Kerim Hestnes Nisancioglu

• Zhang, Zhongshi; Li, Xiangyu; Guo, Chuncheng; Otterå, Odd Helge; Nisancioglu, Kerim Hestnes & Tan, Ning [Show all 29 contributors for this article] (2021). Mid-Pliocene Atlantic Meridional Overturning Circulation simulated in PlioMIP2. Climate of the Past. ISSN 1814-9324. 17(1), p. 529–543. doi: 10.5194/cp-17-529-2021.
• Oldeman, Arthur M.; Baatsen, Michiel L. J.; Von Der Heydt, Anna S.; Dijkstra, Henk A.; Tindall, Julia C. & Abe-Ouchi, Ayako [Show all 33 contributors for this article] (2021). Reduced El Niño variability in the mid-Pliocene according to the PlioMIP2 ensemble. Climate of the Past. ISSN 1814-9324. 17(6), p. 2427–2450. doi: 10.5194/cp-17-2427-2021.
• Han, Zixuan; Zhang, Qiong; Li, Qiang; Feng, Ran; Haywood, Alan M. & Tindall, Julia C. [Show all 29 contributors for this article] (2021). Evaluating the large-scale hydrological cycle response within the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble. Climate of the Past. ISSN 1814-9324. 17(6), p. 2537–2558. doi: 10.5194/cp-17-2537-2021.
• Bretones, Anais Magali; Nisancioglu, Kerim Hestnes; Jensen, Mari Fjalstad; Brakstad, Ailin & Yang, Shuting (2021). Transient Increase in Arctic Deep-Water Formation and Ocean Circulation under Sea Ice Retreat . Journal of Climate. ISSN 0894-8755. 35(1), p. 109–124. doi: 10.1175/JCLI-D-21-0152.1.
• Plach, Andreas; Vinther, Bo M.; Nisancioglu, Kerim Hestnes; Vudayagiri, Sindhu & Blunier, Thomas (2021). Greenland climate simulations show high Eemian surface melt which could explain reduced total air content in ice cores. Climate of the Past. ISSN 1814-9324. 17(1), p. 317–330. doi: 10.5194/cp-17-317-2021. Show summary

  This study presents simulations of Greenland surface melt for the Eemian interglacial period (∼130 000 to 115 000 years ago) derived from regional climate simulations with a coupled surface energy balance model. Surface melt is of high relevance due to its potential effect on ice core observations, e.g., lowering the preserved total air content (TAC) used to infer past surface elevation. An investigation of surface melt is particularly interesting for warm periods with high surface melt, such as the Eemian interglacial period. Furthermore, Eemian ice is the deepest and most compressed ice preserved on Greenland, resulting in our inability to identify melt layers visually. Therefore, simulating Eemian melt rates and associated melt layers is beneficial to improve the reconstruction of past surface elevation. Estimated TAC, based on simulated melt during the Eemian, could explain the lower TAC observations. The simulations show Eemian surface melt at all deep Greenland ice core locations and an average of up to ∼30 melt days per year at Dye-3, corresponding to more than 600 mm water equivalent (w.e.) of annual melt. For higher ice sheet locations, between 60 and 150 mmw.e.yr−1 on average are simulated. At the summit of Greenland, this yields a refreezing ratio of more than 25 % of the annual accumulation. As a consequence, high melt rates during warm periods should be considered when interpreting Greenland TAC fluctuations as surface elevation changes. In addition to estimating the influence of melt on past TAC in ice cores, the simulated surface melt could potentially be used to identify coring locations where Greenland ice is best preserved.

• Morée, Anne; Sun, Tianyi; Bretones, Anaïs; Straume, Eivind Olavson; Nisancioglu, Kerim Hestnes & Gebbie, Geoffrey (2021). Cancellation of the precessional cycle in δ18O records during the Early Pleistocene . Geophysical Research Letters. ISSN 0094-8276. doi: 10.1029/2020GL090035. Show summary

  The dominant pacing of glacial‐interglacial cycles in deep‐ocean δ18O records changed substantially during the Mid‐Pleistocene Transition. The precessional cycle (∼23 ky) is absent during the Early Pleistocene, which we show can be explained by cancellation of the hemispherically anti‐phased precessional cycle in the Early Pleistocene interior ocean. Such cancellation develops due to mixing of North Atlantic and Southern Ocean δ18O signals at depth, and shows characteristic spatial patterns. We explore the cancellation potential for different North Atlantic and Southern Ocean deep‐water source δ18O values using a tracer transport ocean model. Cancellation of precession occurs for all signal strengths and is widespread for a signal strength typical for the Early Pleistocene. Early Pleistocene precessional power is therefore likely incompletely archived in deep‐sea δ18O records, concealing the true periodicity of the glacial cycles in the two hemispheres.

