Greenland and its ice capsImportant contributors to sea level rise
The Greenland ice sheet (GrIS) and peripheral glaciers and ice caps (GICs) contribute 43% to contemporary sea level rise. Although Greenland's GICs only cover 90,000 km2, or 5% of the total ice sheet area, they account for 14% of Greenland total mass loss.
Important but uncertain
Despite the importance of the GICs for sea level rise, our current understanding of their mass loss is limited to sparse field campaigns, some dating back to the early 20th century, and satellite- or model-based estimates. However, these approaches often fail to account for the variations over the steep and complex terrain in which most GICs are located. They also fail to provide insight in the drivers of mass loss.
A high-resolution approach
In our study we use a novel, high-resolution (1 km) climate model product that allows for the first time to identify the physical processes behind the GICs mass loss. Evaluation of this product using in-situ data and satellite observations shows very good agreement.
A dry climate
Most of Greenland's GICs are found in dry regions, where summer melt (ME) exceeds precipitation (PR) from snow and rain. However, a large part of the meltwater is refrozen (RF) in the snow and firn layers covering the ice caps; the remainder runs off to the ocean (RU). When an ice cap is healthy, refreezing buffers any increase in meltwater, and the ice cap's mass remains stable. Therefore, refreezing of meltwater is a key process to sustain these ice caps.
Discover the change in mean SMB before/after 1997 by pushing the play button at bottom left of the page
1997 as a tipping point
A healthy ice cap has a large inland region where mass is gained at the surface (blue), and narrow marginal regions where mass is lost (red).
Before 1997, these two areas were in balance and the ice caps remained stable. After 1997, this situation changed dramatically and Greenland's GICs crossed a tipping point; mass gain in the interior no longer compensated mass loss at the margins. Increased meltwater runoff tripled the mass loss of Greenland's GICs to 36 +-16 Gt/yr.
Changes in the firn layer
The main reason for the post-1997 increase in mass loss is the reduced refreezing capacity of the firn layer, the porous, multiyear snow layer
that covers the ice. A healthy firn layer acts as a meltwater sponge, retaining the meltwater before it refreezes.
Decades of increased melt have saturated the firn and its refreezing capacity has decreased to such a degree that refreezing can no longer compensate for increased meltwater production. As a result, more meltwater runs off to the ocean, accelerating the mass loss.
Ice caps versus ice sheet
We found a striking difference between the response of the GICs and the main Greenland ice sheet to atmospheric warming. Since 1997, refreezing continued to increase on the main ice sheet, buffering an important fraction (22%) of the excess meltwater. In contrast, refreezing decreased on the GICs. As a result, runoff there outpaced meltwater production by 65% since 1997.
All regions in Greenland experienced warming (higher T2m), reduced refreezing capacity (lower RF/ME) and an increase in runoff (RU), but a marked contrast is found between GICs in north (orange boxes) and south Greenland (black boxes). Northern GICs experienced a significantly greater warming and increase in runoff than southern GICs.
Discover the change in mean runoff before/after 1997 by pushing the play button at bottom left of the page
A saturated firn layer
In north Greenland, where the changes are largest, all but the very highest parts of the GICs now regularly experience runoff. This means that the firn layer is no longer capable of refreezing all meltwater that is produced in summer.
From this state, it would take decades to regrow a healthy firn layer. That is why we call it a tipping point.