Kerim Hestnes Nisancioglu

• Haywood, Alan M.; Tindall, Julia C.; Dowsett, Harry J.; Dolan, Aisling M.; Foley, Kevin M.; Hunter, Stephen J.; Hill, Daniel J.; Chan, Wing-Le; Abe-Ouchi, Ayako; Stepanek, Christian; Lohmann, Gerrit; Chandan, Deepak; Peltier, W. Richard; Tan, Ning; Contoux, Camille; Ramstein, Gilles; Li, Xiangyu; Zhang, Zhongshi; Guo, Chuncheng; Nisancioglu, Kerim Hestnes; Zhang, Qiong; Li, Qiang; Kamae, Youichi; Chandler, Mark A.; Sohl, Linda E.; Otto-Bliesner, Bette L.; Feng, Ran; Brady, Esther C.; von der Heydt, Anna S.; Baatsen, Michiel L. J. & Lunt, Daniel J (2020). A return to large-scale features of Pliocene climate: the Pliocene Model Intercomparison Project Phase 2. Climate of the Past.  ISSN 1814-9324. . doi: 0.5194/cp-2019-145 Show summary

  The Pliocene epoch has great potential to improve our understanding of the long-term climatic and environmental consequences of an atmospheric CO2 concentration near ~ 400 parts per million by volume. Here we present the large-scale features of Pliocene climate as simulated by a new ensemble of climate models of varying complexity and spatial resolution and based on new reconstructions of boundary conditions (the Pliocene Model Intercomparison Project Phase 2; PlioMIP2). As a global annual average, modelled surface air temperatures increase by between 1.4 and 4.7 °C relative to pre-industrial with a multi-model mean value of 2.8 °C. Annual mean total precipitation rates increase by 6 % (range: 2 %–13 %). On average, surface air temperature (SAT) increases are 1.3 °C greater over the land than over the oceans, and there is a clear pattern of polar amplification with warming polewards of 60° N and 60° S exceeding the global mean warming by a factor of 2.4. In the Atlantic and Pacific Oceans, meridional temperature gradients are reduced, while tropical zonal gradients remain largely unchanged. Although there are some modelling constraints, there is a statistically significant relationship between a model's climate response associated with a doubling in CO2 (Equilibrium Climate Sensitivity; ECS) and its simulated Pliocene surface temperature response. The mean ensemble earth system response to doubling of CO2 (including ice sheet feedbacks) is approximately 50 % greater than ECS, consistent with results from the PlioMIP1 ensemble. Proxy-derived estimates of Pliocene sea-surface temperatures are used to assess model estimates of ECS and indicate a range in ECS from 2.5 to 4.3 °C. This result is in general accord with the range in ECS presented by previous IPCC Assessment Reports.

• Haywood, Alan M.; Tindall, Julia C.; Dowsett, Harry J.; Dolan, Aisling M.; Foley, Kevin M.; Zhang, Zhongshi; Guo, Chuncheng & Nisancioglu, Kerim Hestnes (2020). A return to large-scale features of Pliocene climate: the Pliocene Model Intercomparison Project Phase 2. Climate of the Past Discussions.  ISSN 1814-9340. . doi: 10.5194/cp-2019-145 Show summary

  The Pliocene epoch has great potential to improve our understanding of the long-term climatic and environmental consequences of an atmospheric CO2 concentration near ~ 400 parts per million by volume. Here we present the large-scale features of Pliocene climate as simulated by a new ensemble of climate models of varying complexity and spatial resolution and based on new reconstructions of boundary conditions (the Pliocene Model Intercomparison Project Phase 2; PlioMIP2). As a global annual average, modelled surface air temperatures increase by between 1.4 and 4.7 °C relative to pre-industrial with a multi-model mean value of 2.8 °C. Annual mean total precipitation rates increase by 6 % (range: 2 %–13 %). On average, surface air temperature (SAT) increases are 1.3 °C greater over the land than over the oceans, and there is a clear pattern of polar amplification with warming polewards of 60° N and 60° S exceeding the global mean warming by a factor of 2.4. In the Atlantic and Pacific Oceans, meridional temperature gradients are reduced, while tropical zonal gradients remain largely unchanged. Although there are some modelling constraints, there is a statistically significant relationship between a model's climate response associated with a doubling in CO2 (Equilibrium Climate Sensitivity; ECS) and its simulated Pliocene surface temperature response. The mean ensemble earth system response to doubling of CO2 (including ice sheet feedbacks) is approximately 50 % greater than ECS, consistent with results from the PlioMIP1 ensemble. Proxy-derived estimates of Pliocene sea-surface temperatures are used to assess model estimates of ECS and indicate a range in ECS from 2.5 to 4.3 °C. This result is in general accord with the range in ECS presented by previous IPCC Assessment Reports.

• Kajanto, Karita Anneli; Seroussi, Hélène; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2020). Present day Jakobshavn Isbræ close to the Holocene minimum extent. Quaternary Science Reviews.  ISSN 0277-3791.  246 . doi: 10.1016/j.quascirev.2020.106492 Show summary

  Marine terminating glaciers evolve on millenial timescales in response to changes in oceanic and atmospheric conditions. However, the relative role of oceanic and atmospheric drivers remains uncertain. The evolution of marine terminating glaciers under the warmer than present Holocene Climate Optimum climate can give important insights into the dynamics of ice streams as the climate evolves. The early Holocene evolution of Jakobshavn Isbræ, from the Last Glacial Maximum extent up to 8.2 ka BP is well constrained by geomorphological studies in the area. However, the Holocene minimum extent of the glacier is unknown. Here, we use a high-resolution regional ice sheet model to study the retreat and readvance of Jakobshavn Isbræ from the Mid-Holocene to the Little Ice Age. This model of Jakobshavn Isbræ accurately tracks the terrestrial ice margin and agrees with available estimates of marine grounding line evolution. We find that the Holocene minimum extent of both the terrestrial ice margin and the grounding line, reached at 6–5 ka BP, is close to the present day extent of the glacier. We also find that the glacier is currently located close to a tipping point, from beyond which readvance would require a longer and more significant cooling than the Little Ice Age. We assess the importance of the ocean forcing in explaining the Holocene evolution of Jakobshavn, and find that cooling within the fjord during the Mid-Holocene is critical for the glacier to readvance. This finding emphasizes the role of ocean forcing when trying to understand the millenial scale evolution of marine terminating glaciers.

• Loose, Nora; Heimbach, Patrick; Pillar, H.R. & Nisancioglu, Kerim Hestnes (2020). Quantifying Dynamical Proxy Potential Through Shared Adjustment Physics in the North Atlantic. Journal of Geophysical Research (JGR): Oceans.  ISSN 2169-9275.  125 . doi: 10.1029/2020JC016112 Show summary

  Oceanic quantities of interest (QoIs), for example, ocean heat content or transports, are often inaccessible to direct observation, due to the high cost of instrument deployment and logistical challenges. Therefore, oceanographers seek proxies for undersampled or unobserved QoIs. Conventionally, proxy potential is assessed via statistical correlations, which measure covariability without establishing causality. This paper introduces an alternative method: quantifying dynamical proxy potential. Using an adjoint model, this method unambiguously identifies the physical origins of covariability. A North Atlantic case study illustrates our method within the ECCO (Estimating the Circulation and Climate of the Ocean) state estimation framework. We find that wind forcing along the eastern and northern boundaries of the Atlantic drives a basin‐wide response in North Atlantic circulation and temperature. Due to these large‐scale teleconnections, a single subsurface temperature observation in the Irminger Sea informs heat transport across the remote Iceland‐Scotland ridge (ISR), with a dynamical proxy potential of 19%. Dynamical proxy potential allows two equivalent interpretations: Irminger Sea subsurface temperature (i) shares 19% of its adjustment physics with ISR heat transport and (ii) reduces the uncertainty in ISR heat transport by 19% (independent of the measured temperature value), if the Irminger Sea observation is added without noise to the ECCO state estimate. With its two interpretations, dynamical proxy potential is simultaneously rooted in (i) ocean dynamics and (ii) uncertainty quantification and optimal observing system design, the latter being an emerging branch in computational science. The new method may therefore foster dynamics‐based, quantitative ocean observing system design in the coming years.