• Otto-Bliesner, Bette L.; Brady, Esther C.; Zhao, Anni; Brierley, Chris M.; Axford, Yarrow & Capron, Emilie [Show all 39 contributors for this article] (2021). Large-scale features of Last Interglacial climate: results from evaluating the lig127k simulations for the Coupled Model Intercomparison Project (CMIP6)–Paleoclimate Modeling Intercomparison Project (PMIP4). Climate of the Past. ISSN 1814-9324. 17(1), p. 63–94. doi: 10.5194/cp-17-63-2021. Full text in Research Archive
• Kageyama, Masa; Sime, Louise C.; Sicard, Marie; Guarino, Maria-Vittoria; De Vernal, A & Stein, Ruediger [Show all 36 contributors for this article] (2021). A multi-model CMIP6-PMIP4 study of Arctic sea ice at 127 ka: sea ice data compilation and model differences. Climate of the Past. ISSN 1814-9324. 17(1). doi: 10.5194/cp-17-37-2021. Show summary

  The Last Interglacial period (LIG) is a period with increased summer insolation at high northern latitudes, which results in strong changes in the terrestrial and marine cryosphere. Understanding the mechanisms for this response via climate modelling and comparing the models' representation of climate reconstructions is one of the objectives set up by the Paleoclimate Modelling Intercomparison Project for its contribution to the sixth phase of the Coupled Model Intercomparison Project. Here we analyse the results from 16 climate models in terms of Arctic sea ice. The multi-model mean reduction in minimum sea ice area from the pre industrial period (PI) to the LIG reaches 50 % (multi-model mean LIG area is 3.20×106 km2, compared to 6.46×106 km2 for the PI). On the other hand, there is little change for the maximum sea ice area (which is 15–16×106 km2 for both the PI and the LIG. To evaluate the model results we synthesise LIG sea ice data from marine cores collected in the Arctic Ocean, Nordic Seas and northern North Atlantic. The reconstructions for the northern North Atlantic show year-round ice-free conditions, and most models yield results in agreement with these reconstructions. Model–data disagreement appear for the sites in the Nordic Seas close to Greenland and at the edge of the Arctic Ocean. The northernmost site with good chronology, for which a sea ice concentration larger than 75 % is reconstructed even in summer, discriminates those models which simulate too little sea ice. However, the remaining models appear to simulate too much sea ice over the two sites south of the northernmost one, for which the reconstructed sea ice cover is seasonal. Hence models either underestimate or overestimate sea ice cover for the LIG, and their bias does not appear to be related to their bias for the pre-industrial period. Drivers for the inter-model differences are different phasing of the up and down short-wave anomalies over the Arctic Ocean, which are associated with differences in model albedo; possible cloud property differences, in terms of optical depth; and LIG ocean circulation changes which occur for some, but not all, LIG simulations. Finally, we note that inter-comparisons between the LIG simulations and simulations for future climate with moderate (1 % yr−1) CO2 increase show a relationship between LIG sea ice and sea ice simulated under CO2 increase around the years of doubling CO2. The LIG may therefore yield insight into likely 21st century Arctic sea ice changes using these LIG simulations.

• Mojtabavi, Seyedhamidreza; Wilhelms, Frank; Cook, Eliza; Davies, Siwan M.; Sinnl, Giulia & Skov Jensen, Mathias [Show all 20 contributors for this article] (2020). A first chronology for the East Greenland Ice-core Project (EGRIP) over the Holocene and last glacial termination. Climate of the Past. ISSN 1814-9324. 16(6), p. 2359–2380. doi: 10.5194/cp-16-2359-2020. Full text in Research Archive
• Åkesson, Henning; Gyllencreutz, Richard; Mangerud, Jan; Svendsen, John-Inge; Nick, Faezeh M. & Nisancioglu, Kerim Hestnes (2020). Rapid retreat of a Scandinavian marine outlet glacier in response to warming at the last glacial termination. Quaternary Science Reviews. ISSN 0277-3791. 250. doi: 10.1016/j.quascirev.2020.106645. Full text in Research Archive Show summary

