The Greenland ice sheet in a warming climate

Abstract

In this thesis we assess multiple aspects of the Greenland climate, including the surface energy and mass balance of the ice sheet for the contemporary and near future climate. For these purposes we used output of the extensively and well-evaluated regional atmospheric climate model RACMO2. The relatively high horizontal resolution (11 km) enables us to explicitly solve the atmospheric momentum budget and explain both small and large-scale wind patterns over the Greenland ice sheet and its surrounding seas. In the surface layer the katabatic pressure gradient force dominates the momentum budget resulting in strong and persistent cross-slope winds. Along the northeast coast we explain the presence of a persistent thermally induced northerly boundary layer jet (the Greenland Sea Jet), which is the main driver of sea ice export out of the Arctic Ocean through Fram Strait. The most important energy source for surface melting at the ice sheet is absorbed solar radiation, which is mainly determined by cloud cover and surface albedo. Due to the relatively high reflectivity of snow and ice, small changes in the surface albedo have a strong impact on the amount of absorbed radiation. Furthermore, the surface albedo is highly variable in space and time making it a challenge to simulate realistically. By implementing a grain size dependent albedo scheme and a background ice albedo field based on satellite observations in RACMO2, we are able to better resemble measured albedos. We show that small changes in the parameter settings of the albedo scheme have a large impact on the surface mass balance and individual components. Using measurements of albedo and melt extent we were able to strongly reduce the uncertainty in the scheme. Greenland has been subject to strong warming over the last two decades and especially for the last few years, with a record low surface mass balance year in 2010 and a melt event affecting almost the entire ice sheet in July 2012. A combination of stronger than average warming in this part of the Arctic region due to reduced sea ice cover, lower surface albedo over the ice sheet and advection of relatively warm air from southern latitudes explain the anomalously strong melting over Greenland. To assess the impact of 21st century climate change over Greenland, we forced RACMO2 with a mid-range warming scenario in which average summer temperatures over the ice sheet rise almost 3 °C towards the end of the century. This induces a strong increase in surface melting and runoff, which is only partly compensated by enhanced precipitation. The total surface mass balance shows a gradual decrease and even turns negative around 2070. Currently, more than 40% of the melt water is refrozen in the snowpack. We show that owing to a strong reduction in pore space in the firn layer upon refreezing, the refreezing capacity of the Greenland ice sheet decreases to less than 30%. This results in an acceleration of mass loss from the Greenland ice sheet and we quantify that the expected contribution to sea level rise will increase to 1.7±0.5 mm/yr, more than four times the current contribution.