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Anthropogenic climate change will drive extensive mass loss across both the Antarctic (AIS) and Greenland Ice Sheets (GrIS), with the potential for global climate system feedbacks, especially in polar regions. Historically, the high‐latitude North Atlantic and Southern Ocean have been critical regions for anthropogenic heat and carbon uptake, but our understanding of how this uptake will be altered by future freshwater discharge is incomplete. We assess each ice sheet's impact on global ocean anthropogenic heat and carbon storage for a high‐emission scenario over the 21st ${21}^{\text{st}}$‐century using a coupled Earth system model. We explore the impact of contemporaneous mass loss from both ice sheets on anthropogenic heat and carbon storage and quantify their linear and nonlinear contributions. Notably, added freshwater reduces ocean heat and carbon storage by 2,100, and the sum of individual freshwater effects differ from those induced by simultaneous freshwater discharge from both ice sheets. Combined AIS and GrIS freshwater engenders distinct anthropogenic storage anomalies—particularly in the high‐latitude Southern Ocean and North Atlantic. From 2080 to 2100, GrIS freshwater exerts primary control on the temporal evolution of global ocean heat storage, while global ocean carbon storage is modulated by the linear AIS and GrIS freshwater impacts. Nonlinear impacts of simultaneous ice sheet discharge have a non‐negligible contribution to the evolution of global ocean heat storage. Further, anthropogenic heat changes are realized more quickly in response to ice sheet discharge than anthropogenic carbon. Our results highlight the need to incorporate both ice sheets actively in climate models to accurately project future global climate. Plain Language Summary: As the globe continues to warm in the next 100 years, the Antarctic and Greenland Ice sheets will continue to melt, adding freshwater to the surrounding ocean regions. This process is often poorly (if at all) represented in global climate models used to make projections about future climate change. Here, we simulate the climate response to melting ice sheets in a global climate model by adding freshwater to the model ocean near the edges of ice sheets. We focus our analysis on the impact of this freshwater addition on the future evolution of heat and carbon in the ocean, because both heat and carbon have the potential to feed back on the climate system (less heat/carbon in the ocean means more heat/carbon in the atmosphere and a warmer climate). By the end of the century, we find that the ocean stores less heat and carbon because of the melting ice sheets. We also find that summing the effects from melt on Antarctica and Greenland separately is not equal to the effect of melting both ice sheets simultaneously. Finally, we show that ocean heat and carbon respond differently to the same amount of ice sheet melt. Key Points: We disentangle the linear and nonlinear effects of Greenland and Antarctic Ice Sheet melt on ocean heat content and anthropogenic carbonFuture anthropogenic carbon storage and ocean heat content have disparate responses to separate and combined ice sheet melt in polar regionsGreenland freshwater is more influential than Antarctic freshwater in driving future changes in anthropogenic carbon and ocean heat content [ABSTRACT FROM AUTHOR]

Earth system models are essential tools for understanding the impacts of a warming world, particularly on the contribution of polar ice sheets to sea level change. However, current models lack full coupling of the ice sheets to the ocean and are typically run at a coarse resolution (1 ∘ grid spacing or coarser). Coarse spatial resolution is particularly a problem over Antarctica, where sub-grid-scale orography is well-known to influence precipitation fields, and glacier models require high-resolution atmospheric inputs. This resolution limitation has been partially addressed by regional climate models (RCMs), which must be forced at their lateral and ocean surface boundaries by (usually coarser) global atmospheric datasets, However, RCMs fail to capture the two-way coupling between the regional domain and the global climate system. Conversely, running high-spatial-resolution models globally is computationally expensive and can produce vast amounts of data. Alternatively, variable-resolution grids can retain the benefits of high resolution over a specified domain without the computational costs of running at a high resolution globally. Here we evaluate a historical simulation of the Community Earth System Model version 2 (CESM2) implementing the spectral element (SE) numerical dynamical core (VR-CESM2) with an