Mass Loss From Calving in Himalayan Proglacial Lakes

Abstract

The formation and expansion of Himalayan glacial lakes has implications for glacier dynamics, mass balance and glacial lake outburst floods (GLOFs). Subaerial and subaqueous calving is an important component of glacier mass loss but they have been difficult to track due to spatiotemporal resolution limitations in remote sensing data and few field observations. In this study, we used near-daily 3 m resolution PlanetScope imagery in conjunction with an uncrewed aerial vehicle (UAV) survey to quantify calving events and derive an empirical area-volume relationship to estimate calved glacier volume from planimetric iceberg areas. A calving event at Thulagi Glacier in 2017 was observed by satellite from before and during the event to nearly complete melting of the icebergs, and was observed in situ midway through the melting period, thus giving insights into the melting processes. In situ measurements of Thulagi Lake's surface and water column indicate that daytime sunlight absorption heats mainly just the top metre of water, but this heat is efficiently mixed downwards through the top tens of metres due to forced convection by wind-blown icebergs; this heat then is retained by the lake and is available to melt the icebergs. Using satellite data, we assess seasonal glacier velocities, lake thermal regime and glacier surface elevation change for Thulagi, Lower Barun and Lhotse Shar glaciers and their associated lakes. The data reveal widely varying trends, likely signifying divergent future evolution. Glacier velocities derived from 1960/70s declassified Corona satellite imagery revealed evidence of glacier deceleration for Thulagi and Lhotse Shar glaciers, but acceleration at Lower Barun Glacier following lake development. We used published modelled ice thickness data to show that upon reaching their maximum extents, Imja, Lower Barun and Thulagi lakes will contain, respectively, about 90 × 10 6 , 62 × 10 6 and 5 × 10 6 m 3 of additional water compared to their 2018 volumes. Understanding lake-glacier interactions is essential to predict future glacier mass loss, lake formation and associated hazards.

A near 90-year record of the evolution of El Morado Glacier and its proglacial lake, Central Chilean Andes

“…Considering the ice losses presented for El Morado Glacier, it is unsurprising that its proglacial lake has grown in area dramatically since its emergence in the early 1950s. This behaviour is in line with glacial lake trends that have been observed across the Chilean Andes (Wilson and others, 2018) and other glacierised regions elsewhere (Wang and others, 2015; Emmer and others, 2016; Watson and others, 2020). The importance of glacial lakes is threefold: (1) they represent a considerable water resource by storing meltwater (Haeberli and others, 2016); (2) they can exacerbate the glacier mass loss (e.g.…”

“…The development and increase in the size of the lake appear to be linked to subaerial glacier elevation changes, subaqueous processes such as calving and ice-cliff collapse. Glacier calving depends on factors including water temperature, debris cover, glacier velocity and bed topography (Rohl, 2006; Benn and others, 2007; Watson and others, 2020). Our observations between 2000 and 2010 suggests that calving above the waterline was driven by a thermal-erosional notch (Rohl, 2006).…”

Six Decades of Glacier Mass Changes around Mt. Everest Are Revealed by Historical and Contemporary Images

“…Further to King et al, 16 our results show that the elevated rates of mass loss from lake-terminating glaciers could persist for several decades, during which time the rates of glacier terminus retreat and glacier surface velocities could increase. 39,40 After this phase of enhanced flow and ice loss, the rates at which lake-terminating glaciers lose mass diminish to a level comparable to that of land-terminating glaciers ( Figure 7C), presumably because their termini retreat to the periphery of their proglacial lake. 41 An additional consideration that is often not corrected for in the derivation of lake-terminating glacier mass balance is the portion of subaqueous ice that is lost during glacier recession, which is not captured in DEMs that represent the lake surface.…”

Proglacial Lakes Control Glacier Geometry and Behavior During Recession

“…Glaciers in contact with a lake are receding more rapidly than their land‐terminating counterparts, for example, in the Himalaya (Maurer et al, 2019; Tsutaki et al, 2019), in Alaska (Larsen et al, 2007; Willis et al, 2012), and in New Zealand (Chinn et al, 2012). Glaciers terminating in proglacial lakes can lose mass by several mechanisms in addition to melt from energy exchanges at the ice surface, namely, calving and subaqueous melting, collectively known as frontal ablation (Maurer et al, 2016; Sakai et al, 2009; Truffer & Motyka, 2016; Watson et al, 2020). Glaciers hosting proglacial lakes can, therefore, be partially decoupled from climatic forcing due to the effects of frontal ablation (King et al, 2018; Kirkbride, 1993), causing nonlinear responses in ice geometry and behavior (Benn et al, 2012).…”

“…Recent observations, both field‐based (e.g., Watson et al, 2020) and via satellite imagery (e.g., King et al, 2019), have highlighted the spatiotemporal frequency and magnitude of changes in glacial lakes and the associated glaciers that feed meltwater to them. However, field‐based measurements are limited to individual sites, usually spanning a single ablation season (e.g., Purdie & Fitzharris, 1999), and remote sensing investigations are limited to the last decade or so at most.…”