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The future trajectory of the Atlantic Meridional Overturning Circulation (AMOC) and its past and future variability on timescales of centuries to millennia remain a subject of debate. For instance, the mechanism setting up the oscillatory behaviour of the climate system during Marine Isotope Stage 3 (MIS3; about 59.4 - 27.8 thousand of years before present), a period characterized by pronounced Dansgaard-Oeschger variability, has not been disentangled so far.In order to shed more light on millennial-scale variability from a modelling perspective, we have conducted a set of experiments with the Comprehensive Earth System Model v1.2 (CESM1.2). These simulations have been specifically designed with the aim to explore the parameter space of the AMOC with respect to greenhouse gas concentrations, continental ice-sheet topography and orbital parameters representative for the Last Glacial Maximum (LGM) and Marine Isotope Stage 3. Our experiments reveal an increase in mean AMOC strength in response to higher greenhouse-gas concentrations. Furthermore, the relative role of the various boundary conditions in shaping a possible oscillatory behaviour of the AMOC is investigated. Under full LGM conditions, unforced multi-centennial AMOC variations on the order of about 5 Sv are simulated. With the intermediate MIS3 ice-sheet topography, however, the model exhibits well-pronounced millennial-scale AMOC fluctuations of more than 12 Sv at a recurrence time of ~1500 years and reproduces the range of Dansgaard-Oeschger-type surface-temperature fluctuations of more than 10°C recorded in Greenland ice cores. The high temporal covariation between Greenland temperature and AMOC strength in our model setup further mimics MIS3 stadial-interstadial transitions., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Marine Isotope Stage 11c (MIS-11c) interglacial is an enigmatic period characterized by a long duration of relatively weak insolation forcing, but is thought to have been coincident with a large global sea level rise of 6–13 m. The configuration of the Greenland Ice Sheet during the MIS-11c interglacial highstand is therefore of great interest. Given the limited data constraints, model-based analysis may be of use, but only if model uncertainties are adequately accounted for. A particularly under-addressed issue in coupled climate and ice sheet modeling is the coupling of surface air temperatures to the ice model. Many studies apply a uniform “lapse rate” accounting for the temperature differences at different altitudes over the ice surface, but this uniformity neglects both regional and seasonal differences in near-surface temperature changes. Herein we provide the first such analysis for MIS-11c Greenland that addresses these uncertainties by comparing 1-way coupled CESM and ice sheet model results from several different downscaling methodologies. In our study, a spatially- and temporally-varying temperature downscaling method produced the greatest success rate in matching limited paleodata constraints, and suggests a peak ice volume loss from Greenland during MIS-11c of near 50 % compared to present day (~3.9 m contribution to sea level rise). This result is on the lower bound of existing data- and model-based studies, partly as a consequence of the applied one-way coupling methodology which neglects some feedbacks. Additional uncertainties are examined by comparing two different present-day regional climate analyses for bias correction of temperatures and precipitation, a spread of initialization states and times, and different spatial configurations of precipitation bias corrections. No other factor exhibited greater influence over the simulated Greenland ice sheet than the choice of temperature downscaling scheme.

Funding: This research has been supported by the Swedish Research Council (grant no. 2016-04422), the German Research Foundation (grant no. 1158-365737614), the US National Science Foundation (grant no. PLR-1542930), and the Norwegian Polar Institute/NARE (grant no. 2015/38/7/NK/ihs). Jorge Bernales has been supported by the MAGIC-DML project through DFG SPP 1158 (RO 4262/1-6). The article processing charges for this open-access publication were covered by Stockholm University. Studying the response of the Antarctic ice sheets during periods when climate conditions were similar to the present can provide important insights into current observed changes and help identify natural drivers of ice sheet retreat. In this context, the marine isotope substage 11c (MIS11c) interglacial offers a suitable scenario, given that during its later portion orbital parameters were close to our current interglacial. Ice core data indicate that warmer-than-present temperatures lasted for longer than during other interglacials. However, the response of the Antarctic ice sheets and their contribution to sea level rise remain unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three glaciological and one sedimentary proxy records of ice volume. Our results indicate that the East and West Antarctic ice sheets contributed 4.0–8.2 m to the MIS11c sea level rise. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea level reconstructions, the range is reduced to 6.7–8.2 m independently of the choices of external sea level forcing and millennial-scale climate variability. Within this latter range, the main source of uncertainty arises from the sensitivity of the East Antarctic Ice Sheet to a choice of initial ice sheet configuration. We found that the warmer regional climate signal captured by Antarctic ice cores during peak MIS11c is crucial to reproduce the contribution expected from Antarctica during the recorded global sea level highstand. This climate signal translates to a modest threshold of 0.4 °C oceanic warming at intermediate depths, which leads to a collapse of the West Antarctic Ice Sheet if sustained for at least 4000 years. Publisher PDF