• McClymont, Erin L.; Ford, Heather; Ho, Sze Ling; Tindall, Julia C.; Haywood, Alan M.; Alonso-Garcia, Montserrat; Bailey, Ian; Berke, Melissa A.; Littler, Kate; Patterson, Molly O.; Petrick, Benjamin; Peterse, Francien; Ravelo, Ana Christina; Risebrobakken, Bjørg; De Schepper, Stijn; Swann, George E. A.; Kaustubh, Thirumalai; Tierney, Jessica E; Van der Weijst, Carolien; white, sarah; Abe-Ouchi, Ayako; Baatsen, Michiel L. J.; Brady, Esther C.; Chan, Wing-Le; Chandan, Deepak; Ran, Feng; Guo, Chenchen Guo; von der Heydt, Anna S.; Stephen, Hunter; Xiangyi, Li; Lohmann, Gerrit; Nisancioglu, Kerim Hestnes; Otto-Bliesner, Bette L.; Peltier, Richard W.; Stepanek, Christian & Zhang, Zhongshi (2020). Lessons from a high-CO2 world: an ocean view from  ∼ 3 million years ago. Climate of the Past.  ISSN 1814-9324.  16 . doi: 10.5194/cp-16-1599-2020 Show summary

  A range of future climate scenarios are projected for high atmospheric CO2 concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. 3.205±0.01 Ma) when atmospheric CO2 exceeded pre-industrial concentrations, but were similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future CO2. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial values, by ∼2.3 ∘C for the combined proxy data (foraminifera Mg∕Ca and alkenones), or by ∼3.2–3.4 ∘C (alkenones only). Compared to the pre-industrial period, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values. Our results demonstrate that even under low-CO2 emission scenarios, surface ocean warming may be expected to exceed model projections and will be accentuated in the higher latitudes.

• Otto-Bliesner, Bette; Brady, Esther C.; Zhao, Anni; Axford, Yarrow; Capron, Emilie; Guo, Chuncheng; Nisancioglu, Kerim Hestnes & Zhang, Zhongshi (2020). Large-scale features of Last Interglacial climate: Results from evaluating the lig127k simulations for CMIP6-PMIP4. Climate of the Past Discussions.  ISSN 1814-9340. . doi: 10.5194/cp-2019-174 Show summary

  The modeling of paleoclimate, using physically based tools, is increasingly seen as a strong out-of-sample test of the models that are used for the projection of future climate changes. New to CMIP6 is the Tier 1 lig127k experiment, designed to address the climate responses to stronger orbital forcing than the midHolocene experiment, using the same state-of-the-art models and following a common experimental protocol. We present a multi-model ensemble of 17 climate models, all of which (except for two) have also completed the CMIP6 DECK experiments. The Equilibrium Climate Sensitivity (ECS) of these models varies from 2.1 to 5.6 °C. The seasonal character of the insolation anomalies results in strong warming over the Northern Hemisphere (NH) continents in the lig127k ensemble as compared to the piControl in June–July–August and a much-reduced minimum (August–September) summer sea ice extent in the Arctic. The multi-model results indicate enhanced summer monsoonal precipitation and areal extent in the Northern Hemisphere and reductions in the Southern Hemisphere. These responses are greater in the lig127k than midHolocene simulations as expected from the larger insolation anomalies at 127 ka than 6 ka. New syntheses for surface temperature and precipitation, targeted for 127 ka, have been developed for comparison to the multi-model ensemble. The lig127k model ensemble and data reconstructions are in good agreement for summer temperature anomalies over Canada, Scandinavia, and the North Atlantic and precipitation over the Northern Hemisphere continents. The model-data comparisons and mismatches point to further study of the sensitivity of the simulations to uncertainties in the specified boundary conditions and of the uncertainties and sparse coverage in current proxy reconstructions.

• Rheinlænder, Jonathan Winfield; Ferreira, David & Nisancioglu, Kerim Hestnes (2020). Topological Constraints by the Greenland–Scotland Ridge on AMOC and Climate. Journal of Climate.  ISSN 0894-8755.  33(13) . doi: 10.1175/JCLI-D-19-0726.1 Show summary

  Changes in the geometry of ocean basins have been influential in driving climate change throughout Earth’s history. Here, we focus on the emergence of the Greenland–Scotland Ridge (GSR) and its influence on the ocean state, including large-scale circulation, heat transport, water mass properties, and global climate. Using a coupled atmosphere–ocean–sea ice model, we consider the impact of introducing the GSR in an idealized Earth-like geometry, comprising a narrow Atlantic-like basin and a wide Pacific-like basin. Without the GSR, deep-water formation occurs near the North Pole in the Atlantic basin, associated with a deep meridional overturning circulation (MOC). By introducing the GSR, the volume transport across the sill decreases by 64% and deep convection shifts south of the GSR, dramatically altering the structure of the high-latitude MOC. Due to compensation by the subpolar gyre, the northward ocean heat transport across the GSR only decreases by ~30%. As in the modern Atlantic Ocean, a bidirectional circulation regime is established with warm Atlantic water inflow and a cold dense overflow across the GSR. In sharp contrast to the large changes north of the GSR, the strength of the Atlantic MOC south of the GSR is unaffected. Outside the high latitudes of the Atlantic basin, the surface climate response is surprisingly small, suggesting that the GSR has little impact on global climate. Our results suggest that caution is required when interpreting paleoproxy and ocean records, which may record large local changes, as indicators of basin-scale changes in the overturning circulation and global climate.

• Smith-Johnsen, Silje; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2020). The role of subglacial hydrology in ice streams with elevated geothermal heat flux. Journal of Glaciology.  ISSN 0022-1430.  s 1- 10 . doi: doi.org/10.1017/jog.2020.8 Full text in Research Archive. Show summary

  The spatial distribution of geothermal heat flux (GHF) under ice sheets is largely unknown. Nonetheless, it is an important boundary condition in ice-sheet models, and suggested to control part of the complex surface velocity patterns observed in some regions. Here we investigate the effect of including subglacial hydrology when modelling ice streams with elevated GHF. We use an idealised ice stream geometry and a thermomechanical ice flow model coupled to subglacial hydrology in the Ice Sheet System Model (ISSM). Our results show that the dynamic response of the ice stream to elevated GHF is greatly enhanced when including the interactive subglacial hydrology. On the other hand, the impact of GHF on ice temperature is reduced when subglacial hydrology is included. In conclusion, the sensitivity of ice stream dynamics to GHF is likely to be underestimated in studies neglecting subglacial hydrology.

• Smith-Johnsen, Silje; de Fleurian, Basile; Schlegel, Nicole; Seroussi, Hélène & Nisancioglu, Kerim Hestnes (2020). Exceptionally high heat flux needed to sustain the Northeast Greenland Ice Stream. The Cryosphere.  ISSN 1994-0416.  14, s 841- 854 . doi: 10.5194/tc-14-841-2020 Show summary

  The Northeast Greenland Ice Stream (NEGIS) currently drains more than 10 % of the Greenland Ice Sheet area and has recently undergone significant dynamic changes. It is therefore critical to accurately represent this feature when assessing the future contribution of Greenland to sea level rise. At present, NEGIS is reproduced in ice sheet models by inferring basal conditions using observed surface velocities. This approach helps estimate conditions at the base of the ice sheet but cannot be used to estimate the evolution of basal drag in time, so it is not a good representation of the evolution of the ice sheet in future climate warming scenarios. NEGIS is suggested to be initiated by a geothermal heat flux anomaly close to the ice divide, left behind by the movement of Greenland over the Icelandic plume. However, the heat flux underneath the ice sheet is largely unknown, except for a few direct measurements from deep ice core drill sites. Using the Ice Sheet System Model (ISSM), with ice dynamics coupled to a subglacial hydrology model, we investigate the possibility of initiating NEGIS by inserting heat flux anomalies with various locations and intensities. In our model experiment, a minimum heat flux value of 970 mW m−2 located close to the East Greenland Ice-core Project (EGRIP) is required locally to reproduce the observed NEGIS velocities, giving basal melt rates consistent with previous estimates. The value cannot be attributed to geothermal heat flux alone and we suggest hydrothermal circulation as a potential explanation for the high local heat flux. By including high heat flux and the effect of water on sliding, we successfully reproduce the main characteristics of NEGIS in an ice sheet model without using data assimilation.

