatial resolution. We test the performance of different microphysical parametrizations and apply a new state-of-the-art secondary ice parametrization. We find that agreement with observations highly depends on the prescribed cloud condensation nuclei/cloud droplet and ice-nucleating particle concentrations and requires an enhancement of secondary ice production processes. Lowering mass mixing ratio thresholds for rime splintering inside the Morrison microphysics scheme is crucial to enable secondary ice production and thereby match observations for the right reasons. In our case, rime splintering is required to initiate collisional breakup. The simulated contribution from collisional breakup is larger than that from droplet shattering. Simulating ice production correctly for the right reasons is a prerequisite for reliable simulations of Arctic mixed-phase cloud responses to future temperature or aerosol perturbations.

• Svennevik, Hanna; Hicks, Steven; Riegler, Michael; Storelvmo, Trude & Hammer, Hugo Lewi (2024). A dataset for predicting cloud cover over Europe. Scientific Data. 11, p. 1–7. doi: 10.1038/s41597-024-03062-0. Full text in Research Archive Show summary

  Clouds are important factors when projecting future climate. Unfortunately, future cloud fractional cover (the portion of the sky covered by clouds) is associated with significant uncertainty, making climate projections difficult. In this paper, we present the European Cloud Cover dataset, which can be used to learn statistical relations between cloud cover and other environmental variables, to potentially improve future climate projections. the dataset was created using a novel technique called area Weighting Regridding Scheme to map satellite observations to cloud fractional cover on the same grid as the other variables in the dataset. Baseline experiments using autoregressive models document that it is possible to use the dataset to predict cloud fractional cover.

• Skeie, Ragnhild Bieltvedt; Aldrin, Magne Tommy; Berntsen, Terje Koren; Holden, Marit; Huseby, Ragnar Bang & Myhre, Gunnar [Show all 7 contributors for this article] (2024). The aerosol pathway is crucial for observationally constraining climate sensitivity and anthropogenic forcing. Earth System Dynamics (ESD). ISSN 2190-4979. 15(6), p. 1435–1458. doi: 10.5194/esd-15-1435-2024. Full text in Research Archive Show summary

  Climate sensitivity and aerosol forcing are two of the most central, but uncertain, quantities in climate science that are crucial for assessing historical climate as well as future climate projections. Here, we use a Bayesian approach to estimate inferred climate sensitivity and aerosol forcing using observations of temperature and global ocean heat content as well as prior knowledge of effective radiative forcing (ERF) over the industrial period. Due to limited information on uncertainties related to the time evolution of aerosol forcing, we perform a range of sensitivity analyses with idealized aerosol time evolution. The estimates are sensitive to the aerosol forcing pathway, with the mean estimate of inferred effective climate sensitivity ranging from 2.0 to 2.4 K, present-day (2019 relative to 1750) aerosol ERF ranging from −0.7 to −1.1 W m−2, and anthropogenic ERF ranging from 2.6 to 3.1 W m−2. Using observations and forcing up to and including 2022, the inferred effective climate sensitivity is 2.2 K with a 1.6 to 3.0 K 90 % uncertainty range. Analysis with more freely evolving aerosol forcing between 1950 and 2014 shows that a strong negative aerosol forcing trend in the latter part of the 20th century is not consistent with observations. Although we test our estimation method with strongly idealized aerosol ERF pathways, our posteriori estimates of the climate sensitivity consistently end up in the weaker end of the range assessed in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6). As our method only includes climate feedbacks that have occurred over the historical period, it does not include the pattern effect, i.e., where climate feedbacks are dependent on the pattern of warming which will likely change into the future. Adding the best estimate of the pattern effect from IPCC AR6, our climate sensitivity estimate is almost identical to the IPCC AR6 best estimate and very likely range.

• Sokol, Adam Bennett & Storelvmo, Trude (2024). The spatial heterogeneity of cloud phase observed by satellite. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 129(3). doi: 10.1029/2023JD039751. Full text in Research Archive Show summary

  Abstract We conduct a global assessment of the spatial heterogeneity of cloud phase within the temperature range where liquid and ice can coexist. Single‐shot Cloud‐Aerosol Lidar with Orthogonal Polarization lidar retrievals are used to examine cloud phase at scales as fine as 333 m, and horizontal heterogeneity is quantified according to the frequency of switches between liquid and ice along the satellite's path. In the global mean, heterogeneity is greatest between −15 and −4°C with a peak at −5°C, when small patches of ice are prevalent within liquid‐dominated clouds. Heterogeneity “hot spots” are typically found over the extratropical continents, whereas phase is relatively homogeneous over the Southern Ocean and the eastern subtropical ocean basins, where supercooled liquid clouds dominate. Even at a fixed temperature, heterogeneity undergoes a pronounced annual cycle that, in most places, consists of a minimum during autumn or winter and a maximum during spring or summer. Based on this spatial and temporal variability, it is hypothesized that heterogeneity is affected by the availability of ice nucleating particles. These results can be used to improve the representation of subgrid‐scale heterogeneity in general circulation models, which has the potential to reduce longstanding model biases in cloud phase partitioning and radiative fluxes.

• Mostue, Idunn Aamnes; Hofer, Stefan; Storelvmo, Trude & Fettweis, Xavier (2024). Cloud- and ice-albedo feedbacks drive greater Greenland Ice Sheet sensitivity to warming in CMIP6 than in CMIP5. The Cryosphere. ISSN 1994-0416. 18(1), p. 475–488. doi: 10.5194/tc-18-475-2024. Full text in Research Archive Show summary

  The Greenland Ice Sheet (GrIS) has been losing mass since the 1990s as a direct consequence of rising temperatures and has been projected to continue to lose mass at an accelerating pace throughout the 21st century, making it one of the largest contributors to future sea-level rise. The latest Coupled Model Intercomparison Project Phase 6 (CMIP6) models produce a greater Arctic amplification signal and therefore also a notably larger mass loss from the GrIS when compared to the older CMIP5 projections, despite similar forcing levels from greenhouse gas emissions. However, it is also argued that the strength of regional factors, such as melt–albedo feedbacks and cloud-related feedbacks, will partly impact future melt and sea-level rise contribution, yet little is known about the role of these regional factors in producing differences in GrIS surface melt projections between CMIP6 and CMIP5. In this study, we use high-resolution (15 km) regional climate model simulations over the GrIS performed using the Modèle Atmosphérique Régional (MAR) to physically downscale six CMIP5 Representative Concentration Pathway (RCP) 8.5 and five CMIP6 Shared Socioeconomic Pathway (SSP) 5-8.5 extreme high-emission-scenario simulations. Here, we show a greater annual mass loss from the GrIS at the end of the 21st century but also for a given temperature increase over the GrIS, when comparing CMIP6 to CMIP5. We find a greater sensitivity of Greenland surface mass loss in CMIP6 centred around summer and autumn, yet the difference in mass loss is the largest during autumn with a reduction of 27.7 ± 9.5 Gt per season for a regional warming of +6.7 ∘C and 24.6 Gt per season more mass loss than in CMIP5 RCP8.5 simulations for the same warming. Assessment of the surface energy budget and cloud-related feedbacks suggests a reduction in high clouds during summer and autumn – despite enhanced cloud optical depth during autumn – to be the main driver of the additional energy reaching the surface, subsequently leading to enhanced surface melt and mass loss in CMIP6 compared to CMIP5. Our analysis highlights that Greenland is losing more mass in CMIP6 due to two factors: (1) a (known) greater sensitivity to greenhouse gas emissions and therefore warmer temperatures and (2) previously unnotified cloud-related surface energy budget changes that enhance the GrIS sensitivity to warming.

• Toll, Velle; Rahu, Jorma; Keernik, Hannes; Trofimov, Heido; Voormansik, Tanel & Manshausen, Peter [Show all 19 contributors for this article] (2024). Glaciation of liquid clouds, snowfall, and reduced cloud cover at industrial aerosol hot spots. Science. ISSN 0036-8075. 386(6723), p. 756–762. doi: 10.1126/science.adl0303. Full text in Research Archive
• Li, Ryan L.; Studholme, Joshua H. P.; Fedorov, Alexey V. & Storelvmo, Trude (2023). Increasing Precipitation Efficiency Amplifies Climate Sensitivity by Enhancing Tropical Circulation Slowdown and Eastern Pacific Warming Pattern. Geophysical Research Letters. ISSN 0094-8276. 50(2). doi: 10.1029/2022GL100836. Full text in Research Archive Show summary

  The role of precipitation efficiency (PE)—the fraction of column-integrated condensate that reaches the surface as rain—in the global temperature response to CO2 rise is yet to be quantified. Here we employ 36 limited-domain cloud resolving models (CRMs) from the Radiative-Convective Equilibrium Model Intercomparison Project and find that they strongly imply higher PE at warmer temperatures. We then analyze 35 general circulation models (GCMs) from the Coupled Model Intercomparison Project Phase 6 and find that increasing PE is associated with tropical circulation slowdown and greater eastern equatorial Pacific warming. These changes trigger pan-tropical positive cloud feedback through stratiform anvil cloud reduction and stratocumulus suppression, resulting in higher Effective Climate Sensitivity (ECS). We find that in 24 of 35 GCMs matching the CRMs in simulating increasing PE with greenhouse warming, mean ECS is 1 K higher than in PE-decreasing GCMs. Thus, further constraining PE sensitivity to temperature could reduce uncertainty over future climate projections.

