A Greenland glacier is growing. That doesn't mean melting is over.

A pulse of cooler water at its edge let part of the glacier gain some mass. But overall, the melting across Greenland continues apace.

It’s melting and it’s not coming back. That’s something we often hear about our shifting climate and the ice that, in a sense, holds it together. Throughout the Arctic, glaciers are shrinking, right?

Maybe not quite the way we think.

NASA's Oceans Melting Greenland (OMG) project has revealed Greenland’s Jakobshavn Glacier, the island’s biggest, is actually growing, at least at its edge. In research published Monday in Nature Geoscience, researchers report that since 2016, Jakobshavn’s ice has thickened slightly, thanks to relatively cool ocean waters at its base—which have caused the glacier to slow down its melt. This reverses the glacier’s 20-year trend of thinning and retreating. But because of what else is happening on the ice sheet, and the overall climate outlook, that’s not necessarily a good thing for global sea level.

That's because, despite the fact that this particular glacier is growing, the whole Greenland ice sheet is still losing lots and lots of ice. Jakobshavn drains only about seven percent of the entire ice sheet, so even if it were growing robustly, mass loss from the rest of the ice sheet would outweigh its slight expansion.

It may sound a bit confusing, but that’s because the reality of climate change isn’t a straight line, say NASA researchers.

“The thinking was once glaciers start retreating, nothing's stopping them,” explains Josh Willis, an oceanographer at NASA’s Jet Propulsion Laboratory and OMG’s lead scientist. “We've found that that's not true.”

Other Arctic glaciers may be undergoing similar growth. That suggests the ebb and flow of glaciers in a warming world may be more complicated and harder to predict than previously thought, says Willis.

One important factor is that it’s not just about a warming atmosphere. “The water is warming, too,” says Willis. “Oceans are doing a huge part of the work in terms of driving Greenland's ice melt.”

“All this is an indicator of how sensitive glaciers are to ocean temperatures,” says the study’s lead author, Ala Khazendar, a glaciologist with OMG.

Why is Jakobshavn growing? The scientists point to a recent influx of unusually cold water from the north Atlantic pushing into the Arctic. This has been particularly marked in Disko Bay, which spills over into the Illulisat Icefjord, the glacier’s home. At a depth of 820 feet, temperatures have dropped two degrees Celsius since 2014. And that colder water has helped the glacier slow its melt and even grow slightly.

This influx of cold waters isn't an isolated event: Thanks to a natural cycle in the Atlantic Ocean that switches back and forth between warm and cold about once every 20 years, cooler waters are penetrating far up the western coast of Greenland. But the phase will switch again at some point and warmer waters will return.

So, while the overall melt on the continent isn’t on hold—and the overall ice balance of the ice sheet is still declining as the world warms—these cycles show that climate change isn’t necessarily creating impacts in a straight line. It also means that conditions might be a bit more complicated than previously thought for Greenland’s 200-plus glaciers throughout the continent-sized island.

Complex interactions

Glaciers like Jakobshavn extend out into the ocean, which explains how water temperature can impact their size and movement and may mean that the overall melting trend—while still happening—may be happening slower than we thought.

Between 2000 and 2010, Jakobshavn contributed the largest solid ice discharge in all of Greenland’s ice sheet, equivalent to nearly a millimeter to global sea rise. Greenland holds 10 percent of the world's freshwater in a deep freeze. If it melted completely, it could raise oceans about 25 feet (see what would happen if all the world’s ice melted).

Willis says experts had thought the shape of the seafloor is what largely controlled a glacier’s retreat, because the ice can get stuck on undersea ridges that delay its movement. “We found you also have to pay attention to what the ocean’s doing,” he says. And as ice melts on the surface, the water drills through the ice sheet and comes down at the bottom of the glacier.

This mix of fresh and ocean water under the glacier drives melting and increases calving, the real hard-luck consequence of the findings. So, while the glacier is growing, the ice sheet is still losing tremendous mass and contributing to sea-level rise.

“That’s how the warming of the ocean translates,” Khazendar says.

It’s also why despite serious implications for Greenland’s near future ice loss, researchers caution the findings don’t mean we’ve found a magic climate reverse. It’s in fact akin to a pendulum. Jakobshavn had periods of fast retreat in the 20th century, followed by thickening, then retreat, Khazendar says—but on average the air and sea are warming, and that means retreats are always going to be bigger than advances, adds Willis.