• Straume, Eivind Olavson; Gaina, Carmen; Medvedev, Sergei & Nisancioglu, Kerim Hestnes (2020). Global Cenozoic Paleobathymetry with a focus on the Northern Hemisphere Oceanic Gateways. Gondwana Research.  ISSN 1342-937X.  86, s 126- 143 . doi: 10.1016/j.gr.2020.05.011
• Guo, Chuncheng; Nisancioglu, Kerim Hestnes; Bentsen, Mats; Bethke, Ingo & Zhang, Zhongshi (2019). Equilibrium simulations of Marine Isotope Stage 3 climate. Climate of the Past.  ISSN 1814-9324.  15(3), s 1133- 1151 . doi: 10.5194/cp-15-1133-2019 Full text in Research Archive.
• Plach, Andreas; Nisancioglu, Kerim Hestnes; Langebroek, Petra; Born, Andreas & Le clec'h, Sebastien (2019). Eemian Greenland ice sheet simulated with a higher-order model shows strong sensitivity to surface mass balance forcing. The Cryosphere.  ISSN 1994-0416.  13(8), s 2133- 2148 . doi: 10.5194/tc-13-2133-2019 Full text in Research Archive. Show summary

  The Greenland ice sheet contributes increasingly to global sea level rise. Its history during past warm intervals is a valuable reference for future sea level projections. We present ice sheet simulations for the Eemian interglacial period (∼130 000 to 115 000 years ago), a period with warmer-than-present summer climate over Greenland. The evolution of the Eemian Greenland ice sheet is simulated with a 3-D higher-order ice sheet model, forced with a surface mass balance derived from regional climate simulations. Sensitivity experiments with various surface mass balances, basal friction, and ice flow approximations are discussed. The surface mass balance forcing is identified as the controlling factor setting the minimum in Eemian ice volume, emphasizing the importance of a reliable surface mass balance model. Furthermore, the results indicate that the surface mass balance forcing is more important than the representation of ice flow for simulating the large-scale ice sheet evolution. This implies that modeling of the future contribution of the Greenland ice sheet to sea level rise highly depends on an accurate surface mass balance.

• Sessford, Evangeline; Jensen, Mari Fjalstad; Tisserand, Amandine Aline; Muschitiello, Francesco; Dokken, Trond Martin; Nisancioglu, Kerim Hestnes & Jansen, Eystein (2019). Consistent fluctuations in intermediate water temperature off the coast of Greenland and Norway during Dansgaard-Oeschger events. Quaternary Science Reviews.  ISSN 0277-3791.  223 . doi: 10.1016/j.quascirev.2019.105887 Full text in Research Archive.
• Smith-Johnsen, Silje; de Fleurian, Basile; Schlegel, Nicole; Seroussi, Helene & Nisancioglu, Kerim Hestnes (2019). Exceptionally High Geothermal Heat Flux Needed to Sustain the Northeast Greenland Ice Stream. The Cryosphere Discussions.  ISSN 1994-0432. . doi: 10.5194/tc-2019-212 Show summary

  The Northeast Greenland Ice Stream (NEGIS) currently drains more than 10 % of the Greenland Ice Sheet area, and has recently undergone significant dynamic changes. It is therefore critical to accurately represent this feature when assessing the future contribution of Greenland to sea level rise. At present, NEGIS is reproduced in ice sheet models by inferring basal conditions using observed surface velocities. This approach helps estimate conditions at the base of the ice sheet, but cannot be used to estimate the evolution of basal drag in time, so it is not a good representation of the evolution of the ice sheet in future climate warming scenarios. NEGIS is suggested to be initiated by a geothermal heat flux anomaly close to the ice divide, left behind by the movement of Greenland over the Icelandic plume. However, the heat flux underneath the ice sheet is largely unknown, except for a few direct measurements from deep ice core drill sites. Using the Ice Sheet System Model (ISSM), with ice dynamics coupled to a subglacial hydrology model, we investigate the possibility of initiating NEGIS by inserting hotspots with various locations and intensities. We find that a minimum geothermal heat flux value of 970 mW/m2 located close to EastGRIP is required locally to reproduce the observed NEGIS velocities, consistent with previous estimates. By including high geothermal heat flux and the effect of water on sliding, we successfully reproduce the main characteristics of NEGIS in an ice sheet model without using data assimilation.

• Smith-Johnsen, Silje; Schlegel, Nicole; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2019). Sensitivity of the Northeast Greenland Ice Stream to Geothermal Heat. Journal of Geophysical Research (JGR): Earth Surface.  ISSN 2169-9003.  125(1) . doi: 10.1029/2019JF005252 Full text in Research Archive. Show summary

  Recent observations of ice flow surface velocities have helped improve our understanding of basal processes on Greenland and Antarctica, though these processes still constitute some of the largest uncertainties driving ice flow change today. The Northeast Greenland Ice Stream is driven largely by basal sliding, believed to be related to subglacial hydrology and the availability of heat. Characterization of the uncertainties associated with Northeast Greenland Ice Stream is crucial for constraining Greenland's potential contribution to sea level rise in the upcoming centuries. Here, we expand upon past work using the Ice Sheet System Model to quantify the uncertainties in models of the ice flow in the Northeast Greenland Ice Stream by perturbing the geothermal heat flux. Utilizing a subglacial hydrology model simulating sliding beneath the Greenland Ice Sheet, we investigate the sensitivity of the Northeast Greenland Ice Stream ice flow to various estimates of geothermal heat flux, and implications of basal heat flux uncertainties on modeling the hydrological processes beneath Greenland's major ice stream. We find that the uncertainty due to sliding at the bed is 10 times greater than the uncertainty associated with internal ice viscosity. Geothermal heat flux dictates the size of the area of the subglacial drainage system and its efficiency. The uncertainty of ice discharge from the Northeast Greenland Ice Stream to the ocean due to uncertainties in the geothermal heat flux is estimated at 2.10 Gt/yr. This highlights the urgency in obtaining better constraints on the highly uncertain subglacial hydrology parameters.

• Jensen, Mari Fjalstad; Nisancioglu, Kerim Hestnes & Spall, Michael A. (2018). Large changes in sea ice triggered by small changes in Atlantic water temperature. Journal of Climate.  ISSN 0894-8755.  31(12), s 4847- 4863 . doi: 10.1175/JCLI-D-17-0802.1 Full text in Research Archive.
• Plach, Andreas; Nisancioglu, Kerim Hestnes; Le clec'h, Sebastien; Born, Andreas; Langebroek, Petra; Guo, Chuncheng; Imhof, Michael & Stocker, Thomas F. (2018). Eemian Greenland SMB strongly sensitive to model choice. Climate of the Past.  ISSN 1814-9324.  14(10), s 1463- 1485 . doi: 10.5194/cp-14-1463-2018 Full text in Research Archive. Show summary

  Understanding the behavior of the Greenland ice sheet in a warmer climate, and particularly its surface mass balance (SMB), is important for assessing Greenland’s potential contribution to future sea level rise. The Eemian interglacial period, the most recent warmer-than-present period in Earth’s history approximately 125 000 years ago, provides an analogue for a warm summer climate over Greenland. The Eemian is characterized by a positive Northern Hemisphere summer insolation anomaly, which complicates Eemian SMB calculations based on positive degree day estimates. In this study, we use Eemian global and regional climate simulations in combination with three types of SMB models – a simple positive degree day, an intermediate complexity, and a full surface energy balance model – to evaluate the importance of regional climate and model complexity for estimates of Greenland’s SMB. We find that all SMB models perform well under the relatively cool pre-industrial and late Eemian. For the relatively warm early Eemian, the differences between SMB models are large, which is associated with whether insolation is included in the respective models. For all simulated time slices, there is a systematic difference between globally and regionally forced SMB models, due to the different representation of the regional climate over Greenland. We conclude that both the resolution of the simulated climate as well as the method used to estimate the SMB are important for an accurate simulation of Greenland’s SMB. Whether model resolution or the SMB method is most important depends on the climate state and in particular the prevailing insolation pattern. We suggest that future Eemian climate model intercomparison studies should include SMB estimates and a scheme to capture SMB uncertainties.

• Steiger, Nadine; Nisancioglu, Kerim Hestnes; Åkesson, Henning; de Fleurian, Basile & Nick, Faezeh M. (2018). Simulated retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry. The Cryosphere.  ISSN 1994-0416.  12(7), s 2249- 2266 . doi: 10.5194/tc-12-2249-2018 Full text in Research Archive. Show summary

  Rapid retreat of Greenland's marine-terminating glaciers coincides with regional warming trends, which have broadly been used to explain these rapid changes. However, outlet glaciers within similar climate regimes experience widely contrasting retreat patterns, suggesting that the local fjord geometry could be an important additional factor. To assess the relative role of climate and fjord geometry, we use the retreat history of Jakobshavn Isbræ, West Greenland, since the Little Ice Age (LIA) maximum in 1850 as a baseline for the parameterization of a depth- and width-integrated ice flow model. The impact of fjord geometry is isolated by using a linearly increasing climate forcing since the LIA and testing a range of simplified geometries. We find that the total length of retreat is determined by external factors – such as hydrofracturing, submarine melt and buttressing by sea ice – whereas the retreat pattern is governed by the fjord geometry. Narrow and shallow areas provide pinning points and cause delayed but rapid retreat without additional climate warming, after decades of grounding line stability. We suggest that these geometric pinning points may be used to locate potential sites for moraine formation and to predict the long-term response of the glacier. As a consequence, to assess the impact of climate on the retreat history of a glacier, each system has to be analyzed with knowledge of its historic retreat and the local fjord geometry.

