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Abstract “Online” data assimilation (DA) is used to generate a seasonal-resolution reanalysis dataset over the last millennium by combining forecasts from an ocean–atmosphere–sea-ice coupled linear inverse model with climate proxy records. Instrumental verification reveals that this reconstruction achieves the highest correlation skill, while using fewer proxies, in surface temperature reconstructions compared to other paleo-DA products, particularly during boreal winter when proxy data are scarce. Reconstructed ocean and sea-ice variables also have high correlation with instrumental and satellite datasets. Verification against independent proxy records shows that reconstruction skill is robust throughout the last millennium. Analysis of the results reveals that the method effectively captures the seasonal evolution and amplitude of El Niño events, seasonal temperature trends that are consistent with orbital forcing over the last millennium, and polar-amplified cooling in the transition from the Medieval Climate Anomaly to the Little Ice Age. 

The Camp Century ice core, collected in 1966 from northwestern Greenland, recovered 3.44 meters of sub-glacial sediment from below nearly 1400 meters of ice. After drilling, the sediment was cursorily investigated and then misplaced for decades. This data set provides water isotope data from pore waters in the sediment, which reflects climate conditions when that sediment was exposed at the surface, about 400,000 years ago. 

Abstract The fate of the West Antarctic Ice Sheet (WAIS)¹is the largest cause of uncertainty in long-term sea-level projections. In the last interglacial (LIG) around 125,000 years ago, data suggest that sea level was several metres higher than today^2–4, and required a significant contribution from Antarctic ice loss, with WAIS usually implicated. Antarctica and the Southern Ocean were warmer than today^5–8, by amounts comparable to those expected by 2100 under moderate to high future warming scenarios. However, direct evidence about the size of WAIS in the LIG is sparse. Here we use sea salt data from an ice core from Skytrain Ice Rise, adjacent to WAIS, to show that, during most of the LIG, the Ronne Ice Shelf was still in place, and close to its current extent. Water isotope data are consistent with a retreat of WAIS⁹, but seem inconsistent with more dramatic model realizations¹⁰in which both WAIS and the large Antarctic ice shelves were lost. This new constraint calls for a reappraisal of other elements of the LIG sea-level budget. It also weakens the observational basis that motivated model simulations projecting the highest end of projections for future rates of sea-level rise to 2300 and beyond. 

The persistence and size of the Greenland Ice Sheet (GrIS) through the Pleistocene is uncertain. This is important because reconstructing changes in the GrIS determines its contribution to sea level rise during prior warm climate periods and informs future projections. To understand better the history of Greenland’s ice, we analyzed glacial till collected in 1993 from below 3 km of ice at Summit, Greenland. The till contains plant fragments, wood, insect parts, fungi, and cosmogenic nuclides showing that the bed of the GrIS at Summit is a long-lived, stable land surface preserving a record of deposition, exposure, and interglacial ecosystems. Knowing that central Greenland was tundra-covered during the Pleistocene informs the understanding of Arctic biosphere response to deglaciation. 

Abstract West Antarctica has experienced dramatic ice losses contributing to global sea-level rise in recent decades, particularly from Pine Island and Thwaites glaciers. Although these ice losses manifest an ongoing Marine Ice Sheet Instability, projections of their future rate are confounded by limited observations along West Antarctica’s coastal perimeter with respect to how the pace of retreat can be modulated by variations in climate forcing. Here, we derive a comprehensive, 12-year record of glacier retreat around West Antarctica’s Pacific-facing margin and compare this dataset to contemporaneous estimates of ice flow, mass loss, the state of the Southern Ocean and the atmosphere. Between 2003 and 2015, rates of glacier retreat and acceleration were extensive along the Bellingshausen Sea coastline, but slowed along the Amundsen Sea. We attribute this to an interdecadal suppression of westerly winds in the Amundsen Sea, which reduced warm water inflow to the Amundsen Sea Embayment. Our results provide direct observations that the pace, magnitude and extent of ice destabilization around West Antarctica vary by location, with the Amundsen Sea response most sensitive to interdecadal atmosphere-ocean variability. Thus, model projections accounting for regionally resolved ice-ocean-atmosphere interactions will be important for predicting accurately the short-term evolution of the Antarctic Ice Sheet. 

