Unraveling a paradox of Himalayan glacier melt

Hexbyte Glen Cove

by Ryan M. Strickland

Ryan Strickland pilots the drone for take-off on one of the 15 survey flights of the Ngozumpa Glacier in December 2019. Credit: Ryan M. Strickland

One in five glaciers on Earth are covered with a layer of rocky debris. The presence of debris influences how glaciers melt. In the Himalaya, debris covers most large glaciers, and it is so thick that it should insulate the ice, slowing the rates of melt. However, many debris-covered glaciers melt just as fast as glaciers with no debris. This anomaly is known as the debris cover paradox.

Understanding the causes of the cover paradox is necessary to predict melt rates and in southern Asia in response to climate change. The paradox is partly explained by the development of ice cliffs and meltwater ponds on the glacier surfaces. Although ponds and ice cliffs occupy only a tiny fraction of the glacier surface, they account for nearly half of glacier melt.

Pond and ice cliff growth occurs simultaneously with the development of hummocky topography on the glacier surface. The hummocky topography is complex. Contrasts in melt rate cause thousands of hills, known as hummocks, to form across the surface of the glaciers. Understanding how the hummocky topography evolves will help scientists more accurately predict the growth of ponds and ice cliffs, and therefore better predict melt rates.

Hexbyte Glen Cove The enigma of hummocky topography

Most attribute the development of hummocky topography to variable debris thickness. When the debris is thinner than a few centimeters, melt rates increase compared to bare ice, but thick debris insulates the ice and decreases melt rates. According to this hypothesis, the hummocks develop because debris is thicker on the hilltops. However, the ruggedness of the terrain and difficulty of conducting research in the thin Himalayan air have made it challenging to test this hypothesis.

During 2018 and 2019 expeditions to the Everest Region in Nepal, Dr. Matt Covington, Dr. Jason Gulley and I used a drone to photograph the Ngozumpa Glacier. We used these data to investigate patterns in the topography to better understand how it evolves. Our research has now been published in the journal Geophysical Research Letters.