The contribution of glaciers to sea-level rise in the 21st century
This research was conducted within the framework of the ice2sea programme, which was funded by the European Union Framework 7 scheme. It aimed for a better understanding of processes taking place at ice sheets, ice caps and glaciers and provided a new estimate of the contribution of these ice masses to sea level rise until the year 2100. Researchers at 24 partner institutes, the majority based in Europe, contributed to ice2sea. I worked in Work Package 5.3, which focused on future projections for glaciers and ice caps outside the vast ice sheets of Greenland and Antarctica.
Estimating the contribution of all glaciers on Earth to sea-level rise is a challenge. Until the Randolph Glacier Inventory was released in 2012, the total number of glaciers was not even known, let alone their geometry and area. Still, the volume of water contained in these glaciers has a large uncertainty. If all glaciers would disappear entirely and the melt water would end up in the world's oceans, the potential sea-level rise would likely range between 0.4 and 0.6 m.
Glaciers are situated in a large variety of climates, from high elevations in the Tropics to low elevations in the Arctic and from warm and wet conditions to cold and dry climates. Since the dominant processes in the surface energy and mass balance differ amongst these climates, a model should resolve all these processes for an accurate simulation of all glaciers. On the other hand, the detailed input data required for such simulations are simply not available, resulting in the use of simplified mass balance models for global applications.
Since increases in air temperature are expected to be the major driver of mass balance changes in the future, it is important that the modelled mass balance sensitivity to temperature changes is realistic. This requires a mass balance model where the contributions by the temperature-dependent energy fluxes (mainly net longwave radiation and the turbulent fluxes) to surface melt are separated from energy supplied by net solar radiation, which is only indirectly dependent on air temperature. A relation for the temperature-dependent fluxes as a function of air temperature was derived from measurements at automatic weather stations (AWS) in different climates.
Mass balance models can only be calibrated and validated for the small number of glaciers (compared to the total) where mass balance measurements have been done. Besides the annual mass gain or loss, winter mass balance measurements are needed to derive the vertical gradient in precipitation, which varies largely over the globe. The parameters in the simplified mass balance model were calibrated for 89 glaciers with available measurements; relations with annual precipitation or latitude could not be established. It was shown that both winter and net mass balance need to be included in the calibration procedure to ensure a realistic mass balance sensitivity to temperature and precipitation changes.
The calibrated mass balance model was forced with projected changes in air temperature and precipitation from eight atmosphere-ocean global circulation models (AOGCM), using the A1B emission scenario. Since net solar radiation was calculated separately in the model, the effect of future changes in atmospheric transmissivity (primarily due to changing cloudiness) was also investigated. Future changes in glacier area as a result of mass balance changes were approximated with a volume-area scaling relation. This resulted in future volume projections for the 89 calibrated glaciers. The results were upscaled to all glaciers on Earth by assuming that each modelled glacier is representative for the glaciers surrounding it, while taking into account differences in initial surface area.
The projected sea-level rise over the period 2012–2099 from glacier volume changes is 0.102±0.028 m (multi-AOGCM mean and standard deviation), corresponding to 18±5% of the estimated total volume of glaciers. Glaciers in the Antarctic, Alaska, Central Asia and Greenland together account for 65±4% of the total multi-model mean projected sea-level rise. The projected sea-level contribution becomes 35±17% larger when only anomalies in air temperature are taken into account, demonstrating an important compensating effect by increased precipitation and possibly reduced atmospheric transmissivity. The variability in projected precipitation and atmospheric transmissivity changes is especially large in the Arctic regions, making the sea-level contribution for these regions particularly sensitive to the climate model used.