On Glacier Surface Mass Balance at the Last Glacial Maximum.

Geomorphological evidence shows that during the last glacial period glaciers reached far intothe foreland of the Alps. How frequent such advances took place, how long the glaciersremained in advanced position and how fast they advanced or retreated, however, is difficultto reconstruct from geomorphology. Models of glacier flow are increasingly used to assess icedynamics during the last glacial period. Driving such models relies on a series ofestimates, including ice rheology, the forcing climate and glacier mass balance.Thereby, the latter strongly depends on climate and the equations used to describe theirconnection. Here, we focus on reconstructing surface melt of the Rhine glacier (∼16,000 km2) at thelast glacial maximum (LGM). Given large uncertainties in our knowledge of the LGMclimate, we base our estimates on (i) analogies from Arctic present-day climate and massbalance and on (ii) theoretical considerations. Thereby we investigate which are the keyclimatic characteristics that need to be specified to simulate LGM glacier melt. We find thatchanges in the amplitude of annual air temperature substantially influence simulatedablation-area mass balance gradients db∕dz, and thus glacier-wide mass balance. While thisinfluence has been shown in earlier research, it has not yet been considered in reconstructionsof glacier mass balance during the last glacial. Paleoclimatic evidence from Central Europe suggests that during the LGM annualamplitude of air temperature was larger than today. We show that computing LGM glaciermass balance while assuming no change in amplitude compared to present-dayclimate can result in an overestimation of db∕dz (∼0.4 m w.e. (100 m)−1 yr−1 vs.∼0.55 m w.e. (100 m)−1 yr−1). It remains to be explored how these differing db∕dz couldinfluence simulated glacier dynamics during the last glacial. [ABSTRACT FROM AUTHOR]