The present and future state of the Antarctic firn layer

Abstract

Firn is the transitional product between fresh snow and glacier ice and acts as a boundary between the atmosphere and the glacier ice of the Antarctic Ice Sheet (AIS). Spatiotemporal variations in firn layer characteristics are therefore important to consider when assessing the mass balance of the AIS. In this thesis, a firn densification model, forced with a realistic climate, is used to examine contemporary (1979-2012) and future (2000-2200) variations in the Antarctic firn layer. Currently, 99% of the AIS is covered with a firn layer of 50-150 m thick. The thickest firn layers occur in the cold interior of the AIS, while thinner firn layers appear along the coastal margins, where regular melt occurs. On the remaining 1-2% of the AIS no firn layer exist; a so-called blue-ice area. Here, annual ablation, by either sublimation or melt, is larger than the annual accumulation, resulting in no long-term firn layer. The presence of a blue-ice area depends on a favorable combination of 1) ice velocity, 2) net surface ablation and 3) the mass of the existing firn layer. Next to the spatial variations, also temporal variations in firn layer characteristics exist. Due to the seasonal cycle in temperature and accumulation, the air content of the Antarctic firn layer grows in winter and shrinks in summer. As a consequence, the surface elevation of the AIS also shows cyclic behavior with a seasonal amplitude of 2.6 cm. In order to simulate the reaction of the current Antarctic firn layer on a warmer and wetter future climate, four simulations with the regional atmospheric climate model RACMO2 are performed. By forcing RACMO2 with two different global climate models (HAdCM3 and ECHAM5) and two different emission scenarios (A1B and E1), the possible spread in future climate is mimicked. The temperature increase over the AIS is similar to the global average; +1.8-3.0 K in 2100 and +2.4-5.3 K in 2200. This warmer climate leads to increased accumulation, as warmer air has a larger water vapor holding capacity. This accumulation increase outweighs the increases in both sublimation and melt, leading to a positive surface mass balance sensitivity: +98 Gt yr-1 K-1. In combination with the simulated temperature increase, this would result to a sea level drop of 73-163 mm by 2200. This is however without taking any ice dynamical response into account. Due to the future increase in snowfall, the air content of the Antarctic firn layer will increase. Roughly half of this effect is counteracted by both enhanced firn densification and a faster firn-to-ice transition at the bottom of the firn layer. Along the coast, firn air content will decrease significantly due to increasing melt. On several ice shelves at the Antarctic Peninsula, this will lead to depleted firn layers and enhanced runoff of meltwater. Averaged over the ice sheet, this decrease in firn air content is however small, resulting in an increase of the total AIS air content with 120-150 km3 yr-1, or +2.1 cm surface elevation per year.