Debris Emergence Elevations and Glacier Change

Debris-covered glaciers represent potentially significant stores of freshwater in river basins throughout High Mountain Asia (HMA). Direct glacier mass balance measurements are extremely difficult to maintain on debris-covered glaciers, and optical remote sensing techniques to evaluate annual equilibrium line altitudes (ELAs) do not work in regions with summer-accumulation type glaciers. Surface elevation and glacier velocity change have been calculated previously for debris-covered glaciers across the region, but the response of debris cover itself to climate change remains an open question. In this research we propose a new metric, i.e. the debris emergence elevation (Z_DE), which can be calculated from a combination of optical and thermal imagery and digital elevation data. We quantify Z_DE for 975 debris-covered glaciers in HMA over three compositing periods (1985–1999, 2000–2010, and 2013–2017) and compare Z_DE against median glacier elevations, modelled ELAs, and observed rates of both mass change and glacier velocity change. Calculated values of Z_DE for individual glaciers are broadly similar to both median glacier elevations and modelled ELAs, but slightly lower than both. Across the HMA region, the average value of Z_DE increased by 70 +/− 126 m over the study period, or 2.7 +/− 4.1 m/yr. Increases in Z_DE correspond with negative mass balance rates and decreases in glacier velocity, while glaciers and regions that show mass gains and increases in glacier velocity experienced decreases in Z_DE. Regional patterns of Z_DE, glacier mass balance, and glacier velocities are strongly correlated, which indicates continued overall increases in Z_DEE and expansion of debris-covered areas as glaciers continue to lose mass. Our results suggest that Z_DE is a useful metric to examine regional debris-covered glacier changes over decadal time scales, and could potentially be used to reconstruct relative mass and ELA changes on debris-covered glaciers using historical imagery or reconstructed debris cover extents.