Despite the warming of Earth's climate and the rapid melting of the Greenland ice sheet in the coastal regions, approximately 40 percent of the ice sheet shows no signs of surface melting. Now, scientists from the University of Copenhagen have revealed the reason for this strange phenomenon: an insulating layer of air that forms near its surface during the winter, insulating the ice sheet from the upper area of the atmosphere and reducing evaporation and precipitation.

The team examined the atmosphere over the three-and-a-half kilometer thick ice sheet instead of using data from direct observations of its surface due to the uncertainty that this form of analysis can create.

"We decided to investigate whether you could find the answer in the atmosphere above the ice sheet by measuring the atmospheric processes directly," said Hans Steen-Larsen of the University of Copenhagen and co-author of the study. "We therefore took measurements of the water vapor in the atmosphere for three years."

Through the examination of stable water isotopes in the water vapor in the atmosphere up to a height of 40 meters above the surface of the Greenland ice sheet, the team was able to obtain that ratio between the most common isotopes of oxygen: 16O and 18O.

The isotope ratios revealed that there are extremely stable weather systems in the atmosphere above the ice sheet during winter periods: near the surface is cold and dry and the air is stagnant, whereas higher up - approximately 100 meters - the air is more variable and warmer.

"By measuring the isotopic composition of the water vapor, we discovered that there was a very clear distinction between the water vapor that was associated with the ice and the water vapor that was blown in higher up in the atmosphere," Steen-Larsen said. "There was a high content of the oxygen isotope 18O both in the lowest layer of air near to the ice and in the higher layer of air. But in between the two air layers at a height of two to four meters there was a layer with a lower content of 18O."

"So there was a boundary layer that separated the two layers in the atmosphere," he added. "We were thus able to see a direct disconnection between the surface of the ice and the atmosphere above it - it would not have been possible to discover this if we had only looked at the water vapor amount in the air."

The team's findings reveal why there is less precipitation than expected on the Greenland ice sheet.

"We have simply discovered an important process that helps to explain why there is no relationship between temperature and precipitation on the Greenland ice sheet," Steen-Larsen concluded.

The findings were published in the April 29 issue of the journal Science Advances.