Consistent evidence of increasing Antarctic accumulation with warming

Projections of changes in Antarctic Ice Sheet (AIS) surface mass balance indicate a negative contribution to sea level becauseof theexpected increase inprecipitationdue to thehigher moisture holding capacity of warmer air1. Observations over the past decades, however, are unable to constrain the relation between temperature and accumulation changes because both are dominated by strong natural variability2–5. Here we derive a consistent continental-scale increase in accumulation of approximately 5±1%K−1, through the assessment of ice-core data (spanning the large temperature change during the last deglaciation, 21,000 to 10,000 years ago), in combination with palaeo-simulations, future projections by 35 general circulation models (GCMs), and one high-resolution future simulation. The ice-core data and modelling results for the last deglaciation agree, showing uniform local sensitivities of ∼6%K−1. The palaeo-simulation allows for a continental-scale aggregation of accumulation changes reaching 4.3%K−1. Despite the di erent timescales, these sensitivities agreewith themulti-modelmean of 6.1±2.6%K−1 (GCMprojections) and the continental-scale sensitivity of 4.9%K−1 (high-resolution future simulation). Because some of the mass gain of the AIS is o set by dynamical losses induced by accumulation6,7, we provide a response function allowing projections of sea-level fall in terms of continental-scale accumulation changes that competewith surfacemelting and dynamical losses induced by other mechanisms6,8,9. General Circulation Models and high-resolution atmospheric regional climate models (RCMs) consistently project increasing AIS accumulation (herein defined as precipitation–sublimation) over the twenty-first century5,10–14. Continental-scale increases aremainly attributed to increasing precipitation due to higher atmospheric moisture concentrations in a warmer atmosphere, whereas regional patterns result mainly from the interaction between ice-sheet topography and circulation-driven changes in meridional moisture transport14–16. The surface topography of the AIS leads to a spatially variable distribution of precipitation,with lowprecipitation rates (<50mmyr−1) over the high-elevation inner plateau and a rapid increase in precipitation towards the lower elevation coastal regions4,11,17,18. The projected continental-scale change in precipitation is also dominated by an increase in the coastal regions. Based on a GCM with regional zoom capacity, the mean absolute increase in precipitation over coastal areas (surface elevation < 2,250m) is projected to be three times larger than the mean increase over the inner ice sheet10. In contrast, the projected relative increase in precipitation over the twenty-first century is much more uniformly distributed and even tends to be slightly higher in the interior than in the coastal regions10,13,19. Despite model simulations consistently showing an increase in continental-scale accumulation with regional warming, individual estimates of the sensitivities (herein accumulation sensitivity) have a wide range, from 3.7%K−1 estimated from one GCM over the twenty-first century20, to 5.5%K−1 derived from simulations of the historical period provided by five GCMs (ref. 5) within the Coupled Model Intercomparison Project phase 3 (CMIP3), 7%K−1 based on high-resolution model simulations by the end of the twenty-first century11, and 13%K−1 based on simulations from 15 CMIP3 GCMs through the twenty-first century10, although the high sensitivity in the latter studymay be largely due to the empirical correction factor used to adjust for resolution effects. Moreover, because high-resolution RCMs better resolve the steep coastal topography anduplift of airmasses, adiabatic cooling and associated precipitation than lower-resolution global models, they often result in higher projected continental-scale precipitation changes for the same amount of warming10,12. There are few observational data to evaluate these model simulations. Linear regression analysis of present-day observations21 suggests a sensitivity of 4%K−1 for the Antarctic continent. However, because of the large inter-annual variability of snowfall on a continental scale4, long-term records are required to infer significant accumulation trends3. The analysis of a current 50-year benchmark data set has not shown a significant trend in Antarctic accumulation with time3. In combination with temperature observations, the accumulation sensitivity reaches 4.9±4.9%K−1, in close agreement with a GCM-derived value of 5.5± 0.8%K−1 (ref. 5) and the early estimate by Fortuin and Oerlemans21. However, the simulated sensitivities are based on significant increases in accumulation rates (17± 4mm century−1) and temperatures that are not seen in the observational data. Ice cores provide information about accumulation changes during the period of warming associated with the last deglaciation (∼21–10 ka; Fig. 1), thus providing a unique opportunity to evaluate accumulation sensitivities independent of model simulations. At the same time, however, these records identify only local changes, and thus do not allow an assessment of the continental-scale relationship between integrated accumulation changes across the AIS and continental-mean temperatures that is critical for estimates of sea-level rise. We thus use results from a transient simulation with the coupled atmosphere–ocean Community Climate System Model version 3 (CCSM3) that spans much of the last deglaciation (22.0–14.3 ka; refs 22,23) to derive associated