• Gettelman, Andrew; Morrison, Hugh; Eidhammer, Trude; Thayer-Calder, Katherine; Sun, Jian & Forbes, Richard [Show all 10 contributors for this article] (2023). Importance of ice nucleation and precipitation on climate with the Parameterization of Unified Microphysics Across Scales version 1 (PUMASv1). Geoscientific Model Development. ISSN 1991-959X. 16(6), p. 1735–1754. doi: 10.5194/gmd-16-1735-2023. Full text in Research Archive Show summary

  Abstract. Cloud microphysics is critical for weather and climate prediction. In this work, we document updates and corrections to the cloud microphysical scheme used in the Community Earth System Model (CESM) and other models. These updates include a new nomenclature for the scheme, now called Parameterization of Unified Microphysics Across Scales (PUMAS), and the ability to run the scheme on graphics processing units (GPUs). The main science changes include refactoring an ice number limiter and associated changes to ice nucleation, adding vapor deposition onto snow, and introducing an implicit numerical treatment for sedimentation. We also detail the improvements in computational performance that can be achieved with GPU acceleration. We then show the impact of these scheme changes on the (a) mean state climate, (b) cloud feedback response to warming, and (c) aerosol forcing. We find that corrections are needed to the immersion freezing parameterization and that ice nucleation has important impacts on climate. We also find that the revised scheme produces less cloud liquid and ice but that this can be adjusted by changing the loss process for cloud liquid (autoconversion). Furthermore, there are few discernible effects of the PUMAS changes on cloud feedbacks but some reductions in the magnitude of aerosol–cloud interactions (ACIs). Small cloud feedback changes appear to be related to the implicit sedimentation scheme, with a number of factors affecting ACIs.

• Wall, Casey James; Storelvmo, Trude & Possner, Anna (2023). Global observations of aerosol indirect effects from marine liquid clouds. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 23(20), p. 13125–13141. doi: 10.5194/acp-23-13125-2023. Full text in Research Archive Show summary

  Abstract. Interactions between aerosols and liquid clouds are one of the largest sources of uncertainty in the historical radiative forcing of climate. One widely shared goal to reduce this uncertainty is to decompose radiative anomalies arising from aerosol–cloud interactions into components associated with changes in cloud-droplet number concentration (Twomey effect), liquid-water-path adjustments, and cloud-fraction adjustments. However, there has not been a quantitative foundation for simultaneously estimating these components with global satellite observations. Here we present a method for assessing shortwave radiative flux anomalies from the Twomey effect and cloud adjustments over ocean between 55∘ S and 55∘ N. We find that larger aerosol concentrations are associated with widespread cloud brightening from the Twomey effect, a positive radiative adjustment from decreasing liquid water path in subtropical stratocumulus regions, and a negative radiative adjustment from increasing cloud fraction in the subtropics and midlatitudes. The Twomey effect and total cloud adjustment have contributed −0.77 ± 0.25 and −1.02 ± 0.43 W m−2, respectively, to the effective radiative forcing since 1850 over the domain (95 % confidence). Our findings reduce uncertainty in these components of aerosol forcing and suggest that cloud adjustments make a larger contribution to the forcing than is commonly believed.

• McGraw, Zachary; Storelvmo, Trude; Polvani, Lorenzo M.; Hofer, Stefan; Shaw, Jonah K. & Gettelman, Andrew (2023). On the Links Between Ice Nucleation, Cloud Phase, and Climate Sensitivity in CESM2. Geophysical Research Letters. ISSN 0094-8276. 50(17). doi: 10.1029/2023GL105053. Full text in Research Archive Show summary

  Abstract Ice nucleation in mixed‐phase clouds has been identified as a critical factor in projections of future climate. Here we explore how this process influences climate sensitivity using the Community Earth System Model 2 (CESM2). We find that ice nucleation affects simulated cloud feedbacks over most regions and levels of the troposphere, not just extratropical low clouds. However, with present‐day global mean cloud phase adjusted to replicate satellite retrievals, similar total cloud feedback is attained whether ice nucleation is simulated as aerosol‐sensitive, insensitive, or absent. These model experiments all result in a strongly positive total cloud feedback, as in the default CESM2. A microphysics update from CESM1 to CESM2 had substantially weakened ice nucleation, due partly to a model issue. Our findings indicate that this update reduced global cloud phase bias, with CESM2's high climate sensitivity reflecting improved mixed‐phase cloud representation.

• Kok, Jasper F.; Storelvmo, Trude; Karydis, Vlassis A.; Adebiyi, Adeyemi A.; Mahowald, Natalie M. & Evan, Amato T. [Show all 8 contributors for this article] (2023). Mineral dust aerosol impacts on global climate and climate change. Nature Reviews Earth & Environment. 4, p. 71–86. doi: 10.1038/s43017-022-00379-5. Full text in Research Archive Show summary

  Mineral dust aerosols impact the energy budget of Earth through interactions with radiation, clouds, atmospheric chemistry, the cryosphere and biogeochemistry. In this Review, we summarize these interactions and assess the resulting impacts of dust, and of changes in dust, on global climate and climate change. The total effect of dust interactions on the global energy budget of Earth — the dust effective radiative effect — is −0.2 ± 0.5 W m−2 (90% confidence interval), suggesting that dust net cools the climate. Global dust mass loading has increased 55 ± 30% since pre-industrial times, driven largely by increases in dust from Asia and North Africa, leading to changes in the energy budget of Earth. Indeed, this increase in dust has produced a global mean effective radiative forcing of −0.07 ± 0.18 W m−2, somewhat counteracting greenhouse warming. Current climate models and climate assessments do not represent the historical increase in dust and thus omit the resulting radiative forcing, biasing climate change projections and assessments of climate sensitivity. Climate model simulations of future changes in dust diverge widely and are very uncertain. Further work is thus needed to constrain the radiative effects of dust on climate and to improve the representation of dust in climate models.

• Bjordal, Jenny; Storelvmo, Trude & Jr., Anthony A Smith (2022). Quantifying uncertainty about global and regional economic impacts of climate change. Environmental Research Letters. 17(9). doi: 10.1088/1748-9326/ac8ab1. Full text in Research Archive Show summary

  Abstract The economic impacts of climate change are highly uncertain. Two of the most important uncertainties are the sensitivity of the climate system and the so-called damage functions, which relate climate change to economic costs and benefits. Despite broad awareness of these uncertainties, it is unclear which of them is most important, especially at the regional level. Here we construct regional damage functions, based on two different global damage functions, and apply them to two climate models with vastly different climate sensitivities. We find that uncertainty in both climate sensitivity and aggregate economic damages per degree of warming are of similar importance for the global economic impact of climate change, with the decrease in global economic productivity ranging between 4% and 24% by the end of the century under a high-emission scenario. At the regional level, however, the effects of climate change can vary even more substantially, depending both on a region’s initial temperature and the amount of warming it experiences, with some regions gaining in productivity and others losing. The ranges of uncertainty are therefore potentially much larger at a regional level. For example, at the end of the century, under a high-emission scenario, we find that India’s productivity decreases between 13% and 57% and Russia’s increases between 24% and 74%, while Germany’s change in productivity ranges from an increase of 8% to a decrease of 4%. Our findings emphasise the importance of including these uncertainties in estimates of future economic impacts, as they are vital for the resulting impacts and thus policy implications.

• Pasquier, Julie T.; David, Robert Oscar; Freitas, Gabriel; Gierens, Rosa; Gramlich, Yvette & Haslett, Sophie [Show all 44 contributors for this article] (2022). The Ny-Ålesund Aerosol Cloud Experiment (NASCENT) Overview and First Results. Bulletin of The American Meteorological Society - (BAMS). ISSN 0003-0007. 103(11), p. E2533–E2558. doi: 10.1175/BAMS-D-21-0034.1. Full text in Research Archive Show summary

  Abstract The Arctic is warming at more than twice the rate of the global average. This warming is influenced by clouds which modulate the solar and terrestrial radiative fluxes, and thus, determine the surface energy budget. However, the interactions among clouds, aerosols, and radiative fluxes in the Arctic are still poorly understood. To address these uncertainties, the Ny-Ålesund AeroSol Cloud ExperimeNT (NASCENT) study was conducted from September 2019 to August 2020 in Ny-Ålesund Svalbard. The campaign’s primary goal was to elucidate the life cycle of aerosols in the Arctic and to determine how they modulate cloud properties throughout the year. In-situ and remote sensing observations were taken on the ground at sea-level and at a mountaintop station, and with a tethered balloon system. An overview of the meteorological and the main aerosol seasonality encountered during the NASCENT year is introduced, followed by a presentation of first scientific highlights. In particular, we present new findings on aerosol physicochemical properties which also include molecular properties. Further, the role of cloud droplet activation and ice crystal nucleation in the formation and persistence of mixed-phase clouds, and the occurrence of secondary ice processes, are discussed and compared to the representation of cloud processes within the regional Weather Research and Forecasting model. The paper concludes with research questions that are to be addressed in upcoming NASCENT publications.

• Schäfer, Britta; Carlsen, Tim; Hanssen, Ingrid; Gausa, Michael & Storelvmo, Trude (2022). Observations of cold-cloud properties in the Norwegian Arctic using ground-based and spaceborne lidar. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 22(14), p. 9537–9551. doi: 10.5194/acp-22-9537-2022. Full text in Research Archive Show summary

  Abstract. The role of clouds in the surface radiation budget is particularly complex in the rapidly changing Arctic. However, despite their importance, long-term observations of Arctic clouds are relatively sparse. Here, we present observations of cold clouds based on 7 years (2011–2017) of ground-based lidar observations at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in Andenes in the Norwegian Arctic. In two case studies, we assess (1) the agreement between a co-located cirrus cloud observations from the ground-based lidar and the spaceborne lidar aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite and (2) the ground-based lidar's capability to determine the cloud phase in mixed-phase clouds from depolarization measurements. We then compute multiyear statistics of cold clouds from both platforms with respect to their occurrence, cloud top and base height, cloud top temperature, and thermodynamic phase for the 2011–2017 period. We find that satellite- and ground-based observations agree well with respect to the coincident cirrus measurement and that the vertical phase distribution within a liquid-topped mixed-phase cloud could be identified from depolarization measurements. On average, 8 % of all satellite profiles were identified as single-layer cold clouds with no apparent seasonal differences. The average cloud top and base heights, combining the ground-based and satellite measurements, are 9.1 and 6.9 km, respectively, resulting in an average thickness of 2.2 km. Seasonal differences between the average top and base heights are on the order of 1–2 km and are largest when comparing fall (highest) and spring (lowest). However, seasonal variations are small compared with the observed day-to-day variability. Cloud top temperatures agree well between both platforms, with warmer cloud top temperatures in summer. The presented study demonstrates the capabilities of long-term cloud observations in the Norwegian Arctic from the ground-based lidar at Andenes.

