Some parts of the Greenland ice sheet are even receiving rain in winter, and these events will spread as climate change continues, say researchers. The research was published on March 7, 2019, in the journal The Cryosphere.
To put it bluntly, rain melts ice. Researchers with Germany’s GEOMAR Centre for Ocean Research and the Columbia University Earth Institute gathered satellite images and data from automated weather stations to learn that Greenland ice melt triggered by winter rain tripled between 1979 and 2012.
Rainfall during the summer months doubled during the same time period. The scientists found that on more than 300 occasions between 1979-2012, rainfall events triggered ice melts. Most of the melting events did take place during the summer months when the temperature gets above zero.
But the study found that winter rain events grew from about two rainfall spells in the winter in the early phase of the study to 12 winter rain events in 2012. Total precipitation over the ice sheet did not change; what did change was the form of precipitation and the increase in winter rain events.
The lead author of the study, Dr. Marilena Oltmanns of the GEOMAR ocean research center in Germany, told BBC News: “We were surprised that there was rain in the winter. It does make sense because we’re seeing flows of warm air coming up from the South, but it’s still surprising to see that associated with rainfall.”
The researchers estimated that nearly a third of the total runoff they observed was due to rainfall. Professor Marco Tedesco of Columbia University in New York said the increased rainfall also has important implications.
He points out that even if it does rain on the ice sheet during the winter – and then refreezes – the rain changes the characteristics of the ice sheet surface. It becomes smoother and darker, pre-conditioning the ice to melt quicker during the summer months.
Remember, the darker the ice, the more solar radiation it absorbs. This, in turn, causes the ice to melt faster. “This opens a door to a world that is extremely important to explore,” Prof. Tedesco said. “The potential impact of changes taking place in the winter and spring on what happens in summer needs to be understood.”
Cyclonic weather events trigger the ice melt
For the time being, the winter rainfall appears to be confined to lower elevations in the south and southwest of Greenland. The rainfall is being brought in by relatively warm ocean winds from the south. The winds are triggered by climate-induced shifts in the jet stream, causing what is called cyclonic weather events.
In other words, the rain is the product of moist, warm wind blowing in from the south, which the researchers say is being propelled by changes in the jet stream caused by climate change. This is important because if average temperatures continue to increase as expected, the line where the moisture comes down as rain instead of snow will rapidly move inward, upward and northward.
“The ice should be gaining mass in winter when it snows,” said Oltmanns, reports Quartz. “Instead, the gains are being overwhelmed by melt.” The research concludes that “despite the involved mass gain, year-round precipitation events are contributing to the ice sheet’s decline.”
