Glacial lakes exacerbate Himalayan glacier mass loss

Abstract

Heterogeneous glacier mass loss has occurred across High Mountain Asia on a multi-decadal timescale. Contrasting climatic settings influence glacier behaviour at the regional scale, but high intra-regional variability in mass loss rates points to factors capable of amplifying glacier recession in addition to climatic change along the Himalaya. Here we examine the influence of surface debris cover and glacial lakes on glacier mass loss across the Himalaya since the 1970s. We find no substantial difference in the mass loss of debris-covered and clean-ice glaciers over our study period, but substantially more negative (-0.13 to -0.29 m w.e.a^-1) mass balances for lake-terminating glaciers, in comparison to land-terminating glaciers, with the largest differences occurring after 2000. Despite representing a minor portion of the total glacier population (~10%), the recession of lake-terminating glaciers accounted for up to 32% of mass loss in different sub-regions. The continued expansion of established glacial lakes, and the preconditioning of land-terminating glaciers for new lake development increases the likelihood of enhanced ice mass loss from the region in coming decades; a scenario not currently considered in regional ice mass loss projections.

Figure 1

Regional glacier mass balance estimates across the Himalaya over the period ~1974–~2015, subdivided depending on glacier terminus type. Regional and land-terminating glacier mass balance estimates are the same for the period 2000–~2015 in Central West 1, and there are no lake-terminating glaciers covered in our ~1974–2000 dataset in the West Himalaya. Black boxes mark Hexagon footprint extent, which are lacking for Central 2 and Far East study areas due to inadequate quality of available Hexagon data. Orange polygons are from the Randolph Glacier Inventory version 6.0. Country boundaries are tentative and for orientation only. This figure was generated using ArcGIS, vers. 10.3 (http://www.esri.com/software/arcgis/arcgis-for-desktop) and Inkscape, vers. 0.92.4 (https://inkscape.org/).

Figure 2

The distribution of glacier mass balance estimates for clean-ice (<19% debris cover) and debris-covered (>19% debris cover) glaciers over the period ~1974–2000 (A), and 2000–~2015 (B), lake-terminating (C) and land-terminating glaciers (D) across both study periods. Note variable scaling on y-axis.

Figure 3

The cumulative ice front retreat (dTerm) of a subset (n = 325) of lake (A) and land-terminating (B) glaciers located across the Himalaya between 1973 and 2018. The relationship between glacier surface elevation change and elevation for lake-terminating (C) and land-terminating (D) glaciers with and without substantial debris cover.