• Julsrud, Ingeborg Rian; Storelvmo, Trude; Schulz, Michael; Moseid, Kine Onsum & Wild, Martin (2022). Disentangling Aerosol and Cloud effects on Dimming and Brightening in Observations and CMIP6. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 127(21). doi: 10.1029/2021JD035476. Full text in Research Archive Show summary

  Periods of dimming and brightening have been recorded in observational datasets of surface solar radiation (SSR) between the mid-20th century and present day. Atmospheric components affect SSR, including aerosols and clouds, though studies disagree somewhat about the relative effect of each component in different regions. Current Earth system models (ESMs) are unable to simulate observed trends in SSR. This study includes an investigation into observed SSR variations between 1961 and 2014 and an evaluation of the effects of cloud cover variations and impacts of aerosol extinction, using timeseries of SSR and cloud cover from in-situ measurements. Historical simulations by 42 ESMs participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) have also been studied and compared to observations. The observational study indicates that cloud cover has had a dampening effect on the variations of SSR and that emissions of aerosol and aerosol precursors are the main cause of the general trends in observed SSR in four regions—China, Japan, Europe and the United States—during 1961-2014. The study of simulated SSR in CMIP6 yields the conclusion that current ESMs remain unable to simulate the magnitude of observed dimming and brightening in China, Japan and the United States, but that the European SSR trends between 1961 and 2014 are fairly well reproduced in the ESMs. A rough quantification of the regional surface radiation extinction efficiency of aerosol and precursor emissions in the simulations is found to agree with observed values in Europe, but not in the other three regions.

• Shaw, Jonah; McGraw, Zachary; Bruno, O.; Storelvmo, Trude & Hofer, Stefan (2022). Using Satellite Observations to Evaluate Model Microphysical Representation of Arctic Mixed-Phase Clouds. Geophysical Research Letters. ISSN 0094-8276. 49(3). doi: 10.1029/2021GL096191. Full text in Research Archive Show summary

  Mixed-phase clouds play an important role in determining Arctic warming, but are parametrized in models and difficult to constrain with observations. We use two satellite-derived cloud phase metrics to investigate the vertical structure of Arctic clouds in two global climate models that use the Community Atmosphere Model version 6 (CAM6) atmospheric component. We report a model error limiting ice nucleation, produce a set of Arctic-constrained model runs by adjusting model microphysical variables to match the cloud phase metrics, and evaluate cloud feedbacks for all simulations. Models in this small ensemble uniformly overestimate total cloud fraction in the summer, but have variable representation of cloud fraction and phase in the winter and spring. By relating modeled cloud phase metrics and changes in low-level liquid cloud amount under warming to longwave cloud feedback, we show that mixed-phase processes mediate the Arctic climate by modifying how wintertime and springtime clouds respond to warming.

• Engdahl, Bjørg Jenny Kokkvoll; Carlsen, Tim; Køltzow, Morten Andreas Ødegaard & Storelvmo, Trude (2022). The Ability of the ICE-T Microphysics Scheme in HARMONIE-AROME to Predict Aircraft Icing. Weather and forecasting. ISSN 0882-8156. 37(2), p. 205–217. doi: 10.1175/WAF-D-21-0104.1. Full text in Research Archive Show summary

  Abstract In-cloud icing is a major hazard for aviation traffic and forecasting of these events is an important task for weather agencies worldwide. A common tool utilized by aviation forecasters is an icing intensity index based on supercooled liquid water from numerical weather prediction models. We seek to validate the modified microphysics scheme, ICE-T, in the HARMONIE-AROME numerical weather prediction model with respect to aircraft icing. Icing intensities and supercooled liquid water derived from two 3-month winter season simulations with the original microphysics code, CTRL, and ICE-T are compared with pilot reports of icing and satellite retrieved values of liquid and ice water content from CloudSat–CALIPSO and liquid water path from AMSR-2. The results show increased supercooled liquid water and higher icing indices in ICE-T. Several different thresholds and sizes of neighborhood areas for icing forecasts were tested out, and ICE-T captures more of the reported icing events for all thresholds and nearly all neighborhood areas. With a higher frequency of forecasted icing, a higher false alarm ratio cannot be ruled out, but is not possible to quantify due to the lack of no-icing observations. The increased liquid water content in ICE-T shows a better match with the retrieved satellite observations, yet the values are still greatly underestimated at lower levels. Future studies should investigate this issue further, as liquid water content also has implications for downstream processes such as the cloud radiative effect, latent heat release, and precipitation.

• Wall, Casey J.; Storelvmo, Trude; Norris, Joel R. & Tan, Ivy (2022). Observational Constraints on Southern Ocean Cloud-Phase Feedback. Journal of Climate. ISSN 0894-8755. 35(15), p. 5087–5102. doi: 10.1175/JCLI-D-21-0812.1. Full text in Research Archive Show summary

  Abstract Shortwave radiative feedbacks from Southern Ocean clouds are a major source of uncertainty in climate projections. Much of this uncertainty arises from changes in cloud scattering properties and lifetimes that are caused by changes in cloud thermodynamic phase. Here we use satellite observations to infer the scattering component of the cloud-phase feedback mechanism and determine its relative importance by comparing it with an estimate of the overall temperature-driven cloud feedback. The overall feedback is dominated by an optical thinning of low-level clouds. In contrast, the scattering component of cloud-phase feedback is an order of magnitude smaller and is primarily confined to free-tropospheric clouds. The small magnitude of this feedback component is a consequence of counteracting changes in albedo from cloud optical thickening and enhanced forward scattering by cloud particles. These results indicate that shortwave cloud feedback is likely positive over the Southern Ocean and that changes in cloud scattering properties arising from phase changes make a small contribution to the overall feedback. The feedback constraints shift the projected 66% confidence range for the global equilibrium temperature response to doubling atmospheric CO 2 by about +0.1 K relative to a recent consensus estimate of cloud feedback. Significance Statement Understanding how clouds respond to global warming is a key challenge of climate science. One particularly uncertain aspect of the cloud response involves a conversion of ice particles to liquid droplets in extratropical clouds. Here we use satellite data to infer how cloud-phase conversions affect climate by changing cloud albedo. We find that ice-to-liquid conversions increase cloud optical thickness and shift the scattering angles of cloud particles toward the forward direction. These changes in optical properties have offsetting effects on cloud albedo. This finding provides new insight about how changes in cloud phase affect climate change.

• Li, Ryan L.; Studholme, Joshua H. P.; Fedorov, Alexey V. & Storelvmo, Trude (2022). Precipitation efficiency constraint on climate change. Nature Climate Change. ISSN 1758-678X. 12, p. 642–648. doi: 10.1038/s41558-022-01400-x. Full text in Research Archive Show summary

  Precipitation efficiency (PE) relates cloud condensation to precipitation and intrinsically binds atmospheric circulation to the hydrological cycle. Due to PE’s inherent microphysical dependencies, definitions and estimates vary immensely. Consequently, PE’s sensitivity to greenhouse warming and implications for climate change are poorly understood. Here, we quantify PE’s role in climate change by defining a simple index ϵ as the ratio of surface precipitation to condensed water path. This macroscopic metric is reconcilable with microphysical PE measures and higher ϵ is associated with stronger mean Walker circulation. We further find that state-of-the-art climate models disagree on the sign and magnitude of future ϵ changes. This sign disagreement originates from models’ convective parameterizations. Critically, models with increasing ϵ under greenhouse warming, in line with cloud-resolving simulations, show greater slowdown of the large-scale Hadley and Walker circulations and a two-fold greater increase in extreme rainfall than models with decreasing ϵ.

• Sporre, M.K.; Friberg, J.; Svenhag, C.; Sourdeval, O. & Storelvmo, Trude (2022). Springtime Stratospheric Volcanic Aerosol Impact on Midlatitude Cirrus Clouds. Geophysical Research Letters. ISSN 0094-8276. 49(2). doi: 10.1029/2021GL096171. Full text in Research Archive Show summary

  Explosive volcanic eruptions can reach the stratosphere and cause elevated concentrations of sulphate particles for months to years. When these particles descend into the troposphere, they can impact cirrus clouds though to what degree is unknown. In this study, we combine three satellite data sets to investigate the impact of downwelling sulphate aerosol on midlatitude cirrus clouds during springtime. The results show that cirrus clouds in the northern hemisphere (NH) have lower ice water content (IWC), ice crystal number concentrations, and cloud fraction (CF) when the aerosol load in the lowermost stratosphere is elevated by volcanism. These changes are largest for the coldest clouds at the highest altitudes. The cirrus clouds in the southern hemisphere on the other hand show no significant changes with downwelling aerosol levels. The reduction in cirrus IWC and CF in the NH implies that volcanic aerosol can cool the climate through reduced warming from cirrus clouds.

