A heat wave is turning Greenland’s ice to slush. That's bad news.

On the Greenland ice sheet, high at the top of the planet, things are not well.

The heat wave that wreaked havoc on Europe in late July has now migrated northward, parking itself over Greenland. As air temperatures over the ice rise, the ice sheet is responding in the only way it knows how: by melting. By Tuesday, over half of the surface of the Greenland ice sheet had softened to slush.

This is the second major hot stretch to hit the ice sheet this season, and the second to cause melting across major swaths of the ice sheet. The heat waves were particularly impactful because they arrived after mild, dry winter and spring seasons that primed the ice sheet for melt. The result of this brutal setup is a summer melt season so intense that it’s on track to tie or break the record for the most water loss ever recorded.

Already, the melt extent is well outside the range of what’s normally observed at this time of year, said Ruth Mottram, a polar scientist at the Danish Meteorological Institute.

“These are records we don’t want to see broken,” she says.

This will soon be unexceptional

So far, 2019 is shaping up to rival the worst melt year on record: 2012.

At its worst point, in July of 2012, 97 percent of the surface of the Greenland ice sheet was experiencing some sort of melt, scientists estimated—even the icy, more-than-mile-high interior of the ice sheet, which rarely had been seen to soften.

Melt coursing off the ice sheet caused rivers to gush so intensely that they blew out bridges in coastal towns. Scientists stepped outside their stations only to sink deep into the unexpectedly soft surface.

By the end of July 2012, the ice sheet had lost about 250 billion tons of melt, enough to tick global sea levels up by about eight tenths of a millimeter.

That’s pretty close to the estimated 248 billion tons that have been lost so far this year, according to observations made by Marco Tedesco, a polar scientist at Columbia University’s Lamont Doherty Earth Observatory.

“We’re basically on pace. We’re in the ballpark of the 2012 record,” he says.

On Tuesday alone, the ice sheet shed about seven billion tons of melt. That’s enough water to fill almost three billion Olympic swimming pools, and about 1.75 times as much as it might lose per day in the average year.

That may not sound like a lot, but it adds up quickly. Global average sea levels have crept up by 7 to 8 inches over the past century, with about 40 percent of that rise occurring since 1993. Melting ice contributes more and more to that rise each year, and the rate of melt has potentially sextupled since 1979.

By 2100, accelerating ice loss and surface melting from Greenland’s ice could dump somewhere between about 2 to 13 inches of extra water into the seas. If the whole ice sheet were to melt away—not something likely to happen in the near future in any scientific projections—global sea levels would rise by about 23 feet.

This season alone won’t make or break global sea levels. But this season, on top of many others like it, will have an impact.

A self-reinforcing loop

Parts of the Greenland ice sheet melt every summer, as Earth tilts its northern face toward the sun. At the edges of the ice sheet, that translates to gushing rivers and limpid blue pools of meltwater dotting the ice’s surface.

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But usually, the melt is confined to those fringes, and the loss from melt is balanced—at least partially—by snowfall. Fresh snow also helps the ice sheet stay bright and reflective, bouncing away incoming solar radiation. Older snow, explains Mottram, loses the sharp snowflake edges and fluffy texture that help reflect away the light, instead congealing in a more solid, less-reflective mass that soaks up the sun’s heat.

Over the winter, very little snow fell across the western edge of the ice sheet. That meant that great swaths of the ice sheet were already darker than normal when the summer heat started to set in.

So when the first heat wave of the summer arrived in June, up to about 45 percent of the ice sheet hit a melting point—well above the 10 percent usually observed at that time of year. Air temperatures hovered some 10 to 15 degrees Celsius above normal. During that one hot nine-day stretch, the ice sheet lost about 80 billion tons of melt. By the time the heat passed, scientists were speculating that this season could break records.

And because partially melted ice is darker than fresh snow, that event meant that the ice sheet was vulnerable to any more heat waves that might come along.

“It’s like a kind of preconditioning for later melt,” says Mottram.

When the most recent heat wave started to move north, scientists were prepared to see massive melting start up again across the island. So far, the scenario is playing out how they predicted.

Is the future now?

Scientists already know that climate change has a hand in lengthening and intensifying the melt seasons in Greenland; in 2012, melting around the edges of the ice sheet started about a month earlier than it had three decades prior.

The big question, though, is whether this kind of extreme summer melt will soon become commonplace.

Some scientists, like Xavier Fettweis, an ice sheet modeler at the University of Liège, think so.

The volume and intensity of melting occurring today, he says, is on par with what models predict will occur nearly every summer by 2050. So record-breaking summers like this one, or 2012, will cease to be remarkable. We’re already on that path, he says.

“This is not just one hot summer among a lot of cool ones,” he said. “This kind of anomaly has been repeated this year, and similarly in 2016, 2012, 2011, 2009, 2008, and so on.”

The exact details of why melting is accelerating across the ice sheet are still being worked out. But part of the reason, some scientists think, has to do with the jet stream, the wavy band of air currents that snakes around the planet, locking into place weather patterns on either side of it.

The shape of the Northern Hemisphere jet stream is at least partly controlled by the difference in temperature between the equator and the Arctic. The Arctic has warmed much more rapidly than the rest of the planet—some 1.8 degrees Celsius in the last 20 years alone—and some scientists think that the result is a jet stream that is wavier and more sluggish than before.

That means that weather patterns—like the heat wave that baked Europe last week, or the devastating rains that flooded the U.S. Midwest in the spring—get stuck in place for longer, said Jennifer Francis, an atmospheric scientist at Woods Hole Research Center.

And in turn, that means more opportunity for melting when the hot phases get stuck over Greenland.

“We’ve seen these records falling more rapidly as time goes on,” says Francis. “They’re very clearly connected to the extra greenhouse gases in the atmosphere—they’re a very obvious symptom of climate change.”