Description: Heinrich events are among the dominant modes of glacial climate variability. During these events, massive iceberg armadas were released by the Laurentide Ice Sheet, sailed across the Atlantic, and caused large-scale climate changes. We study these events in a fully coupled complex ice sheet–climate model with synchronous coupling between ice sheets and oceans. The ice discharges occur as internal variability of the model with a recurrence period of 5kyr, an event duration of 1–1.5kyr, and a peak discharge rate of about 50mSv, roughly consistent with reconstructions. The climate response shows a two-stage behavior, with freshwater release effects dominating the surge phase and ice-sheet elevation effects dominating in the post-surge phase. As a direct response to the freshwater discharge during the surge phase, the deepwater formation in the North Atlantic decreases and the North Atlantic deepwater cell weakens by 3.5Sv. With the reduced oceanic heat transport, the surface temperatures across the North Atlantic decrease, and the associated reduction in evaporation causes a drying in Europe. The ice discharge lowers the surface elevation in the Hudson Bay area and thus leads to increased precipitation and accelerated ice sheet regrowth in the post-surge phase. Furthermore, the jet stream widens to the north and becomes more zonal. This contributes to a weakening of the subpolar gyre, and a continued cooling over Europe even after the ice discharge. This two-stage behavior can explain previously contradicting model results and understandings of Heinrich Events.

Description: This study analyzes the response of the Atlantic meridional overturning circulation (AMOC) to different CO2 concentrations and two ice sheet configurations in simulations with the coupled climate model MPI-ESM. With preindustrial (PI) ice sheets, there are two different AMOC states within the studied CO2 range: one state with a strong and deep upper overturning cell at high CO2 concentrations and one state with a weak and shallow upper cell at low CO2 concentrations. Changes in AMOC variability with decreasing CO2 indicate two stability thresholds. The strong state is stable above the first threshold near 217 ppm, and the weak state is stable below the second threshold near 190 ppm. Between the two thresholds, both states are marginally unstable, and the AMOC oscillates between them on millennial time scales. The weak AMOC state is stable when Antarctic Bottom Water becomes dense and salty enough to replace North Atlantic Deep Water (NADW) in the deep North Atlantic and when the density gain over the North Atlantic becomes too weak to sustain continuous NADW formation. With Last Glacial Maximum (LGM) ice sheets, the density gain over the North Atlantic and the northward salt transport are enhanced with respect to the PI ice sheet case. This enables active NADW formation and a strong AMOC for the entire range of studied CO2 concentrations. The AMOC variability indicates that the simulated AMOC is far away from a stability threshold with LGM ice sheets. The nonlinear relationship among AMOC, CO2, and prescribed ice sheets provides an explanation for the large intermodel spread of AMOC states found in previous coupled LGM simulations.

Description: This study analyzes the response of the Atlantic meridional overturning circulation (AMOC) to different CO2 concentrations and two ice sheet configurations in simulations with the coupled climate model MPI-ESM. With preindustrial (PI) ice sheets, there are two different AMOC states within the studied CO2 range: one state with a strong and deep upper overturning cell at high CO2 concentrations and one state with a weak and shallow upper cell at low CO2 concentrations. Changes in AMOC variability with decreasing CO2 indicate two stability thresholds. The strong state is stable above the first threshold near 217 ppm, and the weak state is stable below the second threshold near 190 ppm. Between the two thresholds, both states are marginally unstable, and the AMOC oscillates between them on millennial time scales. The weak AMOC state is stable when Antarctic Bottom Water becomes dense and salty enough to replace North Atlantic Deep Water (NADW) in the deep North Atlantic and when the density gain over the North Atlantic becomes too weak to sustain continuous NADW formation. With Last Glacial Maximum (LGM) ice sheets, the density gain over the North Atlantic and the northward salt transport are enhanced with respect to the PI ice sheet case. This enables active NADW formation and a strong AMOC for the entire range of studied CO2 concentrations. The AMOC variability indicates that the simulated AMOC is far away from a stability threshold with LGM ice sheets. The nonlinear relationship among AMOC, CO2, and prescribed ice sheets provides an explanation for the large intermodel spread of AMOC states found in previous coupled LGM simulations.

