An assessment of the disequilibrium of Alaska glaciers

Abstract. The finite response time of alpine glaciers means that glaciers will be in a state of disequilibrium in the presence of a climate trend. Using a simple model of glacier dynamics, we use metrics of glacier geometry to evaluate the present-day disequilibrium for a population of 5200 alpine glaciers in Alaska. Our results indicate that glaciers throughout the region are in a severe state of disequilibrium. We estimate that the median glacier has only undergone 27 % of the retreat necessary to achieve equilibrium with the present-day climate. In general, glaciers with smaller areas have smaller response times, and so are closer to equilibrium than large glaciers. Because much of Alaska's glacier area is contained in a few large glaciers that are far from equilibrium, and because the rate of warming has increased in the last ∼ 50 years, the median equilibration weighted by area is only 16 %. Our estimates are sensitive to uncertainty in response time and to the shape of the warming trend. Uncertainty is greatest for intermediate glacier response times but is small for glaciers with the smallest and largest response times. Finally, we demonstrate that accounting for the increased rate of warming in the late-20th century is important for estimating glacier disequilibrium, whereas the shape of the warming trend in the early-20th century is less relevant. Our results imply substantial future glacier retreat is already guaranteed regardless of the trajectory of future warming.