• Li, Ryan; Fedorov, Alexey; Storelvmo, Trude & Studholme, Joshua (2021). Precipitation efficiency amplifies climate sensitivity by enhancing tropical circulation slowdown and eastern pacific warming. Nature Climate Change. ISSN 1758-678X. Full text in Research Archive
• Vâjâiac, Sorin Nicolae; Calcan, Andreea; David, Robert Oscar; Moaca, Denisa-Elena; Iorga, Gabriela & Storelvmo, Trude [Show all 8 contributors for this article] (2021). Post-flight analysis of detailed size distributions of warm cloud droplets, as determined in situ by cloud and aerosol spectrometers. Atmospheric Measurement Techniques. ISSN 1867-1381. 14(10), p. 6777–6794. doi: 10.5194/amt-14-6777-2021. Full text in Research Archive Show summary

  Warm clouds, consisting of liquid cloud droplets, play an important role in modulating the amount of incoming solar radiation to Earth's surface and thus the climate. The size and number concentration of these cloud droplets control the reflectance of the cloud, the formation of precipitation and ultimately the lifetime of the cloud. Therefore, in situ observations of the number and diameter of cloud droplets are frequently performed with cloud and aerosol spectrometers, which determine the optical diameters of cloud particles (in the range of up to a few tens of micrometers) by measuring their forward-scattering cross sections in visible light and comparing these values with Mie theoretical computations. The use of such instruments must rely on a fast working scheme consisting of a limited pre-defined uneven grid of cross section values that corresponds to a theoretically derived uneven set of size intervals (bins). However, as more detailed structural analyses of warm clouds are needed to improve future climate projects, we present a new numerical post-flight methodology using recorded particle-by-particle sample files. The Mie formalism produces a complicated relationship between a particle's diameter and its forward-scattering cross section. This relationship cannot be expressed in an analytically closed form, and it should be numerically computed point by point, over a certain grid of diameter values. The optimal resolution required for constructing the diagram of this relationship is therefore analyzed. Cloud particle statistics are further assessed using a fine grid of particle diameters in order to capture the finest details of the cloud particle size distributions. The possibility and the usefulness of using coarser size grids, with either uneven or equal intervals, is also discussed. For coarse equidistant size grids, the general expressions of cloud microphysical parameters are calculated and the ensuing relative errors are discussed in detail. The proposed methodology is further applied to a subset of measured data, and it is shown that the overall uncertainties in computing various cloud parameters are mainly driven by the measurement errors of the forward-scattering cross section for each particle. Finally, the influence of the relatively large imprecision in the real and imaginary parts of the refractive index of cloud droplets on the size distributions and on the ensuing cloud parameters is analyzed. It is concluded that, in the presence of high atmospheric loads of hydrophilic and light-absorbing aerosols, such imprecisions may drastically affect the reliability of the cloud data obtained with cloud and aerosol spectrometers. Some complementary measurements for improving the quality of the cloud droplet size distributions obtained in post-flight analyses are suggested.

• Hellmuth, Franziska; Engdahl, Bjørg Jenny Kokkvoll; Storelvmo, Trude; David, Robert Oscar & Cooper, Steven J. (2021). Snowfall Model Validation Using Surface Observations and an Optimal Estimation Snowfall Retrieval. Weather and forecasting. ISSN 0882-8156. 36(5), p. 1827–1842. doi: 10.1175/WAF-D-20-0220.1. Full text in Research Archive Show summary

  In the winter, orographic precipitation falls as snow in the mid to high latitudes where it causes avalanches, affects local infrastructure, or leads to flooding during the spring thaw. We present a technique to validate operational numerical weather prediction model simulations in complex terrain. The presented verification technique uses a combined retrieval approach to obtain surface snowfall accumulation and vertical profiles of snow water at the Haukeliseter test site, Norway. Both surface observations and vertical profiles of snow are used to validate model simulations from the Norwegian Meteorological Institute’s operational forecast system and two simulations with adjusted cloud microphysics. Retrieved surface snowfall is validated against measurements conducted with a double-fence automated reference gauge (DFAR). In comparison, the optimal estimation snowfall retrieval produces + 10.9% more surface snowfall than the DFAR. The predicted surface snowfall from the operational forecast model and two additional simulations with microphysical adjustments (CTRL and ICE-T) are overestimated at the surface with +41.0 %, +43.8 %, and +59.2 %, respectively. Simultaneously, the CTRL and ICE-T simulations underestimate the mean snow water path by -1071.4% and -523.7 %, respectively. The study shows that we would reach false conclusions only using surface accumulation or vertical snow water content profiles. These results highlight the need to combine ground-based in-situ and vertically-profiling remote sensing instruments to identify biases in numerical weather prediction.

• Hofer, Stefan; Amory, Charles; Kittel, Christoph; Carlsen, Tim; Toumelin, Louis Le & Storelvmo, Trude (2021). The Contribution of Drifting Snow to Cloud Properties and the Atmospheric Radiative Budget Over Antarctica. Geophysical Research Letters. ISSN 0094-8276. 48(22). doi: 10.1029/2021GL094967. Full text in Research Archive Show summary

  The Antarctic Ice Sheet experiences perpetual katabatic winds, transporting snow, and moisture from the interior towards the periphery. However, the impacts of Antarctic moisture and drifting snow on cloud structure and surface energy fluxes have not been widely investigated. Here, we use a regional climate model with a newly developed drifting snow scheme to show that accounting for drifting snow notably alters the spatial distribution, vertical structure and radiative effect of clouds over Antarctica. Overall, we find that accounting for drifting snow leads to a greater cloud cover providing an increase of +2.74 Wm−2 in the surface radiative energy budget. Additionally, a comparison with 20 weather stations reveals a 2.17 Wm−2 improvement in representing the radiative energy fluxes. Our results highlight the need to study the impact of drifting snow processes on the future evolution of clouds, the surface energy budget and the vertical atmospheric structure over Antarctica.

• Huusko, Linnea; Bender, Frida A.-M.; Ekman, Annica M. L. & Storelvmo, Trude (2021). Climate sensitivity indices and their relation with projected temperature change in CMIP6 models. Environmental Research Letters. 16(6). doi: 10.1088/1748-9326/ac0748. Full text in Research Archive Show summary

  Abstract Equilibrium climate sensitivity (ECS) and transient climate response (TCR) are both measures of the sensitivity of the climate system to external forcing, in terms of temperature response to CO 2 doubling. Here it is shown that, of the two, TCR in current-generation coupled climate models is better correlated with the model projected temperature change from the pre-industrial state, not only on decadal time scales but throughout much of the 21st century. For strong mitigation scenarios the difference persists until the end of the century. Historical forcing on the other hand has a significant degree of predictive power of past temperature evolution in the models, but is not relevant to the magnitude of temperature change in their future projections. Regional analysis shows a superior predictive power of ECS over TCR during the latter half of the 21st century in areas with slow warming, illustrating that although TCR is a better predictor of warming on a global scale, it does not capture delayed regional feedbacks, or pattern effects. The transient warming at CO 2 quadrupling (T140) is found to be correlated with global mean temperature anomaly for a longer time than TCR, and it also better describes the pattern of regional temperature anomaly at the end of the century. Over the 20th century, there is a weak correlation between total forcing and ECS, contributing to, but not determining, the model agreement with observed warming. ECS and aerosol forcing in the models are not correlated.

• Smith, Christopher J.; Harris, Glen R.; Palmer, Matthew D.; Bellouin, Nicolas; Collins, William J. & Myhre, Gunnar [Show all 11 contributors for this article] (2021). Energy Budget Constraints on the Time History of Aerosol Forcing and Climate Sensitivity. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 126(13). doi: 10.1029/2020JD033622. Full text in Research Archive Show summary

  An observationally constrained time series of historical aerosol effective radiative forcing (ERF) from 1750 to 2019 is developed in this study. We find that the time history of aerosol ERFs diagnosed in CMIP6 models exhibits considerable variation and explore how the time history of aerosol forcing influences the probability distributions of present-day aerosol forcing and emergent metrics such as climate sensitivity. Using a simple energy balance model, trained on CMIP6 climate models and constrained by observed near-surface warming and ocean heat uptake, we derive estimates for the historical aerosol forcing. We find 2005–2014 mean aerosol ERF to be −1.1 (−1.8 to −0.5) W m−2 relative to 1750. Assuming recently published historical emissions from fossil fuel and industrial sectors and biomass burning emissions from SSP2-4.5, aerosol ERF in 2019 is −0.9 (−1.5 to −0.4) W m−2. There is a modest recovery in aerosol forcing (+0.025 W m−2 decade−1) between 1980 and 2014. This analysis also gives a 5%–95% range of equilibrium climate sensitivity of 1.8°C –5.1°C (best estimate 3.1°C) with a transient climate response of 1.2°C –2.6°C (best estimate 1.8°C).

• Svennevik, Hanna; Riegler, Michael A.; Hicks, Steven; Storelvmo, Trude & Hammer, Hugo L. (2021). Prediction of cloud fractional cover using machine learning. Big Data and Cognitive Computing. 5(4). doi: 10.3390/bdcc5040062. Full text in Research Archive Show summary

  Climate change is stated as one of the largest issues of our time, resulting in many unwanted effects on life on earth. Cloud fractional cover (CFC), the portion of the sky covered by clouds, might affect global warming and different other aspects of human society such as agriculture and solar energy production. It is therefore important to improve the projection of future CFC, which is usually projected using numerical climate methods. In this paper, we explore the potential of using machine learning as part of a statistical downscaling framework to project future CFC. We are not aware of any other research that has explored this. We evaluated the potential of two different methods, a convolutional long short-term memory model (ConvLSTM) and a multiple regression equation, to predict CFC from other environmental variables. The predictions were associated with much uncertainty indicating that there might not be much information in the environmental variables used in the study to predict CFC. Overall the regression equation performed the best, but the ConvLSTM was the better performing model along some coastal and mountain areas. All aspects of the research analyses are explained including data preparation, model development, ML training, performance evaluation and visualization

• Bruno, Olimpia; Hoose, Corinna; Storelvmo, Trude; Coopman, Quentin & Stengel, Martin (2021). Exploring the Cloud Top Phase Partitioning in Different Cloud Types Using Active and Passive Satellite Sensors. Geophysical Research Letters. ISSN 0094-8276. 48(2). doi: 10.1029/2020GL089863. Full text in Research Archive Show summary

  One of the largest uncertainties in numerical weather prediction and climate models is the representation of mixed‐phase clouds. With the aim of understanding how the supercooled liquid fraction (SLF) in clouds with temperature from −40°C to 0°C is related to temperature, geographical location, and cloud type, our analysis contains a comparison of four satellite‐based datasets (one derived from active and three from passive satellite sensors), and focuses on SLF distribution near‐globally, but also stratified by latitude and continental/maritime regions. Despite the warm bias in cloud top temperature of the passive sensor compared to the active sensor and the phase mismatch in collocated data, all datasets indicate, at the same height‐level, an increase of SLF with cloud optical thickness, and generally larger SLF in the Southern Hemisphere than in the Northern Hemisphere (up to about 20% difference), with the exception of continental low‐level clouds, for which the opposite is true.