• Åkesson, Henning; Morlighem, Mathieu; Nisancioglu, Kerim Hestnes; Svendsen, John-Inge & Mangerud, Jan (2018). Atmosphere-driven ice sheet mass loss paced by topography: Insights from modelling the south-western Scandinavian Ice Sheet. Quaternary Science Reviews.  ISSN 0277-3791.  195, s 32- 47 . doi: 10.1016/j.quascirev.2018.07.004 Full text in Research Archive. Show summary

  Marine-terminating glaciers and ice streams are important controls of ice sheet mass balance. However, understanding of their long-term response to external forcing is limited by relatively short observational records of present-day glaciers and sparse geologic evidence for paleo-glaciers. Here we use a high-resolution ice sheet model with an accurate representation of grounding line dynamics to study the deglaciation of the marine-based south-western Norwegian sector of the Scandinavian Ice Sheet and its sensitivity to ocean and atmosphere forcing. We find that the regional response to a uniform climate change is highly dependent on the local bedrock topography, consistent with ice sheet reconstructions. Our simulations suggest that ocean warming is able to trigger initial retreat in several fjords, but is not sufficient to explain retreat everywhere. Widespread retreat requires additional ice thinning driven by surface melt. Once retreat is triggered, the underlying bedrock topography and fjord width control the rate and extent of retreat, while multi-millennial changes over the course of deglaciation are modulated by surface melt. We suggest that fjord geometry, ice-ocean interactions and grounding line dynamics are vital controls of decadal-to centennial scale ice sheet mass loss. However, we postulate that atmospheric changes are the most important drivers of widespread ice sheet demise, and will likely trump oceanic influence on future ice sheet mass loss and resulting sea level rise over centennial and longer time scales.

• Åkesson, Henning; Nisancioglu, Kerim Hestnes & Nick, Faezeh M. (2018). Impact of fjord geometry on grounding line stability. Frontiers in Earth Science.  ISSN 2296-6463.  6 . doi: 10.3389/feart.2018.00071 Full text in Research Archive. Show summary

  Recent and past retreat of marine-terminating glaciers are broadly consistent with observed ocean warming, yet responses vary significantly within regions experiencing similar ocean conditions. We assess how fjord geometry modulates glacier response to a regional ocean warming on decadal to millennial time scales, by using an idealized, numerical model of fast-flowing glaciers including a crevasse-depth calving criterion. Our simulations show that, given identical climate forcing, grounding line responses can differ by tens of kilometers due to variations in channel width. We identify fjord mouths and embayments as vulnerable geometries, showing that glaciers in these fjords are prone to rapid, irreversible retreat, independent of the presence of a fjord sill. This irreversible retreat has relevance for the potential future recovery of marine ice sheets, if the current anthropogenic warming is reduced, or reversed, as well as for the response of marine ice sheets to past climate states; including the warm Bølling-Allerød interstadial, the Younger Dryas cold reversal and the Little Ice Age.

• Kageyama, Masa; Albani, Samuel; Braconnot, Pascale; Harrison, Sandy P.; hopcroft, Peter O.; Ivanovic, Ruza F.; Lambert, Fabrice; Marti, Olivier; Peltier, W. Richard; Peterschmitt, Jean-Yves; Roche, Didier M.; Tarasov, Lev; Zhang, Xu; Brady, Esther C.; Haywood, Alan M.; Legrande, Allegra N.; Lunt, Daniel J.; Mahowald, Natalie M.; Mikolajewicz, Uwe; Nisancioglu, Kerim Hestnes; Otto-Bliesner, Bette L.; Renssen, Hans; Tomas, Robert A.; Zhang, Qiong; Abe-Ouchi, Ayako; Bartlein, Patrick J.; Cao, Jian; Li, Qiang; Lohmann, Gerrit; Ohgaito, Rumi; Shi, Xiaoxu; Volodin, Evgeny; Yoshida, Kohei; Zhang, Xiao & Zheng, Weipeng (2017). The PMIP4 contribution to CMIP6 - Part 4: Scientific objectives and experimental design of the PMIP4-CMIP6 Last Glacial Maximum experiments and PMIP4 sensitivity experiments. Geoscientific Model Development.  ISSN 1991-959X.  10(11), s 4035- 4055 . doi: 10.5194/gmd-10-4035-2017 Full text in Research Archive.

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• Frank, Thomas; Åkesson, Henning; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2020). Geometric Controls of Fjord Glacier Dynamics. Show summary

  Retreat of marine outlet glaciers and ice shelves may initiate depletion of inland ice and lead to ice loss that by far exceeds what would be expected from ocean and atmospheric warming alone. Many marine outlet glaciers draining large parts of past and present ice masses have shown non-linear and variable retreat rates, with adjacent glaciers sometimes showing a highly different response to the same large-scale climate forcing. This suggests that individual glacier characteristics play a dominant role in governing retreat. There is widespread evidence that the dynamic glacier adjustment to an external forcing is highly influenced by fjord topography. However, whether this stabilizes the glacier, or promotes enhanced retreat, depends on the shape of the fjord. So far, no rigorous, systematic assessment of the exact influence of certain geometric features such as overdeepenings or embayments has been undertaken in a model framework that incorporates all relevant processes in a 3D layout. Here, we analyze a multitude of topographic settings and scenarios using the Ice Sheet System Model (ISSM), which accounts for all relevant physics in a 3D framework. Using artificial fjord geometries, we investigate glacier-topography interaction and quantify the modeled glacier response directly in relation to topographic features. In light of our modeled topographic influence on glacier retreat, we consider whether we reliably can extrapolate observations from a few well-monitored glaciers to those less studied. Furthermore, we discuss implications for past and future ice sheet mass loss and associated sea-level rise. Finally, a deeper understanding of processes at the glacier front improves confidence in the climate signal derived from the deglacial landscape, as glacier-proximal landforms can more confidently be linked to climate.

• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2020, 22. januar). SnowHow. [TV].  NRK. Show summary

  Vinteren endrer alt. Den byr på store muligheter og en masse trøbbel. Hva har vinteren gjort med oss som bor her nord, og hva gjør vi med vinteren?

• Jensen, Mari Fjalstad; Nisancioglu, Kerim Hestnes; Spall, Michael A. & Våge, Kjetil (2020). Changing sea ice in the Nordic Seas and its effect on water mass transformation.
• Maffezzoli, Niccolo; Patrizia, Ferretti; Delmonte, Barbara; Nisancioglu, Kerim Hestnes & Barbante, Carlo (2020). The ICELEARNING project - Artificial Intelligence techniques for ice core analyses. Show summary

  The detection of insoluble particles trapped in ice cores, like volcanic and dust particles, pollen grains, foraminifera and diatom assemblages, represents the experimental basis for multiple lines of environmental paleoresearch regarding the atmosphere, the biosphere and volcanology. To date, except for ice core dust, the detection of such particles is achieved through observations by manual microscopy. Artificial Intelligence predictive models are already applied to several research fields within geoscience, but up to date its implementation to ice core science is missing. The recently EU funded Marie Curie ICELEARNING project (2020-2022) aims to develop a two-phase routine for the automatic quantification of insoluble particles trapped in ice cores. The routine is based on a commercial Flow Imaging Microscope producing micro-scale images of insoluble particles from melted ice core samples. The image collection of mineral dust, tephra, pollen and marine foraminifera obtained from natural and/or ad-hoc prepared samples will constitute the training datasets. The images will be then analyzed by Pattern Recognition algorithms developed for automatic particle classification and counting. The routine will be specifically developed in order to be implemented in ice core Continuous Flow Analysis (CFA) systems, thus improving the more traditional methods and potentially providing continuous ice core insoluble particle records. The ICELEARNING methodology is suitable for melted ice core samples and any diluted aqueous sample, thus representing a ground-breaking analytical advancement for a wide range of research fields, from ice core science to marine geology. The innovative routine here presented is automatic and non-destructive, imperative prerequisites for future Antarctic ice core projects.

