Firn on the Greenland Ice Sheet (GrIS) buffers meltwater, and has a variable thickness, complicating observations of volume change to mass change. In this study, we use a firn model (IMAU-FDM v1.2G) forced by a regional climate model (RACMO2.3p2) to investigate how the GrIS firn layer thickness and pore space have evolved since 1958 in response to variability in the large-scale atmospheric circulation. On interannual timescales, the firn layer thickness and pore space show a spatially heterogeneous response to variability in the North Atlantic Oscillation (NAO). Notably, a stronger NAO following the record warm summer of 2012 led the firn layer in the south and east of the ice sheet to regain thickness and pore space after a period of thinning and reduced pore space. In the southwest, a decrease in melt dominates, whereas in the east the main driver is an increase in snow accumulation. At the same time, the firn in the northwestern ice sheet continued to lose pore space. The NAO also varies on intra-annual timescales, being typically stronger in winter than in summer. This impacts the amplitude of the seasonal cycle in GrIS firn thickness and pore space. In the wet southeastern GrIS, most of the snow accumulates during the winter, when melting and densification are relatively weak, leading to a large seasonal cycle in thickness and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn. This dampens the seasonal amplitude of firn thickness and pore space.

Projections of the sea level contribution from the Greenland and Antarctic ice sheets rely on atmospheric and oceanic drivers obtained from climate models. The Earth System Models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) generally project greater future warming compared with the previous CMIP5 effort. Here we use four CMIP6 models and a selection of CMIP5 models to force multiple ice sheet models as part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We find that the projected sea level contribution at 2100 from the ice sheet model ensemble under the CMIP6 scenarios falls within the CMIP5 range for the Antarctic ice sheet but is significantly increased for Greenland. Warmer atmosphere in CMIP6 models results in higher Greenland mass loss due to surface melt. For Antarctica, CMIP6 forcing is similar to CMIP5 and mass gain from increased snowfall counteracts increased loss due to ocean warming.

The future mass balance of the Antarctic ice sheet depends for an important part on the stability of the floating ice shelves that surround it, as these buttress the flow of grounded ice. Being situated relatively far north and near sea level, surface melt is a common but otherwise intermittent process on Antarctic ice shelves. Surface meltwater can form meltwater ponds, which can deepen existing crevasses that may eventually penetrate through the entire ice shelf. This process, called hydrofracturing, likely contributed to the recent disintegration of multiple ice shelves in the Antarctic Peninsula, most recently Larsen B ice shelf in 2002. A thorough understanding of surface melt processes is therefore key to improve our ability to predict future ice shelf stability and ice sheet mass loss. The snowmelt–albedo feedback plays a crucial role in Antarctic ice sheet surface melt: when snow melts, meltwater may refreeze in the snowpack, increasing the average grain size and lowering surface albedo. In turn, this enhances the absorption of solar radiation, further increasing surface melt rate. To investigate the importance of the snowmelt–albedo feedback for surface melt in Antarctica, we implemented an albedo parameterization in a surface energy balance model that calculates melt rates. In this parameterization, we can separate the impacts of dry and wet snow metamorphism on albedo evolution and melt rate. This allows us to quantify the snowmelt–albedo feedback, the results of which are presented here. Results for Neumayer Station on the Ekström ice shelf in Dronning Maud Land, East Antarctica, show that the snowmelt–albedo feedback results in a threefold increase in local melt rate compared to calculations in which the effect has been ignored. We also applied this method to weather station data from locations elsewhere in Antarctica. Finally, the same albedo parameterization was implemented in the regional climate model RACMO2. This provides the opportunity to extend this method to the entire Antarctic ice sheet, and to assess the temporal and spatial variability of the magnitude of the snowmelt–albedo feedback.

Surface melt is an important process for the stability of ice shelves, and therewith the Antarctic ice sheet. In Antarctica, absorption of solar radiation is mostly the largest energy source for surface melt, which is further enhanced by the snowmelt-albedo feedback (SMAF): refrozen snow has a lower albedo than new snow, which causes it to absorb more solar radiation, further increasing the energy available for surface melt. This feedback has previously been shown to increase surface melt by approximately a factor of 2.5 at Neumayer Station in East Antarctica. In this study, we use a regional climate model to quantify SMAF for the entire Antarctic ice sheet. We find that it is most effective on ice shelves in East Antarctica, and is less important in the Antarctic Peninsula and on the Ross and Filchner-Ronne ice shelves. We identify a relationship between SMAF and average summer air temperatures, and find that SMAF is most important around 265±2 K. On a sub-seasonal scale, we identify several parameters that contribute to SMAF: the length of dry periods, the time between significant snowfall events and snowmelt events, and prevailing temperatures. We then apply the same temperature-dependency of SMAF to the Greenland ice sheet and find that it is potentially active in a narrow band around the ice sheet, and finally discuss how the importance of SMAF could change in a warming climate.