• Sagoo, Navjit; Storelvmo, Trude; Hahn, Lily Caroline; Tan, Ivy; Danco, James & Raney, Bryan [Show all 7 contributors for this article] (2021). Observationally Constrained Cloud Phase Unmasks Orbitally Driven Climate Feedbacks. Geophysical Research Letters. ISSN 0094-8276. 48(6). doi: 10.1029/2020GL091873. Full text in Research Archive Show summary

  The mechanisms which amplify orbitally driven changes in insolation and drive the glacial cycles of the past 2.6 million years, the Pleistocene, are poorly understood. Previous studies indicate that cloud phase feedbacks oppose ice sheet initiation when orbital configuration supports ice sheet growth. Cloud phase was observationally constrained in a recent study and provides evidence for a weaker negative cloud feedback in response to carbon dioxide doubling. We observationally constrain cloud phase in the Community Earth System Model and explore how changes in orbital configuration impact the climate response. Constraining cloud phase weakens the negative high latitude cloud phase feedback and unmasks positive water vapor and cloud feedbacks (amount and optical depth) that extend cooling to lower latitudes. Snowfall accumulation and ablation metrics also support ice sheet expansion as seen in proxy records. This indicates that well‐known cloud and water vapor feedbacks are the mechanisms amplifying orbital climate forcing.

• Karset, Inger Helene H.; Gettelman, Andrew; Storelvmo, Trude; Alterskjær, Kari & Berntsen, Terje Koren (2020). Exploring Impacts of Size-Dependent Evaporation and Entrainment in a Global Model. Journal of Geophysical Research (JGR): Space Physics. ISSN 2169-9380. 125(4). doi: 10.1029/2019JD031817. Full text in Research Archive Show summary

  Aerosol radiative forcing can influence climate both locally and far outside the emission region. Here we investigate black carbon (BC) aerosols emitted in four major emission areas and evaluate the importance of emission location and magnitude as well as the concept of the absolute regional temperature-change potentials (ARTP). We perform simulations with a climate model (NorESM) with a fully coupled ocean and with fixed sea surface temperatures. BC emissions for year 2000 are increased by factors of 10 and 20 in South Asia, North America, and Europe, respectively, and by 5 and 10 in East Asia (due to higher emissions there). The perturbed simulations and a reference simulation are run for 100 years with three ensemble members each. We find strikingly similar regional surface temperature responses and geographical patterns per unit BC emission in Europe and North America but somewhat lower temperature sensitivities for East Asian emissions. BC emitted in South Asia shows a different geographical pattern in surface temperatures, by changing the Indian monsoon and cooling the surface. We find that the ARTP approach rather accurately reproduces the fully coupled temperature response of NorESM. Choosing the highest emission rate results in lower surface temperature change per emission unit compared to the lowest rate, but the difference is generally not statistically significant except for the Arctic. An advantage of high-perturbation simulations is the clearer emergence of regional signals. Our results show that the linearity of normalized temperature effects of BC is fairly well preserved despite the relatively large perturbations but that regional temperature coefficients calculated from high perturbations may be a conservative estimate. Regardless of emission region, BC causes a northward shift of the ITCZ, and this shift is apparent both with a fully coupled ocean and with fixed sea surface temperatures. For these regional BC emission perturbations, we find that the effective radiative forcing is not a good measure of the climate response. A limitation of this study is the uncertainties in BC–cloud interactions and the amount of BC absorption, both of which are model dependent.

• McGraw, Zachary; Storelvmo, Trude; David, Robert Oscar & Sagoo, Navjit (2020). Global Radiative Impacts of Mineral Dust Perturbations Through Stratiform Clouds. Journal of Geophysical Research (JGR): Space Physics. ISSN 2169-9380. 125(23). doi: 10.1029/2019JD031807. Full text in Research Archive Show summary

  Airborne mineral dust influences cloud occurrence and optical properties, which may provide a pathway for recent and future changes in dust concentration to alter the temperature at Earth's surface. However, despite prior suggestions that dust‐cloud interactions are an important control on the Earth's radiation balance, we find global mean cloud radiative effects to be insensitive to widespread dust changes. Here we simulate uniformly applied shifts in dust amount in a present‐day atmosphere using a version of the CAM5 atmosphere model (within CESM v1.2.2) modified to incorporate laboratory‐based ice nucleation parameterizations in stratiform clouds. Increasing and decreasing dustiness from current levels to paleoclimate extremes caused effective radiative forcings through clouds of +0.02 ± 0.01 and −0.05 ± 0.02 W/m2, respectively, with ranges of −0.26 to +0.13 W/m2 and −0.21 to +0.39 W/m2 from sensitivity tests. Our simulations suggest that these forcings are limited by several factors. Longwave and shortwave impacts largely cancel, particularly in mixed‐phase clouds, while in warm and cirrus clouds opposite responses between regions further reduce each global forcing. Additionally, changes in dustiness cause opposite forcings through aerosol indirect effects in mixed‐phase clouds as in cirrus, while in warm clouds indirect effects are weak at nearly all locations. Nevertheless, regional forcings and global impacts on longwave and shortwave radiation were found to be nonnegligible, suggesting that cloud‐mediated dust effects have significance in simulations of present and future climate.

• Moseid, Kine Onsum; Schulz, Michael; Storelvmo, Trude; Julsrud, Ingeborg Rian; Oliviè, Dirk Jan Leo & Nabat, Pierre [Show all 12 contributors for this article] (2020). Bias in CMIP6 models as compared to observed regional dimming and brightening. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 20(24), p. 16023–16040. doi: 10.5194/acp-20-16023-2020. Full text in Research Archive Show summary

  Abstract. Anthropogenic aerosol emissions have increased considerably over the last century, but climate effects and quantification of the emissions are highly uncertain as one goes back in time. This uncertainty is partly due to a lack of observations in the pre-satellite era, making the observations we do have before 1990 additionally valuable. Aerosols suspended in the atmosphere scatter and absorb incoming solar radiation and thereby alter the Earth's surface energy balance. Previous studies show that Earth system models (ESMs) do not adequately represent surface energy fluxes over the historical era. We investigated global and regional aerosol effects over the time period 1961–2014 by looking at surface downwelling shortwave radiation (SDSR). We used observations from ground stations as well as multiple experiments from eight ESMs participating in the Coupled Model Intercomparison Project Version 6 (CMIP6). Our results show that this subset of models reproduces the observed transient SDSR well in Europe but poorly in China. We suggest that this may be attributed to missing emissions of sulfur dioxide in China, sulfur dioxide being a precursor to sulfate, which is a highly reflective aerosol and responsible for more reflective clouds. The emissions of sulfur dioxide used in the models do not show a temporal pattern that could explain observed SDSR evolution over China. The results from various aerosol emission perturbation experiments from DAMIP, RFMIP and AerChemMIP show that only simulations containing anthropogenic aerosol emissions show dimming, even if the dimming is underestimated. Simulated clear-sky and all-sky SDSR do not differ greatly, suggesting that cloud cover changes are not a dominant cause of the biased SDSR evolution in the simulations. Therefore we suggest that the discrepancy between modeled and observed SDSR evolution is partly caused by erroneous aerosol and aerosol precursor emission inventories. This is an important finding as it may help interpret whether ESMs reproduce the historical climate evolution for the right or wrong reason.

• Bellouin, Nicolas; Quaas, Johannes; Gryspeerdt, Edward; Kinne, Stephan A.; Stier, Philip & Watson-Parris, Duncan [Show all 33 contributors for this article] (2020). Bounding global aerosol radiative forcing of climate change. Reviews of Geophysics. ISSN 8755-1209. 58. doi: 10.1029/2019RG000660. Full text in Research Archive Show summary

  Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol‐radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol‐driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed‐phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of ‐1.6 to ‐0.6 W m−2, or ‐2.0 to ‐0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial‐era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.

• Gasparini, Blaž; McGraw, Zachary; Storelvmo, Trude & Lohmann, Ulrike (2020). To what extent can cirrus cloud seeding counteract global warming? Environmental Research Letters. 15(5). doi: 10.1088/1748-9326/ab71a3. Full text in Research Archive Show summary

  The idea of modifying cirrus clouds to directly counteract greenhouse gas warming has gained momentum in recent years, despite disputes over its physical feasibility. Previous studies that analyzed modifications of cirrus clouds by seeding of ice nucleating particles showed large uncertainties in both cloud and surface climate responses, ranging from no effect or even a small warming to a globally averaged cooling of about 2.5 °C. We use two general circulation models that showed very different responses in previous studies, ECHAM6-HAM and CESM-CAM5, to determine which radiative and climatic responses to cirrus cloud seeding in a 1.5 × CO2 world are common and which are not. Seeding reduces the net cirrus radiative effect for −1.8 W m−2 in CESM compared with only −0.8 W m−2 in ECHAM. Accordingly, the surface temperature decrease is larger in CESM, counteracting about 70% of the global mean temperature increase due to CO2 and only 30% in ECHAM. While seeding impacts on mean precipitation were addressed in past studies, we are the first to analyze extreme precipitation responses to cirrus seeding. Seeding decreases the frequency of the most extreme precipitation globally. However, the extreme precipitation events occur more frequently in the Sahel and Central America, following the mean precipitation increase due to a northward shift of the Intertropical Convergence Zone. In addition, we use a quadratic climate damage metric to evaluate the amount of CO2-induced damage cirrus seeding can counteract. Seeding decreases the damage by about 50% in ECHAM, and by 85% in CESM over the 21 selected land regions. Climate damage due to CO2 increase is significantly reduced as a result of seeding in all of the considered land regions.