• Nisancioglu, Kerim Hestnes (2020, 23. januar). Forskere deler sine klimabekymringer – Når isen beveger seg, er den ustoppelig. [Internett].  ABC Nyheter. Show summary

  Årlige besøk til Grønlandsisen har fått klimaforsker Kerim Hestnes Nisancioglu til å innse at det er umulig å stoppe de menneskeskapte, globale klimaendringene. Det beste vi kan håpe på er å bremse det litt.

• Nisancioglu, Kerim Hestnes (2020, 06. august). Hun elsket kjøttdeig, plastposer og shopping. Slik fikk hun familien på fem til å leve mer miljøvennlig.  Aftenposten.
• Nisancioglu, Kerim Hestnes (2020, 08. juni). Klimaforskere: – Fra nå vil norske utslipp bare synke. [Internett].  NRK.no. Show summary

  Norske klimagassutslipp gikk ned med to prosent det siste året, viser foreløpig statistikk. Flere klimaforskere tror trenden er kommet for å bli.

• Nisancioglu, Kerim Hestnes (2020). SDG213 Climate Action - Causes and Consequences of Climate Change.
• Bretones, Anaïs; Nisancioglu, Kerim Hestnes; Jensen, Mari Fjalstad & Ilicak, Mehmet (2019). Arctic summer sea ice reduction projection: consequences for the hydrography. Show summary

  The Arctic region is experiencing a rapid warming which endangers its sea ice cover, a key component of the deep Arctic Ocean. The summer sea ice has indeed shrunk significantly during the last decades and prediction forecast its acceleration. On the contrary, the winter sea ice has been spared so far. The asymmetry between summer and winter sea ice reduction has been shown to feed back on the sea ice cover stability. Moreover, in a 2×CO2 experiment with CCSM (Bitz et al. 2006), this type of phenomenon was suggested to be responsible for extending the Atlantic meridional overturning circulation into the deep Arctic Ocean. While the summer sea ice decreases, this kind of mechanism could then dampen the ongoing slowdown of the Atlantic meridional overturning circulation, slowdown that is expect to enhance the sea ice reduction. Similarly we would then expected the acceleration of the winter sea ice melting to put an end to this negative feedback. We decide to work on a more realistic experiment forced with the RCP8.5 scenario and based on the CMIP6 recommendations. Although this experiment was ran with multiple models, we focus here on one of them, EC- Earth, to get a better understanding of the underlying dynamic accompanying the abrupt reduction. Ran from 1850 to 2300, EC-Earth forecasts an acceleration of the summer sea ice reduction leading to a summer ice free Arctic in 2060 whereas the winter extent only abruptly reduces in 2140. Do we also observe an enhancement of vertical convection in the Arctic Ocean contemporary with summer sea ice reduction, and can we link it with the Atlantic meridional overturning circulation? For how long will this feedback operate? results: In EC-Earth, we observed a steady increase in winter brine rejection in the deep Arctic Ocean until 2060, when the Arctic Ocean becomes sea ice free during summers. This extra brine turned out to be mostly located in the Canadian archipelago but also on the Siberian Shelf, a key location for deep water formation. Although we did not see its signature in the water column there, it remains to be evaluated if the vertical mixing is enhanced on the Siberian shelf and if if feeds the Atlantic overturning cell.

• Conrad, Clinton Phillips; Selway, Kate; Weerdesteijn, Maaike; Smith-Johnsen, Silje; Nisancioglu, Kerim Hestnes & Karlsson, Nanna B (2019). Magnetotelluric constraints on upper mantle viscosity structure and basal melt beneath the Greenland ice sheet.
• Faber, Anne-Katrine; Li, Camille; Guo, Chuncheng & Nisancioglu, Kerim Hestnes (2019). Investigating mechanisms connected to the warming signal in Greenland ice cores using model simulations. Show summary

  Ice cores allow us to investigate the past abrupt climate changes. The improved spatial distribution of ice core locations in recent years provide a deeper insight into the glacial conditions and the characteristics of Dansgard- Oeschger (D-O) events, as recorded from the Greenland Ice Sheet. In this study the atmospheric climate conditions during D-O events are explored. Using a set of coupled The Norwegian Earth System Model (NorESM) model simulations that simulate the stadial and interstadial parts of D-O-events, it is investigated how changes in sea ice and ocean conditions influence the Greenland climate conditions. A particular focus is on the atmospheric dynamics relevant for Greenland conditions, such as the jet stream. The seasonal and interannual variability in the simulations are explored and compared to information from ice core records in order to benchmark the model simulations. The use of model simulations makes it possible to explore the dynamics of the D-O events, as recorded by Greenland ice cores, in greater detail.

• Guo, Chuncheng; Nisancioglu, Kerim Hestnes; Sadatzki, Henrik; Dokken, Trond Martin & Jansen, Eystein (2019). Dynamical sequence of ocean, atmosphere, and sea ice changes during an abrupt stadial-to-interstadial climate transition. Show summary

  Marine Isotope Stage 3 (about 60 to 30 ka BP) was punctuated by abrupt climate transitions between colder stadial and warmer interstadial climate states. The fluctuations are known as Dansgaard–Oeschger events, and are featured by a rapid warming from stadial to interstadial in several decades as recorded by the Greenland ice cores, followed by gradual return to stadial conditions. Using a state-of-the-art climate model, the Norwegian Earth System Model configured for fast paleoclimate simulations with a two-degree atmosphere and one-degree ocean, we investigate the transient response of the atmosphere/ocean/sea ice processes from a stadial to interstadial climate state. The stadial state is achieved by applying freshwater forcing to a Marine Isotope Stage 3 control simulation. With support from a high-resolution marine sediment core in the Nordic Seas (MD99-2284), we addressed the key role played by sea ice in modulating the Greenland temperature change at the transition, and identify a sequence of changes in the ocean and its interactions with sea ice and the atmosphere. We found that the recovery of open ocean deep convection is asynchronous in the North Atlantic region, and in agreement with proxy reconstructions, changes in the ocean (e.g. AMOC) precede melting of sea ice in the Nordic Seas, with the latter process occurring simultaneously with rapid changes in Greenland temperature.

• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2019). Abrupt changes in sea ice.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2019, 21. april). Lyden av is. [TV].  NRK.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2019). Unforced Oscillations in Sea Ice.
• Jensen, Mari Fjalstad; Nisancioglu, Kerim Hestnes & Spall, Michael A. (2019). Large changes in sea ice triggered by small changes in Atlantic Water temperature.
• Kajanto, Karita Anneli; Seroussi, Hélène; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2019). Holocene retreat of Jakobshavn Isbræ. Show summary

  Marine terminating glaciers, such as Jakobshavn Isbræ in West Greenland, are complex systems driven by a combination of changes in oceanic and atmospheric climate. The recent retreat and speedup of Jakobshavn Isbræ is mainly attributed to oceanic warming, while the general retreat of the Greenland ice sheet since the last glacial maximum is attributed to changes in surface mass balance. However, the relative impact of atmospheric and oceanic forcing on marine terminating glaciers of the Greenland ice sheet, and the corresponding time scales of the response remain unknown. We use a thermo-mechanical 3D ice sheet model to study the retreat history of Jakobshavn Isbræ during the Holocene (9500 BP to present) and compare it with available observational data. The glacier retreat is simulated with the Ice Sheet System Model (ISSM) in a thermo-mechanically cou- pled 3D setup. The resolution is enhanced in areas with complicated topography and fast flowing ice. The model is run forward with different oceanic forcings starting from a recorded margin position at 9500 a BP. The simulated evolution of the front is compared to known geomorphological evidence, such as moraines and dated boulders. The results show that the retreat in the fjord is strongly dependent on the bathymetry of the fjord. Further- more, our results suggest that while surface mass balance dominates the retreat of the land based sections of the glacier, the fast flowing ice stream in the fjord is dominated by oceanic forcing and the dynamic response of the marine terminating front. Thus, our results indicate that the evolution of the land based and marine based ice in the area of Jakobshavn are driven by different external forcings, and that it is critical to resolve the detailed fjord topography in order to predict past and future changes.

