J. M. van Wessem

Dynamic thinning of glaciers on the Southern Antarctic Peninsula

Increasingly rapid ice sheet melting Glaciers on the Southern Antarctic Peninsula have begun losing mass at a rapid and accelerating rate. Wouters et al. documented the dramatic thinning of the land-based ice, which began in 2009, using satellite altimetry and gravity observations. The melting and weakening of ice shelves reduce their buttressing effect, allowing the glaciers to flow more quickly to the sea. Science, this issue p. 899 Glaciers on the Southern Antarctic Peninsula are disappearing at increasing rates. Growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice, especially if it is resting on bedrock below sea level. Much of the Southern Antarctic Peninsula satisfies this condition and also possesses a bed slope that deepens inland. Such ice sheet geometry is potentially unstable. We use satellite altimetry and gravity observations to show that a major portion of the region has, since 2009, destabilized. Ice mass loss of the marine-terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of –56 ± 8 gigatons per year, constituting a major fraction of Antarctica’s contribution to rising sea level. The widespread, simultaneous nature of the acceleration, in the absence of a persistent atmospheric forcing, points to an oceanic driving mechanism.

Extreme precipitation and climate gradients in Patagonia revealed by high-resolution regional atmospheric climate modeling

This study uses output of a high-resolution (5.5km) regional atmospheric climate model to describe the present-day (1979‐2012) climate of Patagonia, with a particular focus on the surface mass balance (SMB) of the Patagonian ice fields. Through a comparison with available in situ observations, it is shown that the model is able to simulate the sharp climate gradients in western Patagonia. The southern Andes are an efficient barrier for the prevalent atmospheric flow, generating strong orographic uplift and precipitation throughout the entire year. The model suggests extreme orographic precipitation west of the Andes divide, with annual precipitation rates of .5 to 34mw.e. (water equivalent), and a clear rain shadow east of the divide. These modeled precipitation rates are supportedqualitativelyby availableprecipitationstationsand SMBestimates on the ice fields derived from firn cores. For the period 1979‐2012, a slight atmospheric cooling at upper ice field elevations is found, leading to a small but insignificant increase in the ice field SMB.

A Comparison of Antarctic Ice Sheet Surface Mass Balance from Atmospheric Climate Models and In Situ Observations

AbstractIn this study, 3265 multiyear averaged in situ observations and 29 observational records at annual time scale are used to examine the performance of recent reanalysis and regional atmospheric climate model products [ERA-Interim, JRA-55, MERRA, the Polar version of MM5 (PMM5), RACMO2.1, and RACMO2.3] for their spatial and interannual variability of Antarctic surface mass balance (SMB), respectively. Simulated precipitation seasonality is also evaluated using three in situ observations and model intercomparison. All products qualitatively capture the macroscale spatial variability of observed SMB, but it is not possible to rank their relative performance because of the sparse observations at coastal regions with an elevation range from 200 to 1000 m. In terms of the absolute amount of observed snow accumulation in interior Antarctica, RACMO2.3 fits best, while the other models either underestimate (JRA-55, MERRA, ERA-Interim, and RACMO2.1) or overestimate (PMM5) the accumulation. Despite underestima...

Validation of the summertime surface energy budget of Larsen C Ice Shelf (Antarctica) as represented in three high-resolution atmospheric models

We compare measurements of the turbulent and radiative surface energy fluxes from an automatic weather station (AWS) on Larsen C Ice Shelf, Antarctica with corresponding fluxes from three high-resolution atmospheric models over a 1 month period during austral summer. All three models produce a reasonable simulation of the (relatively small) turbulent energy fluxes at the AWS site. However, biases in the modeled radiative fluxes, which dominate the surface energy budget, are significant. There is a significant positive bias in net shortwave radiation in all three models, together with a corresponding negative bias in net longwave radiation. In two of the models, the longwave bias only partially offsets the positive shortwave bias, leading to an excessive amount of energy available for heating and melting the surface, while, in the third, the negative longwave bias exceeds the positive shortwave bias, leading to a deficiency in calculated surface melt. Biases in shortwave and longwave radiation are anticorrelated, suggesting that they both result from the models simulating too little cloud (or clouds that are too optically thin). We conclude that, while these models may be able to provide some useful information on surface energy fluxes, absolute values of modeled melt rate are significantly biased and should be used with caution. Efforts to improve model simulation of melt should initially focus on the radiative fluxes and, in particular, on the simulation of the clouds that control these fluxes.

A Multidisciplinary Perspective on Climate Model Evaluation For Antarctica

Over the twenty-first century, large changes in climate are projected for Antarctica and the Southern Ocean under scenarios of greenhouse gas increase and stratospheric ozone recovery.These changes would potentially have important environmental and societal implications, affecting, for example, sea level change, global ocean heat and carbon uptake, and ecosystem function. However, our ability to make precise estimates of these impacts is hampered by uncertainties in state-of-the-art climate models. Model evaluation is a key step in reducing this model uncertainty by helping to identify biases and shortcomings common to the current generation of models and highlighting priorities for future model development. The evaluation of climate models’ representation of Antarctic climate from the perspective of long-term twenty-first-century climate change was the subject of a workshop organized by Antarctic Climate 21 (AntClim21), one of six current scientific research programs of the Scientific Committee onAntarctic Research (SCAR; see www.scar.org/srp/antclim21).

A model study of the effect of climate and sea-level change on the evolution of the Antarctic Ice Sheet from the Last Glacial Maximum to 2100

Due to a scarcity of observations and its long memory of uncertain past climate, the Antarctic Ice Sheet remains a largely unknown factor in the prediction of global sea level change. As the history of the ice sheet plays a key role in its future evolution, in this study we model the Antarctic Ice Sheet from the Last Glacial Maximum (21 kyr ago) until the year 2100 with the ice-dynamical model ANICE. We force the model with different temperature, surface mass balance and sea-level records to investigate the importance of these different aspects for the evolution of the ice sheet. Additionally, we compare the model output from 21 kyr ago until the present with observations to assess model performance in simulating the total grounded ice volume and the evolution of different regions of the Antarctic Ice Sheet. Although there are some clear limitations of the model, we conclude that sea-level change has driven the deglaciation of the ice sheet, whereas future temperature change and the history of the ice sheet are the primary cause of changes in ice volume in the future. We estimate the change in grounded ice volume between its maximum (around 15 kyr ago) and the present-day to be between 8.4 and 12.5 m sea-level equivalent and the contribution of the Antarctic Ice Sheet to the global mean sea level in 2100, with respect to 2000, to be −22 to 63 mm.