• Hahn, Lily Caroline; Storelvmo, Trude; Hofer, S; Parfitt, R & Ummenhofer, C. C. (2020). Importance of Orography for Greenland Cloud and Melt Response to Atmospheric Blocking. Journal of Climate. ISSN 0894-8755. 33(10), p. 4187–4206. doi: 10.1175/JCLI-D-19-0527.1. Full text in Research Archive Show summary

  More frequent high pressure conditions associated with atmospheric blocking episodes over Greenland in recent decades have been suggested to enhance melt through large-scale subsidence and cloud dissipation, which allows more solar radiation to reach the ice sheet surface. Here we investigate mechanisms linking high pressure circulation anomalies to Greenland cloud changes and resulting cloud radiative effects, with a focus on the previously neglected role of topography. Using reanalysis and satellite data in addition to a regional climate model, we show that anticyclonic circulation anomalies over Greenland during recent extreme blocking summers produce cloud changes dependent on orographic lift and descent. The resulting increased cloud cover over northern Greenland promotes surface longwave warming, while reduced cloud cover in southern and marginal Greenland favors surface shortwave warming. Comparison with an idealized model simulation with flattened topography reveals that orographic effects were necessary to produce area-averaged decreasing cloud cover since the mid-1990s and the extreme melt observed in the summer of 2012. This demonstrates a key role for Greenland topography in mediating the cloud and melt response to large-scale circulation variability. These results suggest that future melt will depend on the pattern of circulation anomalies as well as the shape of the Greenland Ice Sheet. Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JCLI-D-19-0527.s1. © 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

• Takeishi, Azusa; Storelvmo, Trude & Fierce, Laura (2020). Disentangling the microphysical effects of fire particles on convective clouds through a case study. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 125(12). doi: 10.1029/2019JD031890. Full text in Research Archive Show summary

  Aerosol emissions from forest fires may impact cloud droplet activation through an increase in particle number concentrations (“the number effect”) and also through a decrease in the hygroscopicity κ of the entire aerosol population (“the hygroscopicity effect”) when fully internal mixing is assumed in models. This study investigated these effects of fire particles on the properties of simulated deep convective clouds (DCCs), using cloud‐resolving simulations with the Weather Research and Forecasting model coupled with Chemistry for a case study in a partly idealized setting. We found that the magnitude of the hygroscopicity effect was in some cases strong enough to entirely offset the number/size effect, in terms of its influence on modeled droplet and ice crystal concentrations. More specifically, in the case studied here, the droplet number concentration was reduced by about 37% or more due solely to the hygroscopicity effect. In the atmosphere, by contrast, fire particles likely have a much weaker impact on the hygroscopicity of the pre‐existing background aerosol, as such a strong impact would occur only if the fire particles mixed immediately and uniformly with the background. We also show that the differences in the number of activated droplets eventually led to differences in the optical thickness of the clouds aloft, though this finding is limited to only a few hours of the initial development stage of the DCCs. These results suggest that accurately and rigorously representing aerosol mixing and κ in models is an important step toward accurately simulating aerosol‐cloud interactions under the influence of fires.

• Bjordal, Jenny; Storelvmo, Trude; Alterskjær, Kari & Carlsen, Tim (2020). Equilibrium climate sensitivity above 5 °C plausible due to state-dependent cloud feedback. Nature Geoscience. ISSN 1752-0894. 13, p. 718–721. doi: 10.1038/s41561-020-00649-1. Full text in Research Archive Show summary

  The equilibrium climate sensitivity of Earth is defined as the global mean surface air temperature increase that follows a doubling of atmospheric carbon dioxide. For decades, global climate models have predicted it as between approximately 2 and 4.5 °C. However, a large subset of models participating in the 6th Coupled Model Intercomparison Project predict values exceeding 5 °C. The difference has been attributed to the radiative effects of clouds, which are better captured in these models, but the underlying physical mechanism and thus how realistic such high climate sensitivities are remain unclear. Here we analyse Community Earth System Model simulations and find that, as the climate warms, the progressive reduction of ice content in clouds relative to liquid leads to increased reflectivity and a negative feedback that restrains climate warming, in particular over the Southern Ocean. However, once the clouds are predominantly liquid, this negative feedback vanishes. Thereafter, other positive cloud feedback mechanisms dominate, leading to a transition to a high-sensitivity climate state. Although the exact timing and magnitude of the transition may be model dependent, our findings suggest that the state dependence of the cloud-phase feedbacks is a crucial factor in the evolution of Earth’s climate sensitivity with warming.

• McGraw, Zachary; Storelvmo, Trude; Samset, Bjørn Hallvard & Stjern, Camilla Weum (2020). Global radiative impacts of black carbon acting as ice nucleating particles. Geophysical Research Letters. ISSN 0094-8276. 47(20), p. 1–9. doi: 10.1029/2020GL089056. Full text in Research Archive Show summary

  Black carbon (BC) aerosols from incomplete combustion generally warm the climate, but the magnitudes of their various interactions with climate are still uncertain. A key knowledge gap is their role as ice nucleating particles (INPs), enabling ice formation in clouds. Here we assess the global radiative impacts of BC acting as INPs, using simulations with the Community Earth System Model 2 climate model updated to include new laboratory‐based ice nucleation parameterizations. Overall, we find a moderate cooling through changes to stratiform cirrus clouds, counteracting the well‐known net warming from BC's direct scattering and absorption of radiation. Our best estimates indicate that BC INPs generally thin cirrus by indirectly inhibiting the freezing of solution aerosol, with a global net radiative impact of −0.13 ± 0.07 W/m2. Sensitivity tests of BC amounts and ice nucleating efficiencies, and uncertainties in the environment where ice crystals form, show a potential range of impacts from −0.30 to +0.02 W/m2.

• Tan, Ivy & Storelvmo, Trude (2019). Evidence of Strong Contribution from Mixed-Phase Clouds to Arctic Climate Change. Geophysical Research Letters. ISSN 0094-8276. 46(5), p. 2894–2902. doi: 10.1029/2018GL081871. Full text in Research Archive Show summary

  Underestimation of the proportion of supercooled liquid in mixed‐phase clouds in climate models has called into question its impact on Arctic climate change. We show that correcting for this bias in the CESM model can either enhance or reduce Arctic amplification depending on the microphysical characteristics of the clouds as a corollary to the cloud phase feedback. Replacement of ice with liquid in the cloud phase feedback results in more downward longwave radiation, which is effectively trapped as heat at the surface in the Arctic due to its unique stable stratification conditions, and this ultimately leads to a more positive lapse rate feedback. The larger the ice particles are to begin with, the stronger Arctic amplification becomes due to the lower precipitation efficiency of liquid droplets compared to ice crystals. Our results emphasize the importance of realistic representations of microphysical processes in mixed‐phase clouds, particularly in the Arctic.

• Li, Ryan; Storelvmo, Trude; Fedorov, Alexey & Choi, Yong-Sang (2019). A Positive Iris Feedback: Insights from Climate Simulations with Temperature-Sensitive Cloud–Rain Conversion. Journal of Climate. ISSN 0894-8755. 32(16), p. 5305–5324. doi: 10.1175/JCLI-D-18-0845.1. Full text in Research Archive Show summary

  Estimates for equilibrium climate sensitivity from current climate models continue to exhibit a large spread, from 2.1 to 4.7 K per carbon dioxide doubling. Recent studies have found that the treatment of precipitation efficiency in deep convective clouds—specifically the conversion rate from cloud condensate to rain Cp—may contribute to the large intermodel spread. It is common for convective parameterization in climate models to carry a constant Cp, although its values are model and resolution dependent. In this study, we investigate how introducing a potential iris feedback, the cloud–climate feedback introduced by parameterizing Cp to increase with surface temperature, affects future climate simulations within a slab ocean configuration of the Community Earth System Model. Progressively stronger dependencies of Cp on temperature unexpectedly increase the equilibrium climate sensitivity monotonically from 3.8 to up to 4.6 K. This positive iris feedback puzzle, in which a reduction in cirrus clouds increases surface temperature, is attributed to changes in the opacity of convectively detrained cirrus. Cirrus clouds reduced largely in ice content and marginally in horizontal coverage, and thus the positive shortwave cloud radiative feedback dominates. The sign of the iris feedback is robust across different cloud macrophysics schemes, which control horizontal cloud cover associated with detrained ice. These results suggest a potentially strong but highly uncertain connection among convective precipitation, detrained anvil cirrus, and the high cloud feedback in a climate forced by increased atmospheric carbon dioxide concentrations. Supplemental information related to this paper is available at the Journals Online website: https://doi.org/10.1175/JCLI-D-18-0845.s1. © 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

• Ramaswamy, Vijay; Collins, William D; Haywood, Jim M.; Lean, J.; Mahowald, N. & Myhre, Gunnar [Show all 11 contributors for this article] (2019). Radiative Forcing of Climate: The Historical Evolution of the Radiative Forcing Concept, the Forcing Agents and their Quantification, and Applications. AMS Meteorological Monographs. ISSN 0065-9401. 59, p. 14.1–14.101. doi: 10.1175/amsmonographs-d-19-0001.1. Full text in Research Archive
• Phillips, Peter C. B.; Leirvik, Thomas & Storelvmo, Trude (2019). Econometric estimates of Earth's transient climate sensitivity. Journal of Econometrics. ISSN 0304-4076. 214(1), p. 6–32. doi: 10.1016/j.jeconom.2019.05.002. Full text in Research Archive Show summary

  How sensitive is Earth’s climate to a given increase in atmospheric greenhouse gas (GHG) concentrations? This long-standing question in climate science was recently analyzed by dynamic panel data methods using extensive spatio-temporal data of global surface temperatures, solar radiation, and GHG concentrations over the last half century to 2010 (Storelvmo et al, 2016). Those methods revealed that atmospheric aerosol effects masked approximately one-third of the continental warming due to increasing GHG concentrations over this period, thereby implying greater climate sensitivity to GHGs than previously thought. The present study provides regularity conditions and asymptotic theory justifying the use of time series cointegration-based methods of estimation when there are both stochastic process and deterministic trends in the global forcing variables, such as GHGs, and station-level trend effects from such sources as local aerosol pollutants. The asymptotics validate estimation and confidence interval construction for econometric measures of Earth’s transient climate sensitivity (TCS). The methods are applied to observational data and to data generated from several groups of global climate models (GCMs) that are sampled spatio-temporally and aggregated in the same way as the empirical observations for the time period 1964–2005. The findings indicate that 7 out of 9 of the GCM reported TCS values lie within the 95% empirical confidence interval computed econometrically from the GCM output. The analysis shows the potential of econometric methods to provide empirical estimates and confidence limits for TCS, to calibrate GCM simulation output against observational data in terms of the implied TCS estimates obtained via the econometric model, and to reveal the respective sensitivity parameters (GHG and non-GHG related) governing GCM temperature trends.