• Loose, Nora & Nisancioglu, Kerim Hestnes (2019). Adjoint Modeling and Observing System Design in the Subpolar North Atlantic.
• Nisancioglu, Kerim Hestnes (2019). 1,5-graders-rapporten: Hvor mye olje det vil være plass til i et scenario der vi når halvannengradersmålet?.
• Nisancioglu, Kerim Hestnes (ed.) (2019). ACDC summer school – 10 year anniversary.
• Nisancioglu, Kerim Hestnes (2019). ERC Synergy Project ice2ice.
• Nisancioglu, Kerim Hestnes (2019, 03. august). Farlig svette: Hetebølge gir smelterekord på Grønland.  VG. Show summary

  Hetebølgen som ga rekordvarme flere steder i Europa, beveget seg denne uken nordover til Grønland. Det vil gi målbar havnivåstigning og konsekvenser i tusenvis av år fremover, mener klimaforskere.

• Nisancioglu, Kerim Hestnes (2019). Fra værvarslingsmodeller til klimaforskernes krystallkule. Show summary

  Modellene vi bruker til å forstå klimasystemet, samt gjøre prognoser for framtidige endringer har hatt en rivende utvikling siden utviklingen startet med å simulere polarfrontene til Vilhelm Bjerknes. Hva er status for numerisk modellering 100 år etter Bergensskolen i meteorologi og oppfinnelsen av moderne numerisk værvarsling?

• Nisancioglu, Kerim Hestnes (2019). From Evidence to Action: Moving Knowledge to Policy & Practice.
• Nisancioglu, Kerim Hestnes (2019). Greenland and Nordic Seas/Arctic Variability.
• Nisancioglu, Kerim Hestnes (2019, 26. oktober). Havet stiger - flere nasjoner er i ferd med å forsvinne. [Internett].  TV2.no. Show summary

  Ny forskning fra Bjerknessenteret og Universitetet i Bergen viser langt mer dramatiske konsekvenser av issmeltingen på Grønland enn tidligere kjent.

• Nisancioglu, Kerim Hestnes (2019, 23. august). Reisebyrå: Grønland-interessen tar av. [Internett].  VG. Show summary

  Etter at USAs president Donald Trump offentliggjorde sitt ønske om å kjøpe Grønland har interessen for øya tatt av, forteller flere reisebyråer.

• Nisancioglu, Kerim Hestnes (2019). The Climate of an Ice Free Arctic. Show summary

  In connection with the inauguration of the "JSPS Alumni Club in Norway (ACN)", an academic seminar about collaborative marine research will be organized in the Research Council of Norway. The seminar is jointly organized by JSPS Alumni Club Norway ACN, Research Council of Norway, Norwegian Agency for International Cooperation and Quality Enhancement in Higher Education (Diku) and Japan Society for the Promotion of Science (JSPS) Stockholm Office. It is an excellent opportunity for people working to enhance academic collaboration between Norway and Japan to get together and to enhance a productive partnership.

• Nisancioglu, Kerim Hestnes (2019). The Dynamics of Abrupt Climate Change. Show summary

  The last glacial cycle was punctuated by multiple abrupt climate transitions. These fluctuations, know as Dansgaard-Oeschger (D-O) events, feature rapid warming of up to 15 degrees in a course of a few decades. Why these extreme climate events, recorded in all the Greenland ice core records, occurred, has eluded scientists since their discovery in the 60s. The prevailing hypothesis for D-O events is that they are caused by abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC). Given the abruptness of the events and the mounting evidence for past rapid changes in sea ice, we present a theory for D-O events based on the interplay between Atlantic water temperature, surface fresh water input and the impact of hydrography on the stability of Nordic Seas sea ice. Experiments with an idealised geometry, eddy permitting ocean model, as well as a coupled global climate model, illustrate the high sensitivity of Greenland temperature to changes in Nordic Sea sea ice, as well as the potential for abrupt transitions and unforced oscillations in sea ice given relatively small changes in the hydrography. The question which remains is wether these abrupt changes in Greenland climate are examples of what we have in store for the future.

• Nisancioglu, Kerim Hestnes (2019). The Dynamics of DO-events.
• Nisancioglu, Kerim Hestnes (2019). The Dynamics of DO-events.
• Nisancioglu, Kerim Hestnes (2019). Togradersmålet vs åttegradersrealiteten på Svalbard. Show summary

  Globalt er 2-gradersmålet satt. Lokalt på Svalbard kan man komme til å se temperaturøkning på 8-10 grader. Hvordan vil klimaendringene arte seg på Svalbard? Hvilken rolle spiller forskningen? Hva kan et sterkt kunnskapsmiljø bidra med? Hvilke geopolitiske implikasjoner kan et annerledes Svalbard skape? Hva slags muligheter og begrensninger kommer med det nye klimaet? På dette frokostmøtet skal vi diskutere konsekvensene av at klimaendringene merkes først på Svalbard.

• Nisancioglu, Kerim Hestnes (2019, 13. september). Unge forskere strømmer til "klimalaboratoriet" Svalbard. Men engasjementet gjenspeiles ikke blant beslutningstakerne. [Internett].  High North News. Show summary

  Svalbard blir våtere, varmere og grønnere. Heldigvis er det stadig flere unge mennesker som kommer til øya for å studere endringene i felt. Uheldigvis gjenspeiles ikke interessen blant beslutningstakerne.

• Nisancioglu, Kerim Hestnes (2019, 26. august). Vi burde trekke oss tilbake fra kysten.  Avis. Show summary

  Det sier NASA-oseanografen under et besøk på Grønland i sommer. Vi har underestimert hvor fort isen smelter, sier norsk ekspert.

• Nisancioglu, Kerim Hestnes; Capron, Emilie & Rasmussen, Sune O. (2019). The impact of H-events on the structure of DO-stadials. Show summary

  DO-events, the abrupt warming events seen repeatedly during the Last Glacial, are the most extreme climate changes found in the Greenland ice cores. The repeated warming of up to 15 degrees in the course of a few decade are particularly clear during MIS3 (∼60-30ka BP). The cold stadial periods, preceding each DO-event, exhibit large variations in duration and structure. The dynamics governing these cold stadials, as well as the abrupt transitions to warm interstadials, has alluded climate scientists since their discovery in the 70s. However, new proxy and model data provide an increasingly detailed view of these events. Here, we will summarize existing reconstructions and simulations of MIS3 with a focus on the cold stadials and the contrast between stadials with H-events (H-stadials) and stadials without H-events (DO-stadials). Studies suggest H-stadials exhibit two phases, different from the relatively stable cold phase of most DO-stadials. Here we investigate whether this could be caused by a delayed release of icebergs and meltwater to the North Atlantic, during a period of extensive Arctic and Nordic Seas sea ice cover, giving a significant reduction in ocean overturning circulation towards the end of Greenland stadials.

• Nisancioglu, Kerim Hestnes; Guo, Chuncheng & Jensen, Mari Fjalstad (2019). The Dynamics of DO-events.
• Nisancioglu, Kerim Hestnes; Guo, Chuncheng & Jensen, Mari Fjalstad (2019). The dynamics of abrupt climate changes during the last glacial cycle. Show summary

  The last glacial cycle was punctuated by multiple abrupt climate transitions between cold stadial and warm interstadial climate conditions. These fluctuations are know as Dansgaard-Oeschger (D- O) events, featuring rapid warming from stadial to interstadial states in a few decades, as revealed by the deep Greenland ice cores. D-O events are particularly prevalent during Marine Isotope Stage 3 (MIS3; about 60-30 ka BP), and less so during the seemingly stable Last Glacial Maximum (LGM). Why these extreme climate events, with repeated warming of up to 15 degrees in a course of a few decades, occurred, has eluded scientists since their discovery in the Camp Century and GISP ice core records. Here we present a detailed discussion of the dynamics of these abrupt events as seen in high resolution Greenland ice cores, marine sediment cores, as well as simulations with climate models. The prevailing hypothesis for D-O events is that they are caused by abrupt changes in the Atlantic Meridional Overturning Circulation (AMOC), Nordic Seas sea ice cover, or a combination of these. Given the abruptness of the events and the mounting evidence for past rapid changes in sea ice, we present a theory for D-O events based on the interplay between Atlantic water temperature, surface fresh water input and the impact of hydrography on the stability of Nordic Seas sea ice. These conclusions are corroborated by a series of sensitivity studies with an idealised geometry, eddy permitting ocean model as well as a coupled global climate model forced with MIS3 boundary conditions. The experiments illustrate the high sensitivity of Greenland temperature to changes in Nordic Sea sea ice, as well as the potential for abrupt transitions and unforced oscillations in sea ice given relatively small changes in the hydrography.

