Global-scale modeling of glacier mass balances for water resources assessments: Glacier mass changes between 1948 and 2006

Glaciers play an important role for freshwater resources, but in global-scale freshwater assessments, their impact on river flows has not yet been taken into account. As a first step, we developed a global glacier model that can be coupled to global land surface and hydrological models. With a spatial resolution of 0.5° by 0.5°, the glacier model HYOGA computes glacier mass balance by a simple degree-day approach for 50. m sub-grid elevation bands, modeling all glaciers within a grid cell as one glacier. The model is tuned individually for each grid cell against observed glacier mass balance data. HYOGA is able to compute glacier mass balances reasonably well, even those of summer accumulation type glaciers. Still, model uncertainty is high, which is, among other reasons, due to the uncertainty of global data sets of temperature and precipitation which do not represent well the climatic situation at glacier sites. We developed a 59-yr (1948-2006) time series of global glacier mass balance and glacier area by driving HYOGA with daily near-surface atmospheric data. According to our computations, most glaciers have lost mass during the study period. Compared to estimates derived from a rather small number of observed glacier mass balances, HYOGA computes larger glacier mass losses in Asia, Europe, Canadian Arctic islands and Svalbard. In accordance with the estimates, average annual mass losses have increased strongly after 1990 as compared to the 30. yrs before. The sea level equivalent of the melt water from glaciers is 0.76. mm/yr water equivalent after 1990 as compared to only 0.34. mm/yr water equivalent before. We computed an acceleration of glacier mass losses after 1990 for all world regions except South America, where the number of gauge observations of precipitation is very small after 1980.