We examine results from two transient modeling experiments that simulate the Last Interglacial period (LIG) using the state-of-the-art Community Earth System Model (CESM2), with a focus on climate and ocean changes relevant to the possible collapse of the Antarctic ice sheet. The experiments simulate the early millennia of the LIG warm period using orbital forcing, greenhouse gas concentrations and vegetation appropriate for 127ka; in the first case (127ka) no other changes are made; in the second case (127kaFW), we include a 0.2 Sv freshwater forcing in the North Atlantic. Both are compared with a pre-industrial control simulation (piControl). In the 127ka simulation, the global average temperature is only marginally warmer (0.004 degrees C) than in the piControl. When freshwater forcing is added (127kaFW), there is surface cooling in the NH and warming in the SH, consistent with the bipolar seesaw effect. Near the Antarctic ice sheet, the 127ka simulation generates notable ocean warming (up to 0.4 degrees C) at depths below 200 m compared to the piControl. In contrast, the addition of freshwater in the North Atlantic in the 127kaFW run results in a multi-century subsurface ocean cooling that rebounds slowly over multiple millennia near the Antarctic ice sheet. These results have implications for the thermal forcing (and thereby mass balance) of the Antarctic Ice Sheet. We explore the physical processes that lead to this result and discuss implications for climate forcing of Antarctic ice sheet mass loss during the LIG. 

We examine results from two transient modeling experiments that simulate the Last Interglacial period (LIG) using the state-of-the-art Community Earth System Model (CESM2), with a focus on climate and ocean changes relevant to the possible collapse of the Antarctic ice sheet. The experiments simulate the early millennia of the LIG warm period using orbital forcing, greenhouse gas concentrations and vegetation appropriate for 127ka; in the first case (127ka) no other changes are made; in the second case (127kaFW), we include a 0.2 Sv freshwater forcing in the North Atlantic. Both are compared with a pre-industrial control simulation (piControl). In the 127ka simulation, the global average temperature is only marginally warmer (0.004 degrees C) than in the piControl. When freshwater forcing is added (127kaFW), there is surface cooling in the NH and warming in the SH, consistent with the bipolar seesaw effect. Near the Antarctic ice sheet, the 127ka simulation generates notable ocean warming (up to 0.4 degrees C) at depths below 200 m compared to the piControl. In contrast, the addition of freshwater in the North Atlantic in the 127kaFW run results in a multi-century subsurface ocean cooling that rebounds slowly over multiple millennia near the Antarctic ice sheet. These results have implications for the thermal forcing (and thereby mass balance) of the Antarctic Ice Sheet. We explore the physical processes that lead to this result and discuss implications for climate forcing of Antarctic ice sheet mass loss during the LIG. 

Abstract. We examine results from two transient modeling experiments that simulate the Last Interglacial period (LIG) using the state-of-the-art Community Earth System Model (CESM2), with a focus on climate and ocean changes relevant to the possible collapse of the Antarctic ice sheet. The experiments simulate the early millennia of the LIG warm period using orbital forcing, greenhouse gas concentrations, and vegetation appropriate for 127 ka. In the first case (127ka), no other changes are made; in the second case (127kaFW), we include a 0.2 Sv freshwater forcing in the North Atlantic. Both are compared with a pre-industrial control simulation (piControl). In the 127ka simulation, the global average temperature is only marginally warmer (0.004 °C) than in the piControl. When freshwater forcing is added (127kaFW), there is surface cooling in the Northern Hemisphere (NH) and warming in the Southern Hemisphere (SH), consistent with the bipolar seesaw effect. Near the Antarctic ice sheet, the 127ka simulation generates notable ocean warming (up to 0.4 °C) at depths below 200 m compared to the piControl. In contrast, the addition of freshwater in the North Atlantic in the 127kaFW run results in a multi-century subsurface ocean cooling that rebounds slowly over multiple millennia near the Antarctic ice sheet. These results have implications for the thermal forcing (and thereby mass balance) of the Antarctic ice sheet. We explore the physical processes that lead to this result and discuss implications for climate forcing of Antarctic ice sheet mass loss during the LIG. 

{"Abstract":["This archive includes data and ipython notebooks to create the figures for the manuscript "Response of water isotopes in precipitation to a collapse of the West Antarctic Ice Sheet in high-resolution simulations with the Weather Research and Forecasting Model" submitted to Journal of Climate in August 2022.<\/p>\n\nModel output from WRFwiso and iCAM is in data.zip (saved as monthly means)<\/p>\n\nNotebooks and python modules are in scripts.zip<\/p>\n\nRequired python packages (all included in environment.yml):<\/p>\n\nnumpy<\/li>matplotlib<\/li>netcdf4<\/li>basemap<\/li>scipy<\/li>wrf-python<\/li>windspharm<\/li>metpy<\/li>intergrid<\/li>cmocean<\/li><\/ul>"]}