• Nisancioglu, Kerim Hestnes; Jansen, Eystein; Dokken, Trond Martin; Christensen, Jens Hesselbjerg & Vinther, Bo Møllesøe (2019). A new era of abrupt climate change - perspectives from Marine Isotope Stage 3. Show summary

  Abrupt climate change is a striking feature of many climate records, particularly those documenting temperature changes during the last glacial period. Known as Dansgaard-Oeschger (D-O) events, in Greenland ice core records, these had global repercussions. The abruptness and high amplitude of the D-O warming events are tightly coupled to a rapid retreat of sea ice in the North Atlantic and Nordic Seas. Sea ice loss is also an integral part of the ongoing Arctic warming. We compare current rates of Arctic temperature change with past abrupt temperature changes from the Greenland ice cores and find that modern rates of change are therefore also abrupt. Model simulations of past abrupt changes are commensurable with the reconstructed changes during the last glacial period. Climate models underestimate, however, the current abrupt warming in the Arctic. Only models with high climate sensitivity display abrupt changes within the 21st century, under emissions in accord with the Paris Agreement targets.

• Plach, Andreas; Nisancioglu, Kerim Hestnes; Born, Andreas & Vinther, Bo Møllesøe (2019). Simulation of the Eemian Greenland ice sheet.
• Plach, Andreas; Nisancioglu, Kerim Hestnes; Langebroek, Petra; Born, Andreas & Le clec’h, Sebastien (2019). Eemian Greenland ice sheet simulated with a higher-order model shows strong sensitivity to SMB forcing. Show summary

  The Greenland ice sheet (GrIS) contributes increasingly to global sea level rise and its past history is a valuable reference for future sea level projections. We present ice sheet simulations for the Eemian interglacial period (ap- prox. 125,000 years ago), the period with the most recent warmer-than-present summer climate over Greenland. The evolution of the Eemian GrIS is simulated with a 3D higher-order ice sheet model forced with surface mass balance (SMB) derived from regional climate simulations. Sensitivity experiments with different SMB, basal fric- tion, and ice flow approximations are discussed. We find that the SMB forcing is the controlling factor setting the Eemian minimum ice volume, emphasizing the importance of a reliable SMB model. Our results suggest that when estimating the contribution from the GrIS to sea level rise during warm periods, such as the Eemian interglacial period, the SMB forcing is more important than the representation of ice flow.

• Rheinlænder, Jonathan Winfield; Ferreira, David & Nisancioglu, Kerim Hestnes (2019). Exploring the impact of the rise of Greenland-Scotland Ridge on ocean circulation and climate.
• Rheinlænder, Jonathan Winfield; Ferreira, David & Nisancioglu, Kerim Hestnes (2019). How the Greenland-Scotland Ridge shapes the Atlantic meridional overturning circulation and Northern Hemisphere climate. Show summary

  Changes in the geometry of ocean basins have been influential in driving climate change throughout Earth’s his- tory. Here we focus on the appearance of the Greenland-Scotland Ridge (GSR) and try to understand its impact on climate, including global circulation, heat transport, water mass properties and ventilation timescales, which will be useful for interpreting paleoproxies. To this end, we use a coupled atmosphere-ocean-sea ice model with idealized geometry and consider two geometri- cal configurations. The reference configuration (noridge) comprises two wide strips of land set 90◦ apart extending from the North Pole to 40◦S, separating the Northern Hemisphere ocean into a small «Atlantic-like» and a large «Pacific-like» basin. In the ridge configuration a zonally symmetric oceanic ridge, that extends across the Atlantic- like basin at 60◦N, mimicking the GSR, is added. We consider three background climates of noridge: a warm case where the northern high latitudes are ice-free, an intermediate case with a small seasonal ice cover and a cold case with a perennial sea ice cover. In all three cases of noridge deep-water formation occurs in the northern parts of the Atlantic-like basin, driving a meridional overturning circulation (MOC) akin to the Atlantic MOC of present-day. When the ridge is introduced, the flow of warm Atlantic water to the high latitudes is hampered and the ocean heat transport across 70◦N decreases by ∼40% which causes cooling and freshening of the waters north of the ridge. Consequently, the meridional density structure changes significantly with cold dense water forming north of the ridge and eventually overflows into the North Atlantic. Downwelling shifts south of the ridge, thereby altering the properties of the deep water mass and the structure of the MOC north of 60◦N dramatically, although the maximum MOC strength remains unchanged. Despite these changes, the large-scale surface climate response is surprisingly small for all cases. Our results highlight the possible disconnect between changes in the localization of deep-water formation, the structure of the AMOC and the properties of water masses and changes in Northern Hemisphere surface climate. They also underscore the necessary caution in interpreting paleoproxies in terms of AMOC and climate change.

• Rheinlænder, Jonathan Winfield; Ferreira, David & Nisancioglu, Kerim Hestnes (2019). The impact of vertical mixing on glacial ocean circulation and sea ice.
• Rheinlænder, Jonathan Winfield; Ferreira, David & Nisancioglu, Kerim Hestnes (2019). Topological constraints by the Greenland-Scotland Ridge on AMOC and climate.
• Rheinlænder, Jonathan Winfield; Ferreira, David & Nisancioglu, Kerim Hestnes (2019). Topological constraints by the Greenland-Scotland Ridge on AMOC and climate.
• Selway, Kate; Conrad, Clinton Phillips; Nisancioglu, Kerim Hestnes; Karlsson, Nanna B & Steinberger, Bernhard (2019). The MAGPIE Project: Magnetotelluric Analysis of Greenland and Postglacial Isostatic Evolution.
• Sessford, Evangeline; Jensen, Mari Fjalstad; Tisserand, Amandine Aline; Muschitiello, Francesco; Dokken, Trond Martin; Nisancioglu, Kerim Hestnes & Jansen, Eystein (2019). Intermediate water in the Nordic Seas during Greenland Stadials.
• Smith-Johnsen, Silje; de Fleurian, Basile; Schlegel, Nicole; Seroussi, Hélène & Nisancioglu, Kerim Hestnes (2019). Initiating the Northeast Greenland Ice Stream with Geothermal Heat. Show summary

  The Northeast Greenland Ice Stream (NEGIS) currently drains more than 10% of the area of the Greenland Ice Sheet, making it an important feature to simulate when assessing the potential future contribution of Greenland to sea level rise. At present, NEGIS is mostly reproduced in ice sheet models by inferring for basal conditions using observed surface velocities. NEGIS is suggested to be triggered by a geothermal heat flux anomaly at the ice divide. However, the heat flux underneath the ice sheet is largely unknown, except for a few direct measurements from deep ice core drill sites. Models point to a geothermal heat flux anomaly left behind by movement of Greenland over the Icelandic plume. Using the Ice Sheet System Model (ISSM), with ice dynamics coupled to a subglacial hydrology model, we investigate the possibility of initiating NEGIS by inserting various hotspot configurations with values ranging from 120 mW/m2 (Dahl-Jensen et al. 2017) to 917 mW/m2 (Fahnestock et al. 2001). When inserting the hotspot, we observe a shift in the thermal regime from cold to thawed, followed by an acceleration of the system. The hotspot impacts the downstream flow of the ice in broadly the same region as the present day NEGIS. By including high geothermal heat flux, and the effect of water on sliding, we are one step closer to representing NEGIS in ice sheet models.

• Smith-Johnsen, Silje & Nisancioglu, Kerim Hestnes (2019). Dynamics of the Northeast Greenland Ice Stream: the role of geothermal heat and subglacial hydrology.
• Smith-Johnsen, Silje; Nisancioglu, Kerim Hestnes; de Fleurian, Basile; Schlegel, Nicole & Seroussi, Helene (2019). Exceptionally high Geothermal heat flux needed to sustain the Northeast Greenland Ice Stream.
• Vettoretti, Guido; Pedro, Joel; Rasmussen, Sune O.; Nilsen, Søren; Jochum, Markus & Nisancioglu, Kerim Hestnes (2019). Bipolar Synchronicity: Global Atmosphere and Ocean Teleconnections during Dansgaard-Oeschger Oscillations. Show summary

  Studies using the temporal synchronization of ice cores from Antarctica and Greenland along with coupled climate model simulations of abrupt climate change during the glacial have been used to investigate the interhemispheric climate connection during Dansgaard-Oeschger (D-O) events. It has been proposed that Antarctic climate responds to Northern Hemisphere changes with an oceanic lag of approximately two centuries along with a near instanta- neous reorganization of the global atmospheric circulation. In this presentation, results from coupled climate model simulations that accurately simulate the proxy inferred temperature variability from polar ice cores demonstrate a decadal scale synchronization between the poles during D-O transition points. The atmosphere and oceanic telecon- nections that are responsible for the discrepancies between this study and previous studies are described. Previously published results that presented a process-based understanding for the bipolar seesaw are further elaborated upon with new modelling studies. In addition to this, we present the dominant atmospheric modes of variability that re- sult in Antarctic temperature heterogeneity. These include the influence of the Southern Annular Mode (SAM) and the Pacific South American (PSA) pattern on Antarctic climate variability and their corresponding teleconnection with decadal El Nino-Southern Oscillation (ENSO) variability and the Atlantic Multidecadal Oscillation (AMO).

