As global temperatures rise, rainy weather is becoming increasingly more common over the Greenland ice sheet when it should be snowing, eating away at the ice, triggering sudden melting events, and priming the surface for future melting.
Normally, a falling snowflake stays frozen when it lands on the 1,710,000-square-kilometer (660,000-square-mile) sheet, but when rain falls, the warmer liquid water melts snow and the ice below it. Some parts of the ice sheet are getting rained on even in the winter time – and it’s likely to get worse as climate change continues to warm the planet.
It’s no secret that Greenland has been losing its ice. Since 1990, the average temperature has increased by as much as 1.8°C (3.2°F) in the summer and 3°C (5.4°F) in the winter. Estimates suggest the Arctic nation is losing around 270 billion tons of ice each year, with direct meltwater runoff accounting for as much as 70 percent of the loss.
Publishing their work in Cryosphere, researchers paired satellite imagery, which can show the difference between snow and liquid water, with observations made on-the-ground between 1979 and 2012 in order to determine what was causing melting in specific places. They then looked at observations from automated weather stations that were recording data on temperature, wind, and precipitation and found that more than 300 melting events were triggered by rain over the course of their study period.
Melting can be triggered in a variety of ways, but liquid water in the form of rain is particularly powerful. Warm air associated with rain can melt ice directly, but it creates a snowball effect. Rain has more heat than snow, so when it drops to the surface, it melts the snow and ice around it. According to lead author Marilena Oltmanns, melting associated with rain doubled in the summer and tripled in the winter, despite the fact that total precipitation didn’t change – just the way that it fell.
The researchers note that the cycle comes in pulses, increasing over the last four decades from two to three days in the winter and more than double in the summer, from two to five day, altogether contributing to a vicious cycle. When the meltwater refreezes, it changes the surface of the ice sheet from fluffy white snow to darker ice pools that can better absorb heat from the sun, resulting in faster, heavier melting events when the sun does come out.
“If it rains in the winter, that preconditions the ice to be more vulnerable in the summer,” said glaciologist Marco Tedesco in a statement. “We are starting to realize, you have to look at all the seasons.”
Similar events have been observed in the Canadian and Alaskan tundra, where increasing spring rain thaws permafrost and releases mass amounts of the greenhouse gas methane.