enhanced-horizontal-resolution (0.25 ∘) grid over the Antarctic Ice Sheet and the surrounding Southern Ocean; the rest of the global domain is on the standard 1 ∘ grid. We compare it to 1 ∘ model runs of CESM2 using both the SE dynamical core and the standard finite-volume (FV) dynamical core, both with identical physics and forcing, including prescribed sea surface temperatures (SSTs) and sea ice concentrations from observations. Our evaluation reveals both improvements and degradations in VR-CESM2 performance relative to the 1 ∘ CESM2. Surface mass balance estimates are slightly higher but within 1 standard deviation of the ensemble mean, except for over the Antarctic Peninsula, which is impacted by better-resolved surface topography. Temperature and wind estimates are improved over both the near surface and aloft, although the overall correction of a cold bias (within the 1 ∘ CESM2 runs) has resulted in temperatures which are too high over the interior of the ice sheet. The major degradations include the enhancement of surface melt as well as excessive cloud liquid water over the ocean, with resultant impacts on the surface radiation budget. Despite these changes, VR-CESM2 is a valuable tool for the analysis of precipitation and surface mass balance and thus constraining estimates of sea level rise associated with the Antarctic Ice Sheet. [ABSTRACT FROM AUTHOR]

Owing to drifting snow processes, snow accumulation and surface density in polar environments are variable in space and time. We present new field data of manual measurements, repeat terrestrial laser scanning and snow micro-penetrometry from Dronning Maud Land, Antarctica, showing the density of new snow accumulations. We combine these data with published drifting snow mass flux observations, to evaluate the performance of the 1-D, detailed, physics-based snow cover model SNOWPACK in representing drifting snow and surface density. For two sites in East Antarctica with multiple years of data, we found a coefficient of determination for the simulated drifting snow of r 2 = 0.42 and r 2 = 0.50, respectively. The field observations show the existence of low-density snow accumulations during low wind conditions. Successive high wind speed events generally erode these low-density layers while producing spatially variable erosion/deposition patterns with typical length scales of a few metres. We found that a model setup that is able to represent low-density snow accumulating during low wind speed conditions, as well as subsequent snow erosion and redeposition at higher densities during drifting snow events was mostly able to describe the observed temporal variability of surface density in the field. [ABSTRACT FROM AUTHOR]

Ice sheet surface mass balance describes the net snow accumulation at the ice sheet surface. On the Antarctic ice sheet, winds redistribute snow, resulting in a surface mass balance that is variable in both space and time. Representing wind-driven snow redistribution processes in models is critical for local assessments of surface mass balance, repeat altimetry studies, and interpretation of ice core accumulation records. To this end, we have adapted Alpine3D, an existing distributed snow modeling framework, to downscale Antarctic surface mass balance to horizontal resolutions up to 1 km. In particular, we have introduced a new two-dimensional advection-based wind-driven snow redistribution module that is driven by an offline coupling between WindNinja, a wind downscaling model, and Alpine3D. We then show that large accumulation variability can be at least partially explained by terrain-induced wind speed variations which subsequently redistribute snow around rolling topography. By comparing Alpine3D to airborne-derived snow accumulation measurements within a testing domain over Pine Island Glacier in West Antarctica, we demonstrate that our Alpine3D downscaling approach improves surface mass balance estimates when compared to the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), a global atmospheric reanalysis which we use as atmospheric forcing. In particular, when compared to MERRA-2, Alpine3D reduces simulated surface mass balance root mean squared error by 23.4 mmw.e.yr-1 (13 %) and increases variance explained by 24 %. Despite these improvements, our results demonstrate that considerable uncertainty stems from the employed saltation model, confounding simulations of surface mass balance variability. [ABSTRACT FROM AUTHOR]

