ALBERT

Description: Observations suggest that during the last decades the Greenland Ice Sheet (GrIS) has experienced a gradually accelerating mass loss, in part due to the observed acceleration of several of Greenland’s marine-terminating glaciers. Recent studies directly attribute this to increasing North Atlantic temperatures, which have triggered melting of the GrIS outlet glaciers, grounding-line retreat and enhanced ice discharge into the ocean, contributing to an acceleration of sea level rise. Reconstructions suggest that the influence of the ocean has been of primary importance in the past as well. This was the case not only in interglacial periods, when warmer climates led to a rapid retreat of the GrIS to land above sea level, but also in glacial periods, when the GrIS expanded as far as the continental shelf break, and was thus more directly exposed to ocean changes. However, the GrIS response to paleo oceanic variations has not been investigated from a modelling perspective so far. In this work the evolution of the GrIS over the past two glacial cycles has been studied using a three-dimensional hybrid ice-sheet/ice-shelf model. We assess the effect of the variation of oceanic temperatures on the GrIS evolution on glacial-interglacial timescales through changes in submarine melting. The results show a very high sensitivity of the GrIS to the changing oceanic conditions. Oceanic forcing is found to be the dominant driver of the GrIS expansion in glacial times and retreat in interglacial periods. If switched off, paleo atmospheric variations alone are not able to yield a reliable glacial configuration of the GrIS. This work therefore suggests that considering the ocean as an active forcing should become standard in paleo ice sheet modelling.

Description: The last glacial period (LGP; ca. 110–10 ka BP) was marked by the existence of two types of abrupt climatic changes, Dansgaard-Oeschger (D/O) and Heinrich (H) events. Although the mechanisms behind these are not fully understood, it is generally accepted that the presence of ice sheets played an important role in their occurrence. While an important effort has been made to investigate the dynamics and evolution of the Laurentide Ice Sheet (LIS) during this period, the Eurasian Ice Sheet (EIS) has not received much attention, in particular from a modeling perspective. However, meltwater discharge from this and other ice sheets surrounding the Nordic Seas is often implied as a potential cause of ocean instabilities that lead to glacial abrupt climate changes. Thus, a better understanding of its variations during the LGP is important to understand its role in glacial abrupt climate changes. Here we investigate the response of the EIS to millennial-scale climate variability during the LGP. We use a hybrid, three-dimensional, thermomechanical ice-sheet model that includes ice shelves and ice streams. The model is forced offline through a novel perturbative approach that includes the effect of both atmospheric and oceanic variations and provides a more realistic treatment of millennial-scale climatic variability than conventional methods. Our results show that the EIS responds with enhanced iceberg discharges in phase with interstadial warming in the North Atlantic. Separating the atmospheric and oceanic effects demonstrates the major role of the ocean in controlling the dynamics of the EIS on millennial time scales. While the atmospheric forcing alone is only able to produce modest iceberg discharges, warming of oceanic surface waters leads to much higher rates of iceberg discharges as a result of relatively strong basal melting within the margins of the ice sheet. Together with previous work, our results provide a consistent explanation for the timing of the responses of the LIS and the EIS to glacial abrupt climate changes.

Description: Imposing freshwater flux (FWF) variations in the North Atlantic is an effective method to cause reorganizations of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. Through this approach, models have been able to reproduce the abrupt climate changes of the last glacial period. Such exercises have been useful for gaining insight into a wealth of processes regarding the widespread climatic consequences of AMOC variations. However, an issue that has passed unnoticed is the fact that the timing of the FWF applied in these studies is inconsistent with reconstructions. Here we focus on the deglaciation to show that imposing a FWF that is derived from the sea-level record results in a simulated AMOC evolution in a poor fit with the data, revealing an inconsistency between the generally accepted FWF mechanism and the resulting climatic impacts. Based on these negative results, we propose that the trigger of deglacial abrupt climate changes is not yet fully identified and that mechanisms other than FWF forcing should be explored more than ever.

Description: The aim of this study is to assess and improve the methods currently used to force ice sheet models offline. To this end, three different synthetic transient forcing climatologies are developed for the past 120 kyr following a perturbative approach and applied to an ice-sheet model. The results are used to evaluate their consequences for simulating the paleo evolution of the Northern Hemisphere ice sheets. The first method follows the usual approach in which temperature anomalies relative to present are calculated by combining a present-day climatology with a simulated glacial-interglacial climatic anomaly field interpolated through an index derived from ice-core data. In the second approach the representation of millennial-scale climate variability is improved by incorporating a simulated stadial-interstadial anomaly field. The third is a refinement of the second one in which the amplitudes of both orbital and millennial-scale variations are corrected to provide a perfect agreement with a recent absolute temperature reconstruction over Greenland. The comparison of the three climate forcing methods highlights the tendency of the usual approach to overestimate the temperature variability over North America and Eurasia at millennial timescales. This leads to a relatively high Northern Hemisphere (NH) ice-volume variability on these timescales. Through enhanced ablation, this results in too low an ice volume throughout the last glacial period (LGP), below or at the lower end of the uncertainty range of estimations. Improving the representation of millennial-scale variability alone yields an important increase of ice volume in all NH ice sheets, but especially in the Fennoscandian ice sheet (FIS). Optimizing the amplitude of the temperature anomalies to match the Greenland reconstruction results in a further increase of the simulated ice-sheet volume throughout the LGP. Our new method provides a more realistic representation of orbital and millennial scale climate variability and represents an improvement in the transient forcing of ice sheets during the last glacial period. Interestingly, our new approach underestimates ice-volume variations on millennial timescales as indicated by sea-level records. This suggests that either the origin of the latter is not the NH or that processes not represented in our study, notably variations in oceanic conditions, need to be invoked to account for an important role of millennial-scale climate variability on millennial-scale ice- volume fluctuations. We finally provide here both our derived climate evolution of the LGP using the three methods as well as the resulting ice-sheet configurations. These could be of interest for future studies dealing with the atmospheric or/and oceanic consequences of transient ice-sheet evolution throughout the LGP, and as a source of climate input to other ice sheet models.