Retreating “even faster than it was before”

“Over 90 percent of the heat that’s trapped from greenhouse gases are warming the oceans,” he says. “So we know in the long run, this cooling is going to pass. When it does, the glacier is going to retreat even faster than it was before.”

NASA’s OMG, which began in 2016 to track the ice’s seasonal ebb and flow to help predict global sea-level rise, now plans to determine if that hypothesis is accurate. One way the team determines ice thickness is by flying above the glacier and using an airborne topographic mapper, which employs radar to scan and measure the ice cap at an accuracy of about three feet. Much climate research studies the air. OMG, which is also tagging narwhals to measure ocean depth and temperature, studies the water and the glaciers themselves.

Later this year, NASA’s Grace Follow On mission, which tracks earth’s water in motion, will reveal how much mass Greenland has lost the last two years, which Willis says “may help us learn if this ocean effect is more widespread and could affect mass balance in a positive sense in less ice loss in previous years. We don’t know how to put the Jakobshavn results in context of the whole of Greenland until we get that extra data.”

“I think the story is basically correct,” says David Holland, a New York University professor who was not involved with the research but who has studied ice-ocean interaction at Jakobshavn for 12 years. He thinks this type of ocean interaction could be widespread, from the Arctic to Antarctic.

“You can see in the summer, Jakobshavn’s advancing, not slowing down,” Holland continues. “I think the question is why does the glacier do what it does and I’m of the opinion that the ocean is a dominant player controlling it.”

Some peers aren’t as sure. “I have to say I was a bit surprised by the observation,” admits the University of Alaska’s Martin Truffer, who uses ground-based radar to measure glacial movement.

Truffer believes warmer air temperatures play a role in glacial growth. “What’s important to know is that these glaciers can react much more rapidly to short-term change in temperature. We used to think these ice sheets were pretty slow in reacting but this shows that you can have really fast reactions of glaciers to climate,” he says. “It remains to be seen if this is spreading or not.”

According to Truffer, that spread depends on water temperature, too, which Willis points out will be revealed when the GRACE mission reveals its mass findings in the coming months.

Air temperature probably is important, and so is snowfall, adds Willis. “Warmer air will cause more melting and more ice loss,” he says. “Colder air could cause less ice loss. But we know that what we saw was caused by the ocean because the slowing and thickening is concentrated right where the ice meets the water. The thickening gets smaller and smaller as you move inland.”

A changing planet

The interaction of warm currents eroding ocean-facing glaciers already impacts Antarctica; 10 percent of its coastal glaciers are currently in retreat. Between 1991 and 2016, oceans warmed an average of 60 percent more per year than the Intergovernmental Panel on Climate Change has estimated.

From mineral mining to shipping lanes, fishing and strategic claims ranging from China to Russia, the change in the ice has a myriad of ripples.

A reduction of ice loss in glaciers like Jakobshavn could mean less dangerous icebergs traveling south into the Atlantic, a menace to shipping routes—or it could mean that all the underwater calving could create more ice floes. Twenty billion tons of ice dump into the sea from Jakobshavn annually, more than anywhere besides Antarctica, with bergs scooped by the Labrador Current. Over 1,000 icebergs drifted below 48 degrees N in 2017, and they’re massive—the iceberg that took out the Titanic in 1912 originated here. (Read about a massive new iceberg in Antarctica.)

And to be clear, the Jakobshavn Glacier “is still contributing to global sea rise,” Khazendar says. “That doesn’t stop.”

Adding to the complexity, recent research has shown that from 1979 to 2012, Greenland’s rainfall increased, which triggered sudden melting events. There’s also evidence of warming temperatures moving the position of the snow line, which exposed more bare ice to the atmosphere and contributed to the melt.

Willis says these findings aren’t mutually exclusive to his most recent work. And while it’s hard to argue with the data showing Jakobshavn grew, it’s unclear how widespread that may be and why. “There’s a whole laundry list of possible explanations,” he says.

“But what’s surprising is the ocean; it actually reversed the retreat of this glacier. We didn’t think the ocean could be that important.”

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