The Greenland Ice Sheet's (GrIS) firn layer buffers the ice sheet's contribution to sea level rise by storing meltwater in its pore space. However, available pore space and meltwater retention capability is lost due to ablation of the firn layer and refreezing of meltwater as near-surface ice slabs in the firn. Understanding how firn properties respond to climate is important for constraining the GrIS's future contribution to sea level rise in a warming climate. Observations of firn density provide detailed information about firn properties, but they are spatially and temporally limited. Here we use two firn models, the physics-based SNOWPACK model and the Community Firn Model configured with a semi-empirical densification equation (CFM-GSFC), to quantify firn properties across the GrIS from 1980 through 2020. We use an identical forcing (Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) atmospheric reanalysis) for SNOWPACK and the CFM-GSFC in order to isolate firn model differences. To evaluate the models, we compare simulated firn properties, including firn air content (FAC), to measurements from the Surface Mass Balance and Snow on Sea Ice Working Group (SUMup) dataset of snow and firn density. Both models perform well (mean absolute percentage errors of 14 % in SNOWPACK and 16 % in the CFM-GSFC), though their performance is hindered by the spatial resolution of the atmospheric forcing. In the ice-sheet-wide simulations, the 1980–1995 average spatially integrated FAC (i.e., air volume in the firn) for the upper 100 m is 34 645 km 3 from SNOWPACK and 28 581 km 3 from the CFM-GSFC. The discrepancy in the magnitude of the modeled FAC stems from differences in densification with depth and variations in the sensitivity of the models to atmospheric forcing. In more recent years (2005–2020), both models simulate substantial depletion of pore space. During this period, the spatially integrated FAC across the entire GrIS decreases by 3.2 % (-66.6 km 3 yr -1) in SNOWPACK and 1.5 % (-17.4 km 3 yr -1) in the CFM-GSFC. These differing magnitudes demonstrate how model differences propagate throughout the FAC record. Over the full modeled record (1980–2020), SNOWPACK simulates a loss of pore space equivalent to 3 mm of sea level rise buffering, while the CFM-GSFC simulates a loss of 1 mm. The greatest depletion in FAC is along the margins and especially along the western margin where observations and models show the formation of near-surface, low-permeability ice slabs that may inhibit meltwater storage. [ABSTRACT FROM AUTHOR]

Earth System Models are essential tools for understanding the impacts of a warming world, particularly on the contribution of polar ice sheets to sea level change. However, current models lack full coupling of the ice sheets to the ocean, and are typically run at a coarse resolution (1 degree grid spacing or coarser) to save on computational expense. Coarse spatial resolution is particularly a problem over Antarctica, where sub-gridscale orography is well-known to influence precipitation fields. This resolution limitation has been partially addressed by regional climate models (RCMs), which must be forced at their lateral and ocean surface boundaries by (usually coarser) global atmospheric datasets, However, RCMs fail to capture the coupling between the regional domain and the global climate system. Conversely, running high spatial resolution models globally is computationally expensive, and can produce vast amounts of data. Alternatively, variable-resolution, nested grids are a promising way forward, as they can retain the benefits of high resolution over a specified domain without the computational costs of running at a high resolution globally. Here we evaluate a historical simulation of the Community Earth System Model, version 2, (CESM2) implementing the spectral element (SE) numerical dynamical core with an enhanced-horizontal-resolution (0.25°) grid over the Antarctic Ice Sheet and the surrounding Southern Ocean; the rest of the global domain is on the standard 1° grid. We compare it to a 1° model run of CESM2 using the standard finite-volume dynamical core with identical physics and forcing, including prescribed SSTs and sea ice concentrations from observations. Our evaluation indicates both improvements and degradations in VR-CESM2 performance relative to the 1° CESM2. Surface mass balance estimates are slightly higher, but within one standard deviation of the ensemble mean, except for over the Antarctic Peninsula, which is impacted strongly by better-articulated surface topography. Temperature and wind estimates are improved over both the near-surface and aloft, although the overall correction of a cold bias (within the 1° CESM2 runs) has resulted in temperatures which are too high over the interior of the ice sheet. The major degradations include the enhancement of surface melt as well as excessive liquid cloud water over the ocean, with resultant impacts on the radiation balance. Despite these changes, VR-CESM2 is a valuable tool for the analysis of future estimates of precipitation and surface mass balance, and thus constraining estimates of sea level rise. [ABSTRACT FROM AUTHOR]

