Surface mass balance, thinning and iceberg production, Columbia Glacier, Alaska, 1948-2007

Abstract

A mass-balance model using upper-air meteorological data for input was calibrated with surface mass balance measured mainly during 1977-78 at 67 sites on Columbia Glacier, Alaska, between 135 and 2645ma.s.l. Root-mean-square error, model vs measured, is 1.0mw.e. a-1,with r 2 =0.88. A remarkable result of the analysis was that both precipitation and the factor in the positive degree-day model used to estimate surface ablation were constant with altitude. The model was applied to reconstruct glacier-wide components of surface mass balance over 1948-2007. Surface ablation, 4km3 ice eq. a-1 (ice equivalent), has changed little throughout the period. From 1948 until about 1981, when drastic retreat began, the surface mass balance was positive but changes in glacier geometry were small, so the positive balance was offset by calving, ∼0.9 km3 ice eq. a-1. During retreat, volume loss of the glacier accounted for 92% of the iceberg production. Calving increased to ∼4.3 km3 ice eq. a-1 from 1982 to 1995, and after that until 2007 to ∼8.0 km3 ice eq. a -1, which was about twice the loss by surface ablation, whereas prior to retreat it was only about a quarter as much. Calving is calculated as the difference between glacier-wide surface mass balance and geodetically determined volume change.