Rapid snow retreat amplifies North Greenland mass loss

The Greenland ice sheet has been losing mass at an accelerating rate since the early 1990s. A substantial part of the ice loss is driven by an increase in surface meltwater production that runs off to the ocean. In a study published today in Science Advances, researchers from Utrecht and Delft show that there is a large difference between mass loss in the North and South of Greenland, with runoff increasing twice as fast in North Greenland as in the South. The northern mass loss is triggered by a rapid snowline retreat in early summer, exposing dark bare ice and causing high runoff rates.

The Greenland ice sheet is the second largest ice mass on Earth. Totally melted, it has the potential to raise global sea-level by 7.4 meters. Along the ice sheet margins stands the ablation zone, where summer meltwater runoff exceeds the mass gain from winter snowfall. Historically, the widest ablation zone is found in the warm southwest, contributing one third of the ice sheet runoff total, whereas the North only contributes ~10% annually. After 1991, runoff has increased all over Greenland, but much more so in the North. In the summer of 2012, the ice sheet experienced extreme runoff of 560 km³, about two times the pre-1990s average. The frequency of such extreme melt years has increased over the past decades.

Twice as much, twice as fast

To study this regional contrast in runoff increase, the researchers focused on two large areas, namely North and South Greenland. Brice Noël, postdoc at Utrecht University and lead author of the publication, identified a striking result emerging from the data: “Unexpectedly, runoff increased twice as much in North as in South Greenland after the 1990s.”

At the same time, the ablation zone in North Greenland grew twice faster than in the South due to a rapid snowline retreat in early summer. Noël explains: “North Greenland is dry, and the snow layer covering the ice at the end of winter is rather shallow. A small temperature increase is sufficient to melt the winter snow away and expose dark ice at the surface.” The researchers conclude that the faster ablation zone expansion in the North is responsible for its enhanced contribution to Greenland mass loss.

The role of clouds

Since the 1990s, a persistent high-pressure system forms more frequently in western Greenland and drives warm and moist air toward the North, increasing cloudiness in early summer. Clouds play a critical role in the ice sheet melt climate, especially over bright snow. Noël elaborates: “In the North, early summer clouds act as a blanket that warms the snow by preventing surface cooling through heat loss. Cloud warming can rapidly melt the shallow snow cover, which amplifies runoff in late summer. This is because dark bare ice absorbs much more solar energy than bright snow.”   

Noël concludes: “This large regional difference demonstrates the strong sensitivity of North Greenland to climate warming. At the current rate, the area of the northern ablation zone could equal that of the South in another 45 years.”

Publication

Brice Noël*, Willem Jan van de Berg*, Stef Lhermitte, Michiel van den Broeke*
Rapid ablation zone expansion amplifies north Greenland mass loss
Sciences Advances, 4 september 2019, eaaw0123
* researchers affiliated with Utrecht University