The Greenland Ice Sheet's (GrIS) firn layer buffers the ice sheet's contribution to sea level rise by storing meltwater in its pore space. However, available pore space and meltwater retention capability is lost due to ablation of the firn layer and refreezing of meltwater as near-surface ice slabs in the firn. Understanding how firn properties respond to climate is important for constraining the GrIS's future contribution to sea level rise in a warming climate. Observations of firn density provide detailed information about 5 firn properties, but they are spatially and temporally limited. Here we use two firn models, the physics-based SNOWPACK model and the semi-empirical Community Firn Model (CFM) to quantify firn properties across the GrIS from 1980 through 2020. We use an identical forcing (MERRA-2 atmospheric reanalysis) for SNOWPACK and the CFM in order to isolate model differences. To evaluate the models, we compare simulated firn properties, including firn air content (FAC), to measurements from the SUMup dataset of snow and firn density. Both models perform well, though their performance is hindered by meltwater percolation and the spatial resolution of the atmospheric forcing. In the full ice-sheet simulations, the spatially-integrated FAC (i.e., air volume in the firn) for the upper 100 m is 34,645 km³ from SNOWPACK and 28,581 km³ from the CFM. The discrepancy in the magnitude of the modeled FAC stems from differences in densification with depth and variations in the models' treatment of atmospheric input. In more recent years (2005-2020), both models simulate substantial depletion of pore space. During this period, the spatially-integrated FAC across the entire GrIS decreases by 2.8 % and 1.2 % in SNOWPACK and the CFM, respectively. The differing magnitudes of the 2005-2020 spatially-integrated FAC of -66.6 km³ yr-1 in SNOWPACK and -17.4 km³ yr-1 in the CFM demonstrate how model differences propagate throughout the FAC record. Over the full modeled record (1980-2020), SNOWPACK simulates a loss of pore space equivalent to 3 mm of sea level rise buffering, while the CFM simulates a loss of 1 mm. The greatest depletion in FAC is along the margins, and especially along the western margin where observations and models show the formation of near-surface, low-permeability ice slabs that inhibit meltwater storage. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) transport large amounts of moisture from the mid-to high-latitudes and they are a primary driver of the most extreme snowfall events on Antarctica. ARs also raise surface temperatures when they make landfall over Antarctica, leading to surface melting. In this study, we characterize the climatology and surface impacts of ARs on West Antarctica, focusing on the Amundsen Sea Embayment and Marie Byrd Land. First, we develop a climatology of ARs in this region, using an Antarctic-specific AR detection tool combined with MERRA-2 and ERA5 atmospheric reanalyses. We find that while ARs are infrequent, they cause intense precipitation in short periods of time and account for 11% of the annual surface accumulation. They are driven by the coupling of a blocking high over the Antarctic Peninsula with a low-pressure system known as the Amundsen Sea Low. Next, we use observations from automatic weather stations on Thwaites Eastern Ice Shelf to examine a case study of 3 ARs that made landfall in rapid succession from February 2 to 8, known as an AR family event. We use snow height observations to force the firn model SNOWPACK to reconstruct accumulation and surface melting during the event, and compare these results with accumulation higher up on the glacier derived from surface height changes using interferometric reflectometry. While accumulation dominates the surface impacts of the event on Thwaites Eastern Ice Shelf (>100 kg m-2), we find small amounts of surface melt as well (<5 kg m-2). West Antarctica currently experiences minimal surface melting, most of which is absorbed by the firn, but future atmospheric warming could lead to more widespread surface melting in West Antarctica. Combined with a future increase in AR intensity or frequency, this could limit the ability of the firn layer to absorb melt water, which could harm ice shelf stability, and ultimately accelerate mass loss of the West Antarctic Ice Sheet. The results presented here enable us to quantify the past impacts of ARs on West Antarctic surface mass balance and characterize their interannual variability and trends, enabling a better assessment of future AR-driven changes in the surface mass balance. [ABSTRACT FROM AUTHOR]