The news raised memories of the record melt season in 2012, when the ice sheet as a whole lost 562 gigatons, or billion tons, of freshwater mass to the ocean, enough to raise sea levels the world over by more than a millimeter in that year alone.

That included a moment in July of that year when in just a few days, the vast majority of the ice sheet’s surface became awash in at least some surface water, captured in this now iconic image from NASA:

The 2012 melt season, according to the National Snow and Ice Data Center, experienced melting even at mountainous heights over a mile into the sky atop Greenland. “During a peak melt event in July, even the summit areas of the ice sheet, nearly two miles above sea level, saw snowmelt conditions,” the group wrote. “While this has been observed in ice cores a handful of times in the past 1,000 years, it had not previously occurred in this century.”

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In recent years Greenland has been losing so much mass that, along with changes in Antarctica and the amount of water stored on the globe’s continents, it appears to be having a subtle but measurable effect on the rotation of the Earth itself.

Researchers attributed the surprising early melt this year to weather conditions, and more specifically, a warm midlatitude air mass getting stuck over the ice sheet. “The strong south air flow onto Greenland has produced warm air, rain instead of snow, and melting of snow that is in Nuuk,” Greenland’s capital, Jason Box, a researcher with the Geological Survey of Denmark and Greenland, said by email.

This is similar to what appears to have happened in 2012, according to new research by Box and his colleagues, who found that it wasn’t just direct solar radiation that caused the bulk of the melting in 2012, but rather warm and cloudy weather conditions.

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“Even weather stations quite high up on the ice sheet observed very high temperatures on Monday,” said Robert Fausto, also a researcher with the Geological Survey of Denmark and Greenland, according to Polar Portal.

Granted, with an early season melt event like this, it’s important to emphasize that total mass loss is likely to be low, says Marco Tedesco, a Greenland expert at the Lamont-Doherty Earth Observatory at Columbia University.

“This sort of event, early in the season, has a negligible effect on the mass loss, because of the water refreeze,” Tedesco says. However, it could be very important for later in the melt season, he adds, because of the way an early melt sets the stage for future events.

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“These events are local scale, very short term, it’s like a pulse of energy,” Tedesco says. “But the potential implications, in terms of runoff and so on, they alter the memory of the snowpack, the potential implications can be big either for the same season, or future seasons.” He says researchers need to learn a lot more about how warm mid-latitude air is making its way to Greenland and contributing to these types of melt events.

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Unlike the great ice sheet of Antarctica, the Greenland ice sheet is melting both on its surface and also at outlet glaciers that drain the ice sheet’s mass through deep fjords, where these glaciers extend out into the ocean and often terminate in dynamic calving fronts, giving up gigaton-sized icebergs at times.

Surface melt can interact with this process, because water atop the ice sheet flows in surface rivers and then plunges deep into so-called moulins that lead it below the ice sheet, where it can lubricate and speed up its flow.

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The April melt in itself is still not a huge part of the ice sheet – and not at its thickest portions – but it raises questions about what will happen when summer peaks, and along with it, melting.