  Marine outlet glaciers on Greenland are retreating, yet it is unclear if the recent fast retreat will persist, and how atmosphere and ocean warming will impact future retreat. We show how a marine outlet glacier in Hardangerfjorden retreated rapidly in response to the abrupt warming following the Younger Dryas cold period (approximately 11,600 years before present). This almost 1000 m deep fjord, with several sills at 300–500 m depth, hosted a 175 km long outlet glacier at the western rim of the Scandinavian Ice Sheet. We use a dynamic ice-flow model constrained by well-dated terminal and lateral moraines to simulate the reconstructed 500-year retreat of Hardangerfjorden glacier. The model includes an idealized oceanic and atmospheric forcing based on reconstructions, but excludes the surface mass balance-elevation feedback. Our simulations show a highly episodic retreat driven by surface melt and warming fjord waters, paced by the fjord bathymetry. Warming air and ocean temperatures by 4–5 °C during the period of retreat result in a 125-km retreat of Hardangerfjorden glacier in 500 years. Retreat rates throughout the deglaciation vary by an order of magnitude from 50 to 2500 m a−1, generally close to 200 m a−1, punctuated by brief events of swift retreat exceeding 500 m a−1, each event lasting a few decades. We show that the fastest retreat rates occur in regions of the bed with the largest retrograde slopes; ice shelf length and fjord water depth is less important. Our results have implications for modern glacial fjord settings similar to Hardangerfjorden, where high retreat rates have been observed. Our findings imply that increasing air temperatures and warming subsurface waters in Greenland fjords will continue to drive extensive retreat of marine outlet glaciers. However, the recent high retreat rates are not expected to be sustained for longer than a few decades due to constraints by the fjord bathymetry.

• Jansen, Eystein; Christensen, Jens Hesselbjerg; Dokken, Trond Martin; Nisancioglu, Kerim Hestnes; Vinther, Bo M. & Capron, Emilie [Show all 16 contributors for this article] (2020). Past perspectives on the present era of abrupt Arctic climate change. Nature Climate Change. ISSN 1758-678X. 10, p. 714–721. doi: 10.1038/s41558-020-0860-7.
• Haywood, Alan M.; Tindall, Julia C.; Dowsett, Harry J.; Dolan, Aisling M.; Foley, Kevin M. & Hunter, Stephen J. [Show all 31 contributors for this article] (2020). The Pliocene Model Intercomparison Project Phase 2: large-scale climate features and climate sensitivity. Climate of the Past. ISSN 1814-9324. 16(6). doi: 10.5194/cp-16-2095-2020. 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.

• 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.

• McClymont, Erin L.; Ford, Heather; Ho, Sze Ling; Tindall, Julia C.; Haywood, Alan M. & Alonso-Garcia, Montserrat [Show all 36 contributors for this article] (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.

• Loose, Nora; Heimbach, Patrick; Pillar, Helen R. & Nisancioglu, Kerim Hestnes (2020). Quantifying dynamical proxy potential through shared adjustment pysics in the North Atlantic. Journal of Geophysical Research (JGR): Oceans. ISSN 2169-9275. 125( 9). 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.

• 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.

• 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, p. 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, p. 126–143. doi: 10.1016/j.gr.2020.05.011. Full text in Research Archive
• de Nooijer, W.; Zhang, Q.; Li, Q.; Zhang, Qiong; Li, Xiangyu & Zhang, Zhongshi [Show all 29 contributors for this article] (2020). Evaluation of Arctic warming in mid-Pliocene climate simulations. Climate of the Past. ISSN 1814-9324. 16(6), p. 2325–2341. doi: 10.5194/cp-16-2325-2020.
• 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. 66(256), p. 303–312. doi: 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; 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.

• 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.

