Positive degree-day sums in the Alps: a direct link between glacier melt and international climate policy

Annual balances of eight alpine glaciers were slightly negative for 1961-90 and highly negative for 1991-2018. We explain this by changes in positive degree-day sums and summer temperatures extrapolated to the median altitudes of the glaciers. We test a new way of calculating degreeday sums that performs better than the traditional method which used daily mean temperatures. Annual degree-day sums are highly correlated with May-September temperatures as suggested in 1866 by Karl von Sonklar. We find moderate correlations between annual balances and degree-day sums, and with May-September temperatures. Calculated degree-day factors for the eight glaciers cover the reported range for snow and ice ablation, while the temperature sensitivity of annual balance is from -0.4 to -1.0 m w.e. for a +1 degrees C temperature change. We accurately predict mean balances for 1991-2018 using May-September temperatures in regression models calibrated for 1961-90. May-September temperatures in the Alps have already increased similar to+3 degrees C since 1880 and, if temperatures continue to rise, these glaciers will shrink rapidly. As annual balances are already negative for present-day temperatures, these glaciers will not be 'safe' under the further temperature increase permitted by the Paris Agreement.