• Guo, Chuncheng & Nisancioglu, Kerim Hestnes (2018). Dynamical sequence of ocean, atmosphere, and sea ice changes during an abrupt stadial-to-interstadial climate transition.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2018). Abrupt changes in sea ice.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2018). Abrupt changes in sea ice and dynamics of Dansgaard-Oeschger events.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2018). Dynamics of abrupt changes in Arctic sea ice.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2018). Large changes in sea ice and climate with small changes in Atlantic water temperature.
• Nisancioglu, Kerim Hestnes (2018). ERC SyG ice2ice - deciphering the links between Arctic sea ice and the Greenland ice sheet.
• Nisancioglu, Kerim Hestnes (2018, 23. mai). Exhibition: ‘Prospecting Ocean’ by Armin Linke, in Castello. [Internett].  2018 Biennale of Architecture. Show summary

  The spaces of CNR-ISMAR ( Institute of Marine Sciences of the National Research Council of Italy) serve as the backdrop to an investigative exhibition by filmmaker and photographer Armin Linke exploring contemporary challenges facing our oceans. Born out of an important three-year-research and study phase with TBA21–Academy, Prospecting Ocean gathers rare multimedia footage and archival materials, including precious underwater shootings and documentaries, as well as interviews with leading scientists, policymakers, and experts. The rare images and videos invite the viewer to consider the implications of deep-sea mining on both the environment and communities.

• Nisancioglu, Kerim Hestnes (2018, 04. juli). Forsker på 11,000 år gammel is. [TV].  NRK.
• Nisancioglu, Kerim Hestnes (2018, 03. november). Forsker slår alarm: – Havet kan stige over 15 meter de neste 500 årene. [Internett].  NRK. Show summary

  Alle verdens kystbyer forsvinner hvis isen i Antarktis smelter. – Valgene vi tar det neste tiåret avgjør alt, sier professor.

• Nisancioglu, Kerim Hestnes (2018, 13. desember). Frykter det er for seint å unngå skrekkscenarioet: - Det er en dødsdom.  Dagbladet. Show summary

  Grønland beskrives som en sovende kjempe, som er i ferd med å våkne og skape kaos. Nå frykter forskere det er for seint å skru av vekkerklokka.

• Nisancioglu, Kerim Hestnes (2018, 22. august). Magnhild (31) vurderer å forbli barnløs: – Bør jeg ha dårlig miljøsamvittighet hvis jeg velger å få et barn?.  Stavanger Aftenblad.
• Nisancioglu, Kerim Hestnes (2018, 26. august). Magnhild (31) vurderer å forbli barnløs: – Bør jeg ha dårlig miljøsamvittighet hvis jeg velger å få et barn?.  Aftenposten.
• Nisancioglu, Kerim Hestnes (2018). The impact of H-events on the structure of DO-stadials.
• Nisancioglu, Kerim Hestnes (2018, 09. november). Toppen av innlandsisen.  Dag og Tid. Show summary

  Historikar Harald Dag Jølle drog i vår saman med Vegard Ulvang og Ronny Finsås ut på ein 3000 kilometer lang skitur. Målet var å finne ut kvifor den norske polarfararen Eivind Astrup rauk uklar med amerikanaren Robert Peary i 1890-åra.

• Nisancioglu, Kerim Hestnes (2018). Update on climate research from Bjerknessenteret. Show summary

  Joint initiatives to advance European climate policy: Kerim H Nisancioglu will give us an update on climate research from Bjerknessenteret, one of the world's leading research communities that contributes to the climate panel's reports.

• Nisancioglu, Kerim Hestnes; Capron, Emilie & Rasmussen, Sune O. (2018). The impact of H-events on the structure of Greenland stadials (DO-stadials).
• Nisancioglu, Kerim Hestnes; Åkesson, Henning & Steiger, Nadine (2018). Rapid deglaciation of Norwegian and Greenland fjords.
• Plach, Andreas; Nisancioglu, Kerim Hestnes; Le clec'h, Sebastien; Born, Andreas; Langebroek, Petra; Guo, Chuncheng; Imhof, Michael & Stocker, Thomas (2018). Eemian Greenland Surface Mass Balance strongly sensitive to SMB model setup.
• Plach, Andreas; Nisancioglu, Kerim Hestnes; Le clec'h, Sebastien; Born, Andreas; Langebroek, Petra; Guo, Chuncheng; Imhof, Michael & Stocker, Thomas (2018). Simulated Eemian Greenland Surface Mass Balance shows strong sensitivity to SMB model choice.
• Plach, Andreas; Vinther, Bo Møllesøe; Nisancioglu, Kerim Hestnes; Vudayagiri, Sindhu & Blunier, Thomas (2018). Eemian surface melt on Greenland?.
• Smith-Johnsen, Silje; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2018). The effect of subglacial hydrology when assessing the impact of geothermal heat flux on sliding and ice dynamics.
• Steen-Larsen, Hans Christian; Hörhold, Maria; Madsen, Martin; Kipfstuhl, Sepp; Box, Jason E; Faber, Anne-Katrine; Vladimirova, Diana; White, James C; Capron, Emilie; Jensen, Mari Fjalstad; Koldtoft, Iben; Nisancioglu, Kerim Hestnes & Pilar, Helen (2018). A Comprehensive Study of the Drivers of Surface Snow Isotopes in Greenland.
• Åkesson, Henning; Gyllencreutz, Richard; Mangerud, Jan; Nick, Faezeh M. & Nisancioglu, Kerim Hestnes (2018). Fast retreat of a marine outlet glacier in western Norway at the last glacial termination. Show summary

  Observations of marine outlet glaciers covering the last few decades show speedup and accelerated retreat, and the geologic record testaments that catastrophic changes occurred also in the past. However, the time scales of rapid outlet glacier retreat and the underlying drivers are still unclear. Here, we study the collapse of Hardangerfjorden glacier, an outlet glacier of the Scandinavian Ice Sheet, during the transition from the Younger Dryas cold period to the warming in early Holocene. We use dated terminal and lateral moraines as constraints for a simple ice flow model including calving. Our experiments suggest a fast and highly variable retreat history driven by surface melt and warming fjord waters, paced by fjord bathymetry as well as ice shelf dynamics. Grounding line retreat in response to the rapid, large early Holocene warming is simulated at rates below 200 m/a, punctuated by brief spells of swift retreat exceeding 500 m/a. Our findings imply that warming subsurface waters will continue to drive retreat of marine outlet glaciers in Greenland and elsewhere, but that recent elevated retreat rates observed are not likely to be sustained longer than a few decades. On centennial scales, retreat will likely be increasingly driven by a warming atmosphere.

• Åkesson, Henning; Nisancioglu, Kerim Hestnes & Svendsen, John-Inge (2018). Deglaciation of the Norwegian fjords.
• de Fleurian, Basile; Mouginot, J.; Morlighem, Mathieu; Seroussi, Helene; Rignot, Eric & Nisancioglu, Kerim Hestnes (2017). Evolution of the subglacial water pressure under Russell glacier, a modelling approach..
• Faber, Anne-Katrine; Nisancioglu, Kerim Hestnes & Kjær, Helle Astrid (2017, 18. juli). Ice2ice scientists involved in the EastGRIP ice core drilling projects. [Internett].  https://ice2ice.b.uib.no/news/.
• Guo, Chuncheng; Bentsen, Mats; Nisancioglu, Kerim Hestnes; Bethke, Ingo; Ilicak, Mehmet; Toniazzo, Thomas; Otterå, Odd Helge & Zhang, Zhongshi (2017). Glacial and interglacial simulations with NorESM BCCR fast version.
• Guo, Chuncheng; Nisancioglu, Kerim Hestnes; Bentsen, Mats; Bethke, Ingo & Zhang, Zhongshi (2017). NorESM simulations of Marine Isotope Stage 3 climates.

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