The retention capacity of Greenland’s ice sheets is threatened by the formation of thick ice « slabs ». This phenomenon, which tends to increase with Global Warming, could lead to a faster rise of the sea level than previously predicted by climate models. The record melting of the Greenland ice sheet this summer is likely to add a further boost to this phenomenon, uncovered by an international team of researchers, including scientists from the Climatology Laboratory (SPHERES Research Unit/Faculty of Sciences of ULiège). This research is published in the scientific journal Nature.
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reenland's snowpack is a complex mosaic of frozen textures composed of lakes of meltwater scattered over its surface, snow that falls each winter and old compacted snow (firn) that slowly compresses into ice. The ice sheet, which until now has been able to "retain" around 50% of the meltwater, has gradually turned into impermeable ice lenses in recent years, preventing meltwater from being trapped in the ice sheet and thus promoting its discharge into the ocean. This mechanism, which could accelerate the rise in sea level, has just been identified by a research team led by the University of Boulder (Colorado, USA) and involving Xavier Fettweis, a FNRS Research associate at the Laboratory of Climatology (SPHERES Research Unit /Faculty of Sciences) of the University of Liège.
It was by sampling during an expedition in 2012 (previous melting record before 2019) that scientists accidentally discovered large sections of ice lenses/slabs in ice cores instead of finding small thin lenses as observed each summer. Unfortunately, these melting episodes are becoming more and more frequent in Greenland. By July 2012, snow and ice had melted on 97% of the surface of the ice sheet, an event that had not been observed for 100 years. This summer, once again, after the air mass that generated the heat wave in Belgium at the end of July reached Greenland, the ice cap lost about 13 Giga Tonnes (13,000,000,000,000 kg) of water in a single day (August 1st), which is the highest ablation rate ever observed so far! Integrated throughout the summer, the surface melt anomaly represented around 350 Giga Tonnes (350,000,000,000,000,00 kg) which is what the IPCC predicts for the summers of the 2050s under the worst-case climate scenarios.
On the left : Researchers are analyzing ice cores collected in 2012. (Photo: Babis Charalampidis/Bavarian Academy of Sciences and Humanities, Germany) On the right : Researchers are preparing to operate a snowmobile equipped with a ground penetrating radar to measure the extent of Greenland's ice slabs in 2013. (Photo: Karen Alley/CU Boulder/Wooster College)
Over most of Greenland, the snow melts only partially each summer and then turns into thin ice discs or "lenses" 2.5cm to 5cm deep, embedded in the compacted snow. "Normally, meltwater can seep down and around the ice lenses/slabs and refreeze in the same place in winter without flowing to the sea and thus not contribute to a rise in sea level," explains Michaël MacFerrin, a researcher at the University of Boulder who led the study published in Nature(1). But as melting in the Arctic becomes more frequent and intense, these delicate layers of ice expand and solidify into "patches" 1 to 16 metres thick, creating an impermeable layer just below the surface." Meltwater that can no longer infiltrate the ice sheet then flows along these ice lenses and directly into the ocean. This phenomenon, which is highly likely to accelerate the sea-level rise, shows how much and how quickly Global Warming can change one of the most vulnerable regions of the Earth where we currently observe a warming of +1°C every 10 years, i. e. a warming rate 10 times faster than the one observed in Belgium !
In order to understand this phenomenon, the research team submitted the collected data to various climate models, including the regional MAR climate model, developed at the Climatology Laboratory (SPHERES Research Unit / Faculty of Science). "We have tried to understand how ice lenses have spread over the past few decades in our model in an attempt to predict how they could continue to grow and amplify the melting of the ice sheet," says Xavier Fettweis, director of the Laboratory of Climatology and co-author of the paper.
According to a new assessment by this group of researchers, these ice slabs, which reduce the retention capacity of the ice cap's meltwater, would threaten to increase sea level rise by more than 7 cm by 2100 compared to IPCC projections, which predict a Greenland contribution of ~ 20 cm in the worst case scenario. "It is highly likely that by 2100, as global temperatures continue to rise and these ice patches thicken, the runoff area could increase by a factor of 10, accelerating sea level rise," explains Xavier Fettweis.
This animation explains the change in the composition of Greenland's snowpack due to climate change. Recent atmospheric warming is altering the ability of the ice sheets on the ice cap to store meltwater, resulting in a more rapid release of runoff to the ocean.
Scientific reference
Greenland's sponge effect affected by climate change
MacFerrin & al., Rapid expansion of Greenland’s low-permeability ice slabs, Nature, september 2019
Xavier FETTWEIS I Laboratory of Climatology I SPHERES Research Unit
