Using firn cores, radar observations and regional climate models, the study shows that recent melt increases have formed meters-thick, low permeability ice “slabs” that have expanded the Greenland ice sheet’s total runoff area by 26 % since 2001. Ice "slabs" refer to ice layers over a meter thick that form when water refreezes between preexisting ice layers, annealing them together.
At high elevations, a porous snow layer called firn, up to 80 m thick, overlays the Greenland ice sheet. Normally, meltwater can percolate into this firn and refreeze, thereby preventing immediate runoff, but the ice slabs are preventing that process and instead sending the meltwater into the ocean.
Contribution to sea level rise varies under different climate scenarios
How much the ice slabs will contribute to sea level rise in the coming decades depends on the climate scenario the world chooses to follow.
“Although runoff from ice slabs has added less than a millimeter to global sea level so far, this contribution will grow substantially as ice slabs continue to expand in a warming climate. Even under moderate climate projections, ice slabs could double the size of the runoff zone by 2100,” says Mike MacFerrin, researcher at University of Colorado and lead author of the new study.
Co-author Baptiste Vandecrux, post-doc at GEUS, adds:
“We see a big difference in the contribution of ice slabs to sea level rise under different climate scenarios. Under a high emissions scenario like RCP 8,5 the ice slabs could contribute to a rise in sea level with up to 74 mm by 2100. But even under a moderate emissions scenario like RCP 4,5, the contribution to sea level rise could be as high as 33 mm by 2100.”
Data can help predict future emergence of ice slabs
The creation of ice slabs is fundamentally altering the hydrology of the Greenland ice sheet. The new study contributes to a better understanding of the process, enabling researchers to better predict future contributions of the Greenland ice to sea level rise.
“Before, we only had isolated data documenting the state of the firn and no consistent mapping of the ice features. This new study maps the ice slabs over the entire Greenland ice sheet based on an extensive dataset. These new data allow us to understand how they occurred and what amount of snowfall and melt are needed for the emergence of ice slabs in future years,” says Baptiste Vandecrux.
And this kind of knowledge is crucial in estimating the expected future rise in sea level.
“This research is very important to the climate models, we work with at DMI. Refreezing used to be a process that our models had difficulty imitating, but now we are able to describe the process better and thereby get a more correct idea of what to expect in the future in terms of sea level rise,” says Ruth Mottram, co-author and climate researcher at DMI.