Description: The Northeast Greenland Ice Stream (NEGIS) area has been suffering a significant ice mass loss during the last decades. This is partly due to increasing oceanic temperatures in the subpolar North Atlantic, which enhance submarine basal melting and mass discharge. This demonstrates the high sensitivity of this region to oceanic changes. Alongside, a recent study suggests that the NEGIS grounding line was 20–40 km behind its present-day location for 15 ka during Marine Isotopic Stage (MIS) 3, raising an important conundrum. This retreat has been attributed to a combination of atmospheric and external forcings but a modelling approach to the problem is pending. Here we investigate the sensitivity of the NEGIS to the oceanic forcing during the Last Glacial Period (LGP) using a three-dimensional hybrid ice-sheet-shelf model. We find that a sufficiently high oceanic forcing could account for a NEGIS ice-margin retreat of several tens of km, potentially explaining the recently proposed NEGIS grounding-line retreat during MIS-3.

Description: The Antarctic Ice Sheet (AIS) is the largest ice sheet on Earth and hence a major potential contributor to future global sea-level rise. A wealth of studies suggest that increasing oceanic temperatures could cause a collapse of its marine-based western sector, the West Antarctic Ice Sheet, through the mechanism of marine ice-sheet instability, leading to a sea-level increase of 3–5 m. Thus, it is crucial to constrain the sensitivity of the AIS to rapid climate changes. The last glacial period is an ideal benchmark period for this purpose as it was punctuated by abrupt Dansgaard–Oeschger events at millennial timescales. Because their center of action was in the North Atlantic, where their climate impacts were largest, modeling studies have mainly focused on the millennial-scale evolution of Northern Hemisphere (NH) paleo ice sheets. Sea-level reconstructions attribute the origin of millennial-scale sea-level variations mainly to NH paleo ice sheets, with a minor but not negligible role of the AIS. Here we investigate the AIS response to millennial-scale climate variability for the first time. To this end we use a three-dimensional, thermomechanical hybrid, ice sheet–shelf model. Different oceanic sensitivities are tested and the sea-level equivalent (SLE) contributions computed. We find that whereas atmospheric variability has no appreciable effect on the AIS, changes in submarine melting rates can have a strong impact on it. We show that in contrast to the widespread assumption that the AIS is a slow reactive and static ice sheet that responds at orbital timescales only, it can lead to ice discharges of around 6 m SLE, involving substantial grounding line migrations at millennial timescales.

Description: Temperature reconstructions from Greenland ice-sheet (GrIS) ice cores indicate the occurrence of more than 20 abrupt warmings during the last glacial period (LGP) known as Dansgaard-Oeschger (D-O) events. Although their ultimate cause is still debated, evidence from both proxy data and modelling studies robustly links these to reorganisations of the Atlantic Meridional Overturning Circulation (AMOC). During the LGP, the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes than in the present. Therefore oceanic temperature fluctuations on millennial timescales could have had a non-negligible impact on the GrIS. Here we assess the effect of millennial-scale oceanic variability on the GrIS evolution from the last interglacial to the present day. To do so, we use a three-dimensional hybrid ice-sheet–shelf model forced by subsurface oceanic temperature fluctuations, assumed to increase during D-O stadials and decrease during D-O interstadials. Since in our model the atmospheric forcing follows orbital variations only, the increase in total melting at millennial timescales is a direct result of an increase in basal melting. We show that the GrIS evolution during the LGP could have been strongly influenced by oceanic changes on millennial timescales, leading to oceanically induced ice-volume contributions above 1 m sea level equivalent (SLE). Also, our results suggest that the increased flux of GrIS icebergs as inferred from North Atlantic proxy records could have been triggered, or intensified, by peaks in melting at the base of the ice shelves resulting from increasing subsurface oceanic temperatures during D-O stadials. Several regions across the GrIS could thus have been responsible for ice mass discharge during D-O events, opening the possibility of a non-negligible role of the GrIS in oceanic reorganisations throughout the LGP.

Description: The Antarctic Ice Sheet (AIS) is the largest ice sheet on Earth and hence a major potential contributor to future global sea-level rise. A wealth of studies suggest that increasing oceanic temperatures could cause a collapse of its marine-based western sector, the West Antarctic Ice Sheet, through the mechanism of marine ice-sheet instability, leading to a sea-level increase of 3–5m. Thus, it is crucial to constrain the sensitivity of the AIS to rapid climate changes. The Last Glacial Period is an ideal benchmark period for this purpose as it was punctuated by abrupt Dansgaard-Oeschger events at millennial timescales. Because their centre of action was in the North Atlantic, where their climate impacts were largest, modelling studies have mainly focused on the millennial-scale evolution of Northern Hemisphere (NH) paleo ice sheets. Sea-level reconstructions attribute the origin of millennial-scale sea-level variations mainly to NH paleo ice sheets, with a minor but not negligible role to the AIS. Here we investigate the AIS response to millennial-scale climate variability for the first time. To this end we use a three-dimensional, thermomechanical hybrid, ice-sheet-shelf model. Different oceanic sensitivities are tested and the sea-level equivalent (SLE) contributions computed. We find that whereas atmospheric variability has no appreciable effect on the AIS, changes in submarine melting rates can have a strong impact on it. We show that in contrast to the widespread assumption that the AIS is a slow reactive and static ice sheet that responds at orbital timescales only, it can lead to ice discharges of almost 15m of SLE involving substantial grounding line migrations at millennial timescales.