• Sessford, Evangeline; Jensen, Mari Fjalstad; Tisserand, Amandine Aline; Muschitiello, Francesco; Dokken, Trond Martin & Nisancioglu, Kerim Hestnes [Show all 7 contributors for this article] (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
• 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), p. 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), p. 2133–2148. doi: 10.5194/tc-13-2133-2019. Full text in Research Archive

View all works in Cristin

• Kajanto, Karita Anneli & Nisancioglu, Kerim Hestnes (2021). The role of icebergs in a Greenland fjord.
• Nisancioglu, Kerim Hestnes; Morée, Anne; Sun, Tianyi; Bretones, Anais Magali; Straume, Eivind Olavson & Gebbie, Geoffrey (2021). Cancellation of the precessional cycle in δ18O records during the Early Pleistocene. Show summary

  The dominant pacing of glacial‐interglacial cycles in deep‐ocean δ18O records changed substantially during the Mid‐Pleistocene Transition. The precessional cycle (∼23 ky) is absent during the Early Pleistocene, which we show can be explained by cancellation of the hemispherically anti‐phased precessional cycle in the Early Pleistocene interior ocean. Such cancellation develops due to mixing of North Atlantic and Southern Ocean δ18O signals at depth, and shows characteristic spatial patterns. We explore the cancellation potential for different North Atlantic and Southern Ocean deep‐water source δ18O values using a tracer transport ocean model. Cancellation of precession occurs for all signal strengths and is widespread for a signal strength typical for the Early Pleistocene. Early Pleistocene precessional power is therefore likely incompletely archived in deep‐sea δ18O records, concealing the true periodicity of the glacial cycles in the two hemispheres.

• Nisancioglu, Kerim Hestnes (2021). Bærekraft of klimahandling - hva er utfordingene og hva krever det av oss?
• Nisancioglu, Kerim Hestnes (2021). Communicating the science of ice and climate.
• Nisancioglu, Kerim Hestnes (2021). SDG313 OneOcean Field Course - research and teaching at sea.
• Nisancioglu, Kerim Hestnes (2021). ClimateNarratives - SDG-13 i forskning, utdanning og internasjonalt samarbeid.
• Nisancioglu, Kerim Hestnes; Faber, Anne-Katrine & Zolles, Tobias (2021). IceFinse field course.
• Bretones, Anais Magali; Nisancioglu, Kerim Hestnes; Nummelin, Aleksi & Jensen, Mari Fjalstad (2021). Deep-water formation and Arctic circulation under sea-ice retreat: a comparison between CMIP6 models.
• Nisancioglu, Kerim Hestnes; Bretones, Anais Magali & Jensen, Mari Fjalstad (2021). Ice-ocean interactions and the stability of the Greenland ice sheet. Show summary

  The webinar “The Changing Arctic” for strengthening the Arctic research cooperation between Estonia and Norway will take place on Wednesday, 14 April 2021 at 09:00 (in Norway) / 10:00 (in Estonia). The Arctic has gained increased interest over the last decade as global climate change continues to transform the region. Norway and Estonia are both maritime countries with a long history of polar exploration and research. Now we are looking into new opportunities of Arctic research cooperation. Researchers from Estonia and Norway offer an insight into the state of play and their work in the Arctic – the glaciers and the sea, the indigenous people and the governance, methane seeps, algal blooms and the fungi. The webinar celebrates the 100th anniversary of the diplomatic relations between Estonia and Norway, and the Norwegian-Estonian cooperation in climate change under the EEA and Norway Grants. Organisers: The Estonian Academy of Sciences, Tallinn University of Technology (TalTech), the Bjerknes Centre for Climate Research/University of Bergen and the Academia Europaea Bergen Knowledge Hub in cooperation with the Norwegian Embassy in Tallinn.

• Nisancioglu, Kerim Hestnes (2021). Ekspertintervjuet: Grønlandsisen nær vippepunktet. [Internet]. Energi og Klima. Show summary

  For flere tusen år siden var temperaturen bare litt høyere enn i dag. Da var Grønlandsisen 1/3 mindre enn den er i dag. Forskjellen er at den gangen varte varmeperiode over flere tusen år, og temperaturen steg langsommere enn den gjør i dag, forklarer professor Kerim Nisancioglu ved Universitetet i Bergen og Bjerknessenteret.

