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Melting and refreezing is turning the absorbent surface snow of Greenland into solid ice. This means more water is draining straight into the sea instead of soaking into the snow and refreezing deeper down. Now a study suggests that this will cause an extra sea level rise by 2100 of at least a few centimetres.

“As a human and a father of three, it’s a little terrifying,” says Michael MacFerrin at the University of Colorado, whose team discovered the effect.

The Greenland ice sheet is made of snow. Deeper layers gradually turn to ice, but the surface used to consist almost entirely of porous snow. When parts of it melted, the water sank through the snow and refroze deeper down, forming chunks of ice.

Extracted ice cores and radar observations show that surface melting is becoming so common and widespread in Greenland that these bits of ice are getting larger and joining up to form extensive solid slabs.

“This process really is transforming the surface of the ice sheet in the interior of Greenland,” says MacFerrin.

At present, almost all ice loss from Greenland is a result of glaciers flowing faster into the sea. According to a recent survey of climate scientists, Greenland ice loss alone could add 33 centimetres, or maybe even 100 centimetres, to global sea level by 2100.

In parts of Greenland, however, meltwater now runs over the surface of this ice rather than sinking into snow. This was first observed in 2012, when there was extensive surface melting across Greenland.

Now computer modelling by MacFerrin’s team suggests that meltwater runoff from the interior could add somewhere between 2 and 7 additional centimetres to sea level by 2100 (Nature, ). That is roughly double previous estimates that don’t take this slab-forming effect into account.

The good news is that it isn’t a runaway process that can’t be reversed, such as the now-inevitable collapse of the West Antarctic ice sheet. If surface melting was reduced, a porous snow layer would build up again.

“This is completely dependent on atmospheric temperatures,” says MacFerrin. “If you stop the warming, you stop this effect.”