ALBERT

Description: We present time series of equilibrium-line altitude (ELA) measured from the end-of-summer snow line altitude computed using satellite images, for 43 glaciers in the western Alps over the 1984–2010 period. More than 120 satellite images acquired by Landsat, SPOT and ASTER were used. In parallel, changes in climate variables, summer cumulative positive degree days (CPDD) and winter precipitation, were analyzed over the same time period using 22 weather stations located inside and around the study area. Assuming a continuous linear trend over the study period: (1) the average ELA of the 43 glaciers increased by about 170 m; (2) summer CPDD increased by about 150 PDD at 3000 m a.s.l.; and (3) winter precipitation remained rather stationary. Summer CPDD showed homogeneous spatial and temporal variability; winter precipitation showed homogeneous temporal variability, but some stations showed a slightly different spatial pattern. Regarding ELAs, temporal variability between the 43 glaciers was also homogeneous, but spatially, glaciers in the southern part of the study area differed from glaciers in the northern part, mainly due to a different precipitation pattern. A sensitivity analysis of the ELAs to climate and morpho-topographic variables (elevation, aspect, latitude) highlighted the following: (1) the average ELA over the study period of each glacier is strongly controlled by morpho-topographic variables; and (2) the interannual variability of the ELA is strongly controlled by climate variables, with the observed increasing trend mainly driven by increasing temperatures, even if significant nonlinear, low-frequency fluctuations appear to be driven by winter precipitation anomalies. Finally, we used an expansion of Lliboutry's approach to reconstruct fluctuations in the ELA of any glacier of the study area with respect to morpho-topographic and climate variables, by quantifying their respective weight and the related uncertainties in a consistent manner within a hierarchical Bayesian framework. This method was tested and validated using the ELA measured on the satellite images.

Description: We present time series of equilibrium-line altitude (ELA) measured from the end-of-summer snowline altitude computed using satellite images, for 43 glaciers in the western Alps over the 1984–2010 period. More than 120 satellite images acquired by Landsat, SPOT and ASTER were used. In parallel, changes in climate parameters (summer cumulative positive degree days, CPDD, and winter precipitation) were analyzed over the same time period using 22 weather stations located inside and around the study area. Assuming a continuous linear trend over the study period: (1) the average ELA of the 43 glaciers increased by about 170 m; (2) summer CPDD increased by about 150 PDD at 3000 m a.s.l.; and (3) winter precipitation remained rather stationary. Summer CPDD showed homogeneous spatial and temporal variability; winter precipitation showed homogeneous temporal variability, but some stations showed a slightly different spatial pattern. Regarding ELAs, temporal variability between the 43 glaciers was also homogeneous, but spatially, glaciers in the southern part of the study area differed from glaciers in the northern part, mainly due to a different precipitation pattern. A sensitivity analysis of the ELAs to climate and morpho-topographic parameters (elevation, aspect, latitude) highlighted the following: (1) the average ELA over the study period of each glacier is strongly controlled by morpho-topographic parameters; and (2) the interannual variability of the ELA is strongly controlled by climate parameters, with the observed increasing trend mainly driven by increasing temperatures, even if significant nonlinear low frequency fluctuations appear to be driven by winter precipitation anomalies. Finally, we used an expansion of Lliboutry's approach to reconstruct fluctuations in the ELA of any glacier of the study area with respect to morpho-topographic and climate parameters, by quantifying their respective weight and the related uncertainties in a consistent manner within a hierarchical Bayesian framework. This method was tested and validated using the ELA measured on the satellite images.

Description: The Greenland ice sheet is modelled to simulate its extent and volume in warmer climates, and also to find out whether the ice sheet would re-form on theice-free bedrock under present climatic conditions. The ice sheet model is a three-dimensional thermo-mechanical model with a fine resolution grid. Thebedrock surface beneath the ice sheet was mapped using radio-echo-sounding measurements by the Electromagnetic Institute, Copenhagen. The modelexperiments show that increased temperature will result in ice-margin retreat, but the ice sheet is relatively stable; it takes a rather high temperature rise of atleast 6¡C for the ice sheet to disappear completely, which indicates that the ice sheet probably survived the last interglacial. Also, it appears that the Greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re-form on the bare bedrock under the present, or evenslightly warmer, climatic conditions.

Description: Increased melting on glaciers and ice sheets and rising sea level are often mentioned as important aspects of the anticipated greenhouse warming of the earth'satmosphere. This paper deals with the sensitivity of Greenland's ice mass budget and presents a tentative projection of the Greenland component of future sealevel rise for the next few hundred years. To do this, the 'Villach II temperature scenario' is prescribed, and output from a comprehensive mass balancemodel is used to drive a high-resolution 3-D thermomechanic model of the ice sheet.The mass balance model consists of two parts: the accumulation part is based on presently observed values and is forced by changes in mean annual airtemperature. The ablation model is based on the degree-day method and accounts for the daily and annual temperature cycle, a different degree-day factor forice and snow melting and superimposed ice formation. Under present-day climatic conditions, the following total mass balance results (in ice equivalent peryear): 599.3 109 m3 of accumulation, 281.7 109 m3 of runoff and assuming a balanced budget, 317.6 109 m3 of iceberg calving. A 1K uniform warming isthen calculated to increase the runoff by 119.5 109 m3. Since accumulation also increases by 32 109 m3, this leads to reduction of the total mass balance by87.5 109 m3 of ice, corresponding to a sea level rise of 0.22 mm/year. For a temperature increase larger than 2.7 K, runoff exceeds accumulation, and if icesheet dynamics were to remain unchanged, this would add an extra amount of 0.8 mm/year to the worlds' oceans.Imposing the Villach II scenario (warming up to 4.2 K) and accumulating mass balance changes forward in time (static response) would then result in aglobal sea level rise of 7.1 cm by 2100 AD, but this figure may go up to as much as 40 cm per century in case the warming is doubled. In a subsequentdynamic model run involving the ice flow, the ice sheet is found to produce a counteracting effect by dynamically producing steeper slopes at the margin,thereby reducing the area over which runoff can take place. This effect is particularly apparent in the northeastern part of the ice sheet, and is also morepronounced for the smaller temperature perturbations. Nevertheless, all these experiments certainly highlight the vulnerability of the Greenland ice sheet withrespect to a climatic warming.