• Gruber, Simon; Blahak, Ulrich; Haenel, Florian; Kottmeier, Christoph; Leisner, Thomas & Muskatel, Harel [Show all 8 contributors for this article] (2019). A Process Study on Thinning of Arctic Winter Cirrus Clouds With High‐Resolution ICON‐ART Simulations. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 124(11), p. 5860–5888. doi: 10.1029/2018JD029815. Full text in Research Archive Show summary

  In this study, cloud‐resolving simulations of a case study for a limited area of the hibernal Arctic were performed with the atmospheric modeling system ICON‐ART (ICOsahedral Nonhydrostatic‐Aerosol and Reactive Trace gases). A thorough comparison with data both from satellite as well as aircraft measurement is presented to validate the simulations. In addition, the model is applied to clarify the microphysical processes occurring when introducing artificial aerosol particles into the upper troposphere with the aim of modifying cirrus clouds in the framework of climate engineering. Former modeling studies investigating the climate effect of this method were performed with simplifying assumptions and much coarser resolution, reaching partly contradicting conclusions concerning the method's effectiveness. The primary effect of seeding is found to be a reduction of ice crystal number concentrations in cirrus clouds, leading to increased outgoing longwave radiative fluxes at the top of the atmosphere, thereby creating a cooling effect. Furthermore, a secondary effect is found, as ice crystals formed from the injected seeding aerosol particles lead to enhanced riming of cloud droplets within the planetary boundary layer. This effectively reduces the coverage of mixed‐phase clouds, thus generating additional cooling by increased upward longwave radiative fluxes at the surface. The efficacy of seeding cirrus clouds proves to be relatively independent from the atmospheric background conditions, scales with the number concentrations of seeding particles, and is highest for large aerosol particles.

• Karset, Inger Helene H.; Berntsen, Terje Koren; Storelvmo, Trude; Alterskjær, Kari; Grini, Alf & Oliviè, Dirk Jan Leo [Show all 10 contributors for this article] (2018). Strong impacts on aerosol indirect effects from historical oxidant changes. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 18(10), p. 7669–7690. doi: 10.5194/acp-18-7669-2018. Full text in Research Archive Show summary

  Uncertainties in effective radiative forcings through aerosol–cloud interactions (ERFaci, also called aerosol indirect effects) contribute strongly to the uncertainty in the total preindustrial-to-present-day anthropogenic forcing. Some forcing estimates of the total aerosol indirect effect are so negative that they even offset the greenhouse gas forcing. This study highlights the role of oxidants in modeling of preindustrial-to-present-day aerosol indirect effects. We argue that the aerosol precursor gases should be exposed to oxidants of its era to get a more correct representation of secondary aerosol formation. Our model simulations show that the total aerosol indirect effect changes from −1.32 to −1.07 W m−2 when the precursor gases in the preindustrial simulation are exposed to preindustrial instead of present-day oxidants. This happens because of a brightening of the clouds in the preindustrial simulation, mainly due to large changes in the nitrate radical (NO3). The weaker oxidative power of the preindustrial atmosphere extends the lifetime of the precursor gases, enabling them to be transported higher up in the atmosphere and towards more remote areas where the susceptibility of the cloud albedo to aerosol changes is high. The oxidation changes also shift the importance of different chemical reactions and produce more condensate, thus increasing the size of the aerosols and making it easier for them to activate as cloud condensation nuclei.

• Takeishi, Azusa & Storelvmo, Trude (2018). A Study of Enhanced Heterogeneous Ice Nucleation in Simulated Deep Convective Clouds Observed During DC3. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 123(23), p. 13396–13420. doi: 10.1029/2018JD028889. Full text in Research Archive Show summary

  The impacts of enhanced heterogeneous ice nucleation (HET) on the properties of deep convective clouds (DCCs) have been investigated in cloud‐resolving simulations with the WRF‐CHEM model. The study focuses on a case observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. For the simulated DCCs, which had cold cloud‐base temperatures, an inverse relationship exists between ice crystal mass produced through HET and anvil ice crystal number concentrations. This seems to be due to the indirect competition between HET and subsequent homogeneous freezing (HOM) for liquid droplets. Furthermore, our simulations suggest that HET enhancements at warmer temperatures are more efficient in depleting liquid droplets below and hence have larger impacts on anvil properties than HET enhancements at colder temperatures do. This temperature dependence indicates that similar increases in the number of ice nucleating particles (INPs) may potentially have different impacts on DCCs, depending on the INP type and at which temperatures they can nucleate ice crystals. We also found that the reduced anvil ice number concentrations due to the enhanced HET may lead to optically thinner anvil clouds. The reduction in cloud optical depth comes from a decrease in ice crystal mass concentrations, and in some runs also from an increase in ice crystal sizes. These results suggest potentially large impacts of INPs on the properties of DCCs, especially if precipitation is predominantly produced through ice processes in the DCCs. The results underscore the importance of fully understanding the temperature‐dependent ability of aerosol particles to nucleate ice crystals.

• Huang, Yaoxian; Unger, Nadine; Storelvmo, Trude; Harper, Kandice; Zheng, Yiqi & Heyes, Chris (2018). Global radiative effects of solid fuel cookstove aerosol emissions. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 18(8), p. 5219–5233. doi: 10.5194/acp-18-5219-2018. Full text in Research Archive
• Tan, Ivy; Storelvmo, Trude & Zelinka, Mark (2018). The climatic impact of thermodynamic phase partitioning in mixed-phase clouds. In Andronache, Constantin (Eds.), Mixed-phase clouds - Observations and Modeling. Elsevier. ISSN 9780128105504. p. 237–264. doi: 10.1016/b978-0-12-810549-8.00010-6. Full text in Research Archive

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• Wagner, Johannes; Krinner, Gerhard; Merikanto, Joonas; Laumann, Gregor; McGovern, Frank & Kotova, Lola [Show all 42 contributors for this article] (2026). Maximizing the synergy of European research Governance and Innovation for Climate Action (MAGICA): Strategic Research and Innovation Agenda for Climate Change Science (2026-2035). doi: 10.25424/CMCC-76KB-3540. Full text in Research Archive
• Bjordal, Jenny; Jr., Anthony A. Smith,; Cornec, Henri & Storelvmo, Trude (2026). NorESM2-DIAM: a new coupled climate-economy model for understanding the regional economic impacts of climate change. Full text in Research Archive Show summary

  Climate change poses substantial risks to economies worldwide, yet quantifying these impacts and their regional distribution remains a major challenge. Here we present NorESM2-DIAM, a new modelling framework that—to our knowledge—is the first to fully couple an Earth system model with a high-resolution cost-benefit integrated assessment model. Most modelling frameworks either lack two-way climate-economy interactions or operate at coarse spatial scales, limiting our ability to understand who is affected by climate change, where, and by how much. NorESM2-DIAM links the Norwegian Earth System Model version 2(NorESM2) with the Disaggregated Integrated Assessment Model (DIAM). DIAM represents the global economy across approximately 19,000 regions on a 1°×1° grid, and temperature and CO₂ emissions are exchanged annually at each grid cell. Because the coupling is two-way, the model generates its own emissions pathway rather than relying on prescribed scenarios. A baseline simulation shows large regional differences in climate damages, with the highest impacts in already-hot, low-latitude regions, while the coldest regions actually benefit. The results also show that internal climate variability generates substantial volatility in regional GDP, suggesting that short-term changes—not just long-term trends—matter for economic outcomes. Our results underline the importance of high-resolution, two-way coupled models for understanding the regional economic impacts of climate change and their policy implications.

• Bjordal, Jenny; Storelvmo, Trude; Jr., Anthony A. Smith, & Pedersen, Pål Andreas (2026). The Climate Casino: Teaching Climate Change and Mitigation Through Play. Full text in Research Archive Show summary

  Climate change is a complex problem that spans multiple disciplines, including natural science, socioeconomics, ethics, and politics. This complexity makes it challenging to make well-informed decisions about climate change. We therefore introduce The Climate Casino, an educational game that offers a fun and engaging way to learn about climate change and mitigation. At the start of the game, participants are divided into groups representing different geographic regions. They are given two goals: a primary, shared goal to keep global warming below 1.5°C and a secondary, competitive goal to maximise their own region’s economic development. These competing goals create a tricky balance between cooperation and self-interest, introducing challenges such as the free-riding problem. The groups’ assigned regions differ in economic and climatic conditions, and are responsible for various amounts of greenhouse gas emissions—reflecting real-world inequalities. Each group makes decisions for its region by setting a carbon tax. Higher carbon taxes increase production costs, leading to reduced economic output and, consequently, lower emissions. Thus, the carbon tax influences the economy through two competing mechanisms: reduced productivity, which negatively impacts economic output, and reduced climate change, which can have positive or negative economic effects depending on the region. The consequences of the participants’ decisions are calculated using the coupled climate–economy model NorESM2-DIAM, which provides both climatic and economic outcomes. While necessarily simplified, this approach introduces participants to key concepts in climate modelling and highlights the role of uncertainty and regional differences in climate–economy interactions. Through surveys before and after playing the game, we see that participants change how they think about climate change and mitigation. This demonstrates the potential of The Climate Casino as an engaging educational tool that can promote greater climate literacy.

• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2025). Arctic response to high-latitude effusive volcanic eruptions depends on the climate state. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2024). Climate response to the size and season of high-latitude, effusive volcanic eruptions. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2024). Surface climate response to the size and season of northern hemisphere, high latitude, effusive volcanic eruptions. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2023). Surface Arctic warming from a high latitude effusive volcanic eruption. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2023). Can high latitude effusive volcanic eruptions cause surface winter warming in the Arctic? Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2023). Arctic warming from a high latitude effusive volcanic eruption. Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra Margaretha; Born, Andreas; Haubner, Konstanze; Hofer, Stefan & Storelvmo, Trude (2023). Coupled Greenland ice sheet-climate simulations with the Norwegian Earth System Model (NorESM2). Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra Margaretha; Born, Andreas; Haubner, Konstanze; Hofer, Stefan & Storelvmo, Trude (2023). Coupled Greenland ice sheet-climate simulations with the Norwegian Earth System Model (NorESM2). Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2023). Arctic warming from a high latitude effusive volcanic eruption. Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra; Born, Andreas; Hofer, Stefan & Storelvmo, Trude (2022). Greenland ice sheet coupling in NorESM. Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra; Born, Andreas; Hofer, Stefan & Storelvmo, Trude (2022). Coupled Greenland ice sheet-climate simulations with the Norwegian Earth System Model. Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra; Born, Andreas; Hofer, Stefan & Storelvmo, Trude (2022). Coupled Greenland ice sheet-climate simulations with the Norwegian Earth System Model (NorESM2). Full text in Research Archive
• Haugvaldstad, Ove Westermoen; Oliviè, Dirk Jan Leo; Schulz, Michael & Storelvmo, Trude (2022). Decomposing the direct and indirect radiative effects by mineral dust aerosols in CMIP6. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2022). Modelling the climate response to volcanic eruptions using a variable grid. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2022). Modelling the climate response to the 2014-15 Holuhraun eruption. Full text in Research Archive
• Zoega, Tomas; Storelvmo, Trude & Krüger, Kirstin (2022). Modelling the climate response to the 2014-15 Holuhraun eruption. Full text in Research Archive
• Bjordal, Jenny; Storelvmo, Trude & Jr, Anthony A. Smith, (2022). The climate casino: Winners and losers in a warmer world. Full text in Research Archive
• Yuan, Menghan; Storelvmo, Trude & Leirvik, Thomas (2022). Trend analysis and transient climate sensitivity revealed by CMIP6. Full text in Research Archive
• Bjordal, Jenny; Storelvmo, Trude & Jr, Anthony A. Smith, (2022). The climate casino: Winners and losers in a warmer world. Full text in Research Archive
• Goelzer, Heiko; Langebroek, Petra; Born, Andreas; Hofer, Stefan & Storelvmo, Trude (2021). Progress in Greenland ice sheet coupling in NorESM. Full text in Research Archive
• Bjordal, Jenny; Storelvmo, Trude & Jr, Anthony A. Smith, (2021). Economic winners and losers in a warmer world - Uncertainties in regional economic impacts of climate change. Full text in Research Archive
• Røyne, Anja; Samset, Bjørn Hallvard; Sundal, Anja; Storelvmo, Trude & Joner, Erik (2020). Boklansering: Varm klode, kaldt hode. Løsninger på klimakrisen. Full text in Research Archive
• Bjordal, Jenny; Storelvmo, Trude & Jr, Anthony A. Smith, (2019). Climate change modelling and prediction of economic impact. Full text in Research Archive
• Bjordal, Jenny; Storelvmo, Trude & Jr, Anthony A. Smith, (2019). Climate change modelling and prediction of economic impact. Full text in Research Archive
• Haugnes, Gunhild M.; Tallaksen, Lena M.; Vogt, Rolf David; Storelvmo, Trude; Hessen, Dag Olav & Stordal, Frode [Show all 9 contributors for this article] (2018). Klimaforskerne tar temperaturen på tropesommeren. [Journal]. Titan.uio.no. Full text in Research Archive
• Hellmuth, Franziska; Storelvmo, Trude & Daloz, Anne Sophie (2026). Understanding Snowfall Processes Across Scales: Integrating Microphysical Observations, Remote Sensing, and Atmospheric Models. Universitetet i Oslo. Full text in Research Archive Show summary

  Snowfall plays a critical role in mid- to high-latitude regions because it is a key component of the extra-tropical hydrological cycle and influences important variables like surface albedo, ice sheet mass balance and soil moisture. However, snowfall remains poorly represented in atmospheric models because key processes occur at scales that are smaller than the resolution of numerical weather prediction (NWP) and global climate models (GCMs). This thesis integrates multi-platform observations and atmospheric model simulations to evaluate the physical consistency of snowfall representation across scales. The topics progress from individual snowflake observations at micrometer-scale, to weather models and GCMs at the regional or even global scale. The papers combine ground based observations from field campaigns in the Norwegian mountains and the Arctic, including microphysical observations of snowflakes, vertical profiling radar measurements, and NWP. In addition, satellite observations are used to evaluate cloud- and snowfall representation in reanalysis and GCM simulations in mid-to high-latitude regions. Together, these datasets contribute to enhanced understanding of snowfall processes across microphysical, synoptic, and global climate scales. The results of the papers show that while radar reflectivity alone is non-unique for snowfall, the transferability of retrieval relationships is limited by regional microphysics for the maritime Arctic coastal site. Surface snowfall is overestimated, whereas snow water content aloft is underestimated in NWPs at the Norwegian mountain site. Similar microphysical biases are also seen in reanalysis and GCMs. They overestimate the frequency of supercooled liquid containing clouds and produce too frequent and too light snowfall. The findings of this thesis suggest that biases in snowfall representation originating at the microphysical scale propagate through to NWP and GCM scales. These results highlight the importance of integrating observations across scales to identify biases and improve the representation of mixed-phase clouds and hydrometeors in atmospheric models to eventually reduce uncertainty in mid- to high-latitude climate projections.

• Zoega, Tomas; Krüger, Kirstin & Storelvmo, Trude (2025). The climate response to high-latitude effusive volcanic eruptions. Universitetet i Oslo. Full text in Research Archive Show summary

  Temaet for denne avhandlingen er klimapåvirkningene som følger av effusive vulkanutbrudd på nordlige høye breddegrader. Slike utbrudd kjennetegnes av aktivitet som ikke er eksplosiv, og gasser som slippes ut ved den slags utbrudd holder seg for det meste i troposfæren. Gjennom utslipp av svoveldioksid og påfølgende dannelse av sulfataerosoler kan effusive vulkanutbrudd påvirke klimaet. Direkte interaksjoner mellom disse vulkanske aerosolene og stråling fører vanligvis til nedkjøling av jorda gjennom økt refleksjon av sollys tilbake ut i verdensrommet. Sulfataerosoler fungerer også som skykondensasjonskjerner og kan derfor påvirke klimaet gjennom vekselvirkning mellom aerosoler og skyer (engelsk: aerosol-cloud interactions, ACI). Dette kompliserer klimapåvirkningen fordi ACI er blant de minst forståtte delene av klimasystemet. Resultatene fra denne avhandlingen presenteres i en rekke av tre vitenskapelige artikler. Den første artikkelen (Artikkel I) er en tilfellestudie av Holuhraun-utbruddet på Island som skjedde i årene 2014 til 2015. Gjennom bevis fra observasjoner og klimamodellering argumenteres det for at dette utbruddet bidro til en økning av overflatetemperaturen i Arktis, særlig over Grønlandshavet, fra september til november 2014, ved at lavtliggende skyer fanget en større del av jordas varmestråling. Den andre artikkelen (Artikkel II) bygger videre på den første. Her er en rekke av Holuhraun-lignende utbrudd i ulike størrelser simulert med startdag i hver av de fire årstidene, nemilg i mars, juni, september og desember. I Arktis viser simuleringene overflateoppvarming om vinteren og mild nedkjøling om sommeren, i begge tilfeller gjennom vekselvirkning mellom vulkanske aerosolene og skyer. Lengre sør, på midlere breddegrader, er overflateresponsen om vinteren stort sett ubetydelig, men om sommeren simuleres en betydelig nedkjøling. Når utbruddene blir større med hensyn til utslipp av svoveldioksid, blir klimapåvirkningen stadig mindre følsom til endringer i utslippsstyrken. Ved utbrudd mellom 20 og 30 ganger større enn Holuhraun-utbruddet i 2014-15 utjevner responsen seg. I den siste artikkelen (Artikkel III) blir klimapåvirkningens avhengighet av klimatilstanden undersøkt. For dette formålet er klimapåvirkningen til effusive vulkanutbrudd simulert i førindustriell tid (året 1850), nåtiden (året 2010) og framtiden (året 2090 i SSP3-7.0 scenarioet). Det viser seg at klimatilstanden har betydelige modulerende effekter på klimapåvirkningen. Overflateoppvarmingen om vinteren i Arktis er sterkere under førindustrielle forhold, sammenlignet med nåtidens og framtidens forhold, på grunn av tynnere skyer i den førindustrielle bakgrunnstilstanden. Derimot blir overflatenedkjølingen om sommeren i Arktis forsterket når klimaet blir varmere, på grunn av mindre havis. Ved europeiske midlere breddegrader finner en betydelig vinterkjøling sted under førindustrielle forhold, sammenlignet med en utydelig temperaturrespons i nåtidens og framtidens klima. Styrken på sommernedkjølingen er sammenlignbar for alle tre klimaforholdene. I Arktis er den sterkeste modulerende faktoren et varmere klima som følge av menneskeskapte klimagassutslipp, med påfølgende endringer i skyegenskaper og redusert havis. Derimot er den sterkeste modulerende faktor på midlere breddegrader bakgrunnstilstanden til aerosoler.

• Veland, Siri; Wagner, Penelope Mae; Bailey, David; Everett, Alistair; Goldstein, Michael & Hermann, Roberto Rivas [Show all 14 contributors for this article] (2021). Knowledge needs in sea ice forecasting for navigation in Svalbard and the High Arctic. Nordlandsforskning. ISSN 9788273218193. Full text in Research Archive Show summary

  This white paper reports on two virtual workshops held with representatives of the Norwegian Coastal Administration, the Reference Fleet, sea ice research communities in Norway and the United States, and the Norwegian Ice Service. The workshops were planned as sessions at the High North Dialogue in Bodø in March 2020, with funding from the Svalbard Strategic Grant. The High North Dialogue was cancelled due to the Covid-19 pandemic, and could not be rescheduled for the following meeting. The organizers opted instead to host two virtual meetings. This format permitted a different set of participants to the intended academic audience. This produced more applied discussions of sea ice forecasting needs and product development with the Norwegian Ice Service, the Norwegian Coastal Administration, the Norwegian Reference Fleet, and researchers.

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