Temperatures in Arctic regions are rising about twice as fast as the rest of the world. In Svalbard, changes in precipitation patterns over the last few decades have increased the likelihood of winter rain and snowmelt forming a layer of ice at the base of the snow. Such basal ice has severe consequences for animal species that survive winter by finding plants beneath snow.

Bart Peeters of the Norwegian University of Science and Technology and colleagues from elsewhere in Norway have observed Svalbard’s snowpack since 2000, obtaining in situ results that are lacking elsewhere in the Arctic. By comparing their measurements with contemporary records of winter snow and rainfall, the researchers identified the factors that affect the occurrence and thickness of basal ice.

Winter snowpack can absorb a limited amount of rain or melting snow without much effect, but when the snow becomes saturated, water percolates to the base and refreezes. An increase in precipitation, and a larger proportion of that falling as rain, are known consequences of a warming Arctic, so thicker, more widespread basal ice is not unexpected.

“The surprise,” says Peeters, “is that the frequency of rainy and icy winters has changed rather suddenly.”

The general rule for basal ice, the team found, is that heavy winter rainfall results in a thicker layer, as predicted. The picture is complicated, though, by the depth of the overlying snowpack.

Moderate amounts of rain falling on thick snow can refreeze within the snowpack and never make it to the ground. When the rain is heavy enough, however, latent heat transfer boosts snowmelt, making it more likely that the snowpack becomes saturated. This means that a deeper covering of snow can contribute more water, increasing the thickness of the basal ice.

When the researchers looked at this precipitation-ice link in weather records back to the 1950s, they found a marked increase in the likelihood and thickness of ice after 1998.

“What’s striking is that rainy and icy winters on Svalbard have occurred almost every year since the turn of the century, whereas before that winters without rain and ice occurred about every three to four years on average,” says Peeters.

The rate of high Arctic warming means that the pattern could switch again. Under global average increases of 1.5 or 2 °C, rising permafrost temperatures at sensitive sites like Svalbard could soon reach the point at which percolating rain and meltwater no longer refreeze at the base of the snow layer.

“There is a very large chance that this tipping point will be reached, as winters are getting shorter and much warmer on average,” Peeters says. “This is a reason we did not interpolate our results for the future in this study, as this would require more complex models and a better understanding of the link between the climate and cryosphere.”

However long the current regime persists in Svalbard, it’s clear that what happens there first can be expected elsewhere in the Arctic. “Continued monitoring is therefore crucial as well as increased spatial resolution and quality of meteorological, cryosphere and permafrost data,” says Peeters.

Peeters and colleagues reported their findings in Environmental Research Letters (ERL).