• Nisancioglu, Kerim Hestnes & Jensen, Mari Fjalstad (2020). Changes in Overturning Circulation under Arctic Sea Ice Retreat in EC-Earth.
• Bretones, Anaïs; Nisancioglu, Kerim Hestnes; Jensen, Mari Fjalstad & Brakstad, Ailin Dale (2020). Changes in Overturning Circulation under Arctic Sea Ice Retreat in EC-Earth.
• Kajanto, Karita Anneli; Seroussi, Hélène; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2020). Holocene minimum extent of Jakobshavn Isbræ shows an approaching tipping point.
• Conrad, Clinton Phillips; Selway, Kate; Weerdesteijn, Maaike Francine Maria; Smith-Johnsen, Silje; Nisancioglu, Kerim Hestnes & Karlsson, Nanna B (2020). Magnetotelluric Constraints on Upper Mantle Viscosity Structure and Basal Melt Beneath the Greenland Ice Sheet.
• Conrad, Clinton Phillips; Selway, Kate; Weerdesteijn, Maaike Francine Maria; Smith-Johnsen, Silje; Nisancioglu, Kerim Hestnes & Karlsson, Nanna B (2020). Magnetotelluric Constraints on Upper Mantle Viscosity Structure and Basal Melt Beneath the Greenland Ice Sheet.
• Nisancioglu, Kerim Hestnes (2020). Hun elsket kjøttdeig, plastposer og shopping. Slik fikk hun familien på fem til å leve mer miljøvennlig. [Newspaper]. Aftenposten.
• Nisancioglu, Kerim Hestnes (2020). Klimaforskere: – Fra nå vil norske utslipp bare synke. [Internet]. 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.

• 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.

• 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.

• 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.
• Nisancioglu, Kerim Hestnes (2020). SDG213 Climate Action - Causes and Consequences of Climate Change .
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2020). 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?

• Nisancioglu, Kerim Hestnes (2020). Forskere deler sine klimabekymringer – Når isen beveger seg, er den ustoppelig. [Internet]. 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.

• Conrad, Clinton Phillips; Selway, Kate; Weerdesteijn, Maaike Francine Maria; 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.
• Kajanto, Karita Anneli; Seroussi, Hélène; de Fleurian, Basile & Nisancioglu, Kerim Hestnes (2019). Holocene retreat of Jakobshavn isbrae – matching model results to data.
• Morée, Anne; Sun, Tianyi; Bretones, Anaïs; Straume, Eivind Olavson; Nisancioglu, Kerim Hestnes & Gebbie, Geoffrey (2019). Cancellation of the precessional cycle in δ18O records during the Early Pleistocene. Show summary

  Deep-sea δ18O records show a pronounced difference in Milankovitch periodicity between the Early and Late Pleistocene. δ18O is interpreted as a proxy for ice sheet volume and temperature, which led to the conclusion that glacial-interglacial cycles considerably changed their rhythm during the mid-Pleistocene. This transition is referred to as the mid-Pleistocene Transition (MPT). Specifically, the precessional component of the Milankovitch cycles is absent in Early Pleistocene δ18O records, despite its continuous presence in solar insolation forcing to the ice sheets. Climate feedbacks involving (sea) ice, geological processes and carbon and nutrient cycling have been proposed as causes of this marked change. We however show that the absence of an Early Pleistocene precession signal in deep-sea δ18O records could be the result of destructive interference of the precessional cycle in the interior ocean. Such cancellation is caused by the anti-phasing of the precessional cycle between the North Atlantic and Southern Ocean deep-water sources (see Figure). We explore the potential for cancellation in the transient setup of the Total Matrix Intercomparison model for a wide range of source signal strengths. Our results show that cancellation can cause the absence of the precessional signal due to cancellation in the interior South-Atlantic, Indian and Pacific basins. Cancellation is especially widespread for a relative end-member contribution typical for the Early Pleistocene. We therefore conclude that the precessional component is likely incompletely archived in Early Pleistocene δ18O records, and appears as an actual change in Milankovitch periodicity across the MPT. Proxies not susceptible to cancellation of precession (such as those currently retrieved across the MPT from Antarctica) would be able to verify to what extent deep-sea δ18O correctly represents Pleistocene climate.

• Straume, Eivind Olavson; Morée, Anne; Nisancioglu, Kerim Hestnes & Gebbie, Geoffrey (2019). Orbital Forcing of Climate and the Mid-Pleistocene Transition . Show summary

  Climate archives, based on records of oxygen isotopes from marine sediments, show a pronounced change in the periodicity of the glacial cycles during the mid-Pleistocene (~1 million years ago). Before this time, and possibly from the initiation of major glaciations, the periodicity is dominated by the 41 kyr obliquity cycle of orbital insolation. However, in the past million years the ~21 and ~100 kyr cycles of precession and eccentricity have dominated the records of glacial cycles, and the records of atmospheric greenhouse gasses. This climate transition, the mid-Pleistocene Transition (MPT), is not fully understood. Nor are the dynamics of the glacial cycles. In this session we welcome contributions with perspectives on the MPT as well as the dynamics of the glacial cycles based on proxy records, theory and models. The topic is also timely in light of ongoing efforts to extract a continuous Antarctic ice core record spanning the MPT.

• 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). Topological constraints by the Greenland-Scotland Ridge on AMOC and climate.
• 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.
• Nisancioglu, Kerim Hestnes (2019). Greenland and Nordic Seas/Arctic Variability.
• 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.

• 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.

• 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 .
• 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.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2019). Lyden av is. [TV]. NRK.
• Jensen, Mari Fjalstad; Nisancioglu, Kerim Hestnes & Spall, Michael A. (2019). Large changes in sea ice triggered by small changes in Atlantic Water temperature.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2019). Abrupt changes in sea ice.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2019). Unforced Oscillations in Sea Ice.
• 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). From Evidence to Action: Moving Knowledge to Policy & Practice.
• Nisancioglu, Kerim Hestnes (2019). The Dynamics of DO-events.
• 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 (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). ERC Synergy Project ice2ice.
• 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.

• 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.

• 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.

• 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.

• 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).

• 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.

• 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.

• 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 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 (2019). Havet stiger - flere nasjoner er i ferd med å forsvinne. [Internet]. 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). The Dynamics of DO-events.
• Nisancioglu, Kerim Hestnes (2019). Vi burde trekke oss tilbake fra kysten. [Newspaper]. 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 (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.

• 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.
• Nisancioglu, Kerim Hestnes (2019). Reisebyrå: Grønland-interessen tar av. [Internet]. 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). Unge forskere strømmer til "klimalaboratoriet" Svalbard. Men engasjementet gjenspeiles ikke blant beslutningstakerne. [Internet]. 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). Farlig svette: Hetebølge gir smelterekord på Grønland. [Newspaper]. 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). ACDC summer school – 10 year anniversary.
• Sessford, Evangeline; Jensen, Mari Fjalstad; Tisserand, Amandine Aline; Muschitiello, Francesco; Dokken, Trond Martin & Nisancioglu, Kerim Hestnes [Show all 7 contributors for this article] (2019). Intermediate water in the Nordic Seas during Greenland Stadials.
• Nisancioglu, Kerim Hestnes; Guo, Chuncheng & Jensen, Mari Fjalstad (2019). The Dynamics of DO-events.
• 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 (2018). Frykter det er for seint å unngå skrekkscenarioet: - Det er en dødsdom. [Newspaper]. 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). Toppen av innlandsisen. [Newspaper]. 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). Forsker slår alarm: – Havet kan stige over 15 meter de neste 500 årene. [Internet]. NRK.
• Jensen, Mari Fjalstad & Nisancioglu, Kerim Hestnes (2018). Abrupt changes in sea ice.
• Bjartnes, Anders; Michelsen, Lars-Henrik Paarup; Skaugen, Håvar; Ursin, Lars; Darelius, Elin Maria K. & Kutti, Tina [Show all 12 contributors for this article] (2021). Vippepunkter i klimasystemet. Norsk klimastiftelse.
• Smith-Johnsen, Silje & Nisancioglu, Kerim Hestnes (2019). Dynamics of the Northeast Greenland Ice Stream: the role of geothermal heat and subglacial hydrology. Universitetet i Bergen. ISSN 9788230863008.
• Loose, Nora & Nisancioglu, Kerim Hestnes (2019). Adjoint Modeling and Observing System Design in the Subpolar North Atlantic. Universitetet i Bergen. ISSN 9788230841334.
• Plach, Andreas; Nisancioglu, Kerim Hestnes; Born, Andreas & Vinther, Bo Møllesøe (2019). Simulation of the Eemian Greenland ice sheet. Universitetet i Bergen. ISSN 978-82-308-3543-2.

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