Increase of 10 m ice temperature: climate warming or glacier thinning?

Abstract

AbstractIn 1972 and 1995, shallow ice temperatures were measured at identical locations and depths on polythermal McCall Glacier, Brooks Range, Alaska, U.S.A. Mean annual ice temperatures at 10 m depth have systematically increased by > 1 K for the ablation area (1400–1900 m), while closer to the firn area, where meltwater percolation and refreezing play a role, they remained approximately unchanged. Interpreting these findings in terms of climate change requires careful consideration of the observed thinning of the glacier, which causes lowering of the surface through an existing vertical temperature gradient. Such temperature gradients can be particularly large in the ablation areas of polythermal glaciers; on McCall Glacier they are on the order of 0.2 K m−1. We also study the evolution of a 75 m deep temperature profile measured at one location in 1972 using a one-dimensional heat-diffusion model. We find that this profile was in approximate equilibrium with the mean surface temperature extrapolated from the 1972 data. Using the observed rate of surface lowering and measured rates of vertical advection, we find that both the measured temperature change and the vertical temperature gradient at 10 m depth can be reproduced only if the mean annual surface temperature on McCall Glacier has increased by 1.1 ± 0.3 K between 1972 and 1995. This result is consistent with the observed trend toward more negative mass balances on the glacier in the 1990s.