Global warming17.09.2019

In Greenland, global warming is intensifying the runoff of meltwater towards the ocean

An international team of researchers, including Horst Machguth of the University of Fribourg, has shown that global warming promotes the formation and expansion of of ice slabs embedded within the arctic ice sheet. By preventing the water from melting snow from permeating through them, these slabs increase the rate of runoff and ultimately risk contributing to sea level rise. The team’s observations have been published in the prestigious journal Nature.

This is a phenomenon which is gaining momentum in Greenland: there are more and more thick and impermeable slabs of ice in the interior of the ice sheet. Rendered less porous, it loses its capacity to absorb meltwater which then flows away until it reaches the ocean. This is how – according to a study led by the University of Boulder and in which the University of Fribourg participated – Greenland could bring about an additional sea level rise which could reach 74 mm by 2100.

Currently, the researchers are only attributing a rise of less than 1 mm in global sea levels do to the existence of these ice slabs. However, this could change with global warming: «Even with a moderate rise in temperature, these slabs of ice could double the surface area of the runoff zones by 2100» explains Mike MacFerrin, researcher at the University of Boulder.

Rapid expansion of the runoff zone

In 2000, the Greenland runoff zone was equal to the area of Poland (313,000 km2). From 2001 to 2013, this zone grew by an additional 65,000 km2­, which corresponds to a rate of two football fields a minute. By 2100, assuming continued global warming, the runoff zones could, in a moderate emissions scenario, increase by an area equal to that of Colorado (270,000 km2), which would bring about an additional sea level rise of 7 to 33 mm.

In a scenario with greater greenhouse gas emissions, the runoff zone could increase by an area equal to that of Texas (696,000 km2), contributing to a sea level rise of 17 to 74 mm. To the runoff caused by the ice slabs must be added other factors contributing to ocean level rise, such as iceberg calving.

The ice sheet, a less and less absorbent sponge
The Greenland ice sheet can be compared to a «mille-feuille» pastry:  ponds of meltwater litter its surface, snow falls each winter and the layers of old snow, affected by the pressure, gradually turn to ice. Over the greater part of Greenland, the snow only melts partially each summer, the meltwater then refreezes within the layers of compacted snow in the form of small ice lenses with a thickness of 1 to 5 cm. In the past, thanks to the porosity of the near-surface layer of the ice sheet, meltwater could penetrate and refreeze on the spot instead of reaching the sea.

But episodes of extreme melting are becoming more and more frequent: in July 2012, surface melt affected more than 97% of the surface of the Greenland ice sheet, an event never before observed in 33 years of satellite observation. This spring, which was particularly warm and sunny, brought about the melting of 80 billion tonnes of ice. A record! These extraordinary melt events are not without consequences for the ice lenses which grew over the past decade and have merged into gigantic slabs of 1 to 16 metres in thickness, creating an impermeable lid just below the surface of the ice sheet. Not being able to permeate any deeper, the surface meltwater starts its journey to the ocean.

Theory confirmed by observations in the field
Several decades ago already, researchers had attempted to estimate the impact of a rise in temperature on the meltwater and snow cover of Greenland.

«What is interesting», points out Horst Machguth, a researcher at the University of Fribourg, «is that their hypotheses were based on measurements and theory. And now, our observations in the field support them to a great degree!»

It remains to be established to what extent the ice slabs are going to contribute to sea level rise. A few millimetres or several centimetres? «That’s up to us humans to decide!» concludes MacFerrin.