Description: Simulations with the Max Planck Institute Earth System Model (MPI-ESM) are used to study the sensitivity of the AMOC and the deep-ocean water masses during the Last Glacial Maximum to different sets of forcings. Analysing the individual contributions of the glacial forcings reveals that the ice sheets cause an increase in the overturning strength and a deepening of the North Atlantic Deep Water (NADW) cell, while the low greenhouse gas (GHG) concentrations cause a decrease in overturning strength and a shoaling of the NADW cell. The effect of the orbital configuration is negligible. The effects of the ice sheets and the GHG reduction balance each other in the deep ocean so that no shoaling of the NADW cell is simulated in the full glacial state. Experiments in which different GHG concentrations with linearly decreasing radiative forcing are applied to a setup with glacial ice sheets and orbital configuration show that GHG concentrations below the glacial level are necessary to cause a shoaling of the NADW cell with respect to the pre-industrial state in MPI-ESM. For a pCO2 of 149 ppm, the simulated overturning state and the deep-ocean water masses are in best agreement with the glacial state inferred from proxy data. Sensitivity studies confirm that brine release and shelf convection in the Southern Ocean are key processes for the shoaling of the NADW cell. Shoaling occurs only when Southern Ocean shelf water contributes significantly to the formation of Antarctic Bottom Water.

Description: The second Data Science Symposium at AWI gathered several data science related talks from AWI, GEOMAR and HZG.

Description: Last December, Paris was the host city for the 21st Conference of the Parties (COP21) of the United Nations Framework Convention on Climate Change (UNFCCC). Representatives of 195 countries met to dispute a legally binding climate agreement – a highly complex process involving thousands of politicians, scientists and activists, that to date has taken over two decades. The director ensemble “Rimini Protokoll” re-enacted this mammoth-scale drama of diplomacy in the play “Weltklimakonferenz” (World Climate Conference) at the “Deutsches Schauspielhaus” theatre in Hamburg, Germany. Since the opening night (21st Nov. 2014), the play has been performed 16 times, reaching an audience of over 9000. All performers in the play were experts and scientists at different stages of their careers, including PhD students, journalists and professors. Each spectator took on the identity of a delegate of one of the 195 participating countries. We will present the project and the performance, thereby highlighting the role of and the interaction between the spectators and early career scientists. In a nutshell the play went as follows (https://vimeo.com/137817619); after an opening ceremony, the audience was divided up into seven groups, each of which was given advice by experts in several different briefings. These informed on country-specific challenges caused by the social and economic situation, possible future climatic changes and negotiating tactics. In addition, the delegations had bilateral meetings, enabling them to exchange views and experiences with one another. Towards the end of the play each delegation was asked to submit a national commitment to greenhouse gas reduction and a financial contribution to the Green Climate Fund. Based on these national commitments, the final plenum revealed whether or not the delegations had managed to submit reductions compatible with restricting global warming to 2°C compared to pre-industrial times. Due to their direct personal involvement in the play, each spectator was confronted with the full complexity and challenge of a climate change conference: Revealing the "culprit" - in other words the causes of climate change – is all about facts. In contrast, when developing options for action, the matter at hand is ultimately values. Which risks should the world community take on? What cost is it prepared to accept to protect the population in areas menaced by droughts or floods? These are all decisions based on political, economic, and indeed ethical issues. Based on feedback from spectators both during and after the performance, we will discuss to what extent the performance managed to convey the complexity of the question at hand and to what extent the spectators actually took on the role of their given country. The spectators were not the only ones to be put out of their comfort zone during the performance. Also the experts had to, in cooperation with the directors, think carefully on how to find the right balance between scientific integrity and a captivating dramaturgy, ultimately leading to a performance that would be informative, thought-provoking, as well as enjoyable.