Quantifying changes to debris thickness and extent on Rongbuk Glacier, Tibet, between 2000 and 2021, using an energy balance model, spectral imagery, and climate reanalysis data

Abstract

The evolution of debris cover on debris-covered glaciers is poorly constrained in projections of glacier retreat and melt. Under climate warming, debris cover is projected to expand up-glacier and form at a faster rate. Of particular interest is debris’ initial emergence below the equilibrium line, where clean ice transitions into debris-covered ice. Debris cover has a non-linear effect on ice melt and accelerates ice melt in these transition areas. Producing a time series of debris evolution permits a better understanding of the transition of a glacier surface from clean ice to debris-covered ice.

This study produces a time series of debris cover evolution on Rongbuk Glacier, Tibet, between 2000 and 2021 using a surface temperature inversion energy balance model, Landsat 7 thermal imagery, and ERA-5 climate reanalysis data. Using established methods of quantifying changes in thickness and extent (Stewart et al., 2021), over the study period, Rongbuk Glacier’s debriscovered area increased in size at a rate of 0.70 percentage points per year. The glacier-wide mean thickness rose from 0.14 ± 0.05 m to 0.21 ± 0.06 m.

This study further introduces two novel methods of monitoring debris cover evolution beyond simply analysing the evolution of individual distributed debris thickness maps, which are hampered by variability between maps in the time series. First, it stacks and averages distributed debris thickness maps over 5-year periods. Second, it uses pixel-wise linear regression to define a mean distributed ‘debris accumulation rate’ over the study period. It finds that debris is accumulating in the upper ablation area of Rongbuk Glacier at a rate of 1.3 ± 0.4 mm yr-1 . Debris accumulation in the accumulation zone at the base of mountainsides provides compelling evidence that the delivery of debris from adjacent mountainsides has increased.

This study also finds multiple limitations with energy balance modelling in the upper ablation area and suggests areas for further research. Methods of incorporating fractional debris cover within low-resolution satellite-obtained imagery are needed to better understand and predict the future evolution of debris cover, which would result in better estimates of glacier longevity under climate change.