Like a snake slithering back into its den, Greenland's lengthy Jakobshavn glacier has retreated over 25 miles since the 19th century. And for the last two decades, this warming river of ice has purged more ice into to sea than any other Greenland glacier.

But since 2016 — and after 20 years of unprecedented melting in Greenland — Jakobshavn's rapid retreat has slowed down considerably and the glacier has even grown bigger. This might appear to be a rare dose of good news for the Arctic — a place that's heated up over twice as much as the rest of the planet.

But no.

Instead, a team researchers led by scientists at NASA's Jet Propulsion Laboratory discovered that Jakobshavn's stagnated melt is only a temporary blip brought on by cooler ocean currents. Though worryingly, the recent slowing also carries ominous news for the thawing landmass. The research, published Monday in the journal Nature Geoscience, reveals that the Jakobshavn glacier — whose ice reaches some 2,600 feet under the sea — is extremely sensitive to changes in ocean temperature. That's a big problem because the dynamic ocean currents off western Greenland will naturally warm up again — on top of the reality that Earth's absorbent seas soak up over 90 percent of the planet's accumulating heat. These incessantly warming waters spell a grimmer future for both Jakobshavn and Earth's rising seas.

"The big story here is the ocean," said Josh Willis, a study coauthor who heads NASA's Oceans Melting Greenland mission. "The ocean is playing a powerful role in driving the ice loss in Greenland, particularly Jakobshavn."

"This study does not mean we are out of the water."

"If these deep glaciers are this sensitive to the water, then we could be looking at faster sea level rise out of Greenland than we thought," added Willis, