Strong changes in englacial temperatures despite insignificant changes in ice thickness at Dôme du Goûter glacier (Mont Blanc area)

International audienceThe response of very-high-elevation glaciated areas on Mont Blanc to climate change has been analysed using observations and numerical modelling over the last 2 decades. Unlike the changes at low elevations, we observe very low glacier thickness changes, of about -2.6 m on average since 1993. The slight changes in horizontal ice flow velocities and submergence velocities suggest a decrease of about 10 % in ice flux and surface mass balance. This is due to less snow accumulation and is consistent with the precipitation decrease observed in meteorological data. Conversely, measurements performed in deep boreholes since 1994 reveal strong changes in englacial temperature reaching a 1.5 ∘C increase at a depth of 50 m. We conclude that at such very high elevations, current changes in climate do not lead to visible changes in glacier thickness but cause invisible changes within the glacier in terms of englacial temperatures. Our analysis from numerical modelling shows that glacier near-surface temperature warming is enhanced by increasing melt frequency at high elevations although the impact on surface mass balance is low. This results in a non-linear response of englacial temperature to currently rising air temperatures. In addition, borehole temperature inversion including a new dataset confirms previous findings of similar air temperature changes at high and low elevations in the Alps