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

Abstract

In 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 ( " T IH ) have systematically increased by 41K 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 AE 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.

The near−surface ice thermal structure of the Waldemarbreen, Svalbard

“…It is generally accepted that the thermal regime of a glaciers directly controls glacier mass balance, hydrology and dynamics (Fisher 1955;Baranowski 1977;Paterson 1981;Blatter and Haeberli 1984;Krass et al 1991;Key and Haefliger 1992;Dowdeswell et al 1995;Björnsson et al 1996;Jania 1997;Hodgkins et al1999;Nagornov et al 2001Nagornov et al , 2006Rabus and Echelmeyer 2002;Hooke 2005;Willis et al 2007;Reijmer and Hock 2008). Unfortunately, direct measurements of thermal regimes are only available for very few glaciers worldwide.…”

Modelling historical and recent mass loss of McCall Glacier, Alaska, USA

“…This operation softens the ice in the accumulation area to obtain more realistic simulated surface velocities and ice thicknesses. According to temperature measurements, ice in the accumulation area may be considered isothermal over its whole thickness ( Rabus and Echelmeyer, 2002). Although not parameterized as such, our model experiments demonstrate the existence of a quasi isothermal ice layer in the accumulation area once the glacier is in a transient state (Fig.…”

“…However, in the accumulation area, surface temperatures are perturbed by latent heat release due to melting and refreezing processes. This leads to more or less isothermal conditions, and mean annual temperatures are around −6.3 • C (Rabus and Echelmeyer, 2002). Therefore, surface temperatures are defined as:…”

“…Based on the measurements of Rabus and Echelmeyer (2002), 10 m ice temperatures in the ablation area were parameterized as a linear function of altitude. However, in the accumulation area, surface temperatures are perturbed by latent heat release due to melting and refreezing processes.…”

Optical properties of sea ice doped with black carbon – an experimental and radiative-transfer modelling comparison

“…The temperature is recorded via a series of thermocouples, as used by Rabus and Echelmeyer (2002), Johnston and Timco (2002) and Nomurai et al (2006). Calibrated type T thermocouples are each inserted into a thin plastic sleeve and then a white polypropylene pole at regular depths (every 2 cm) into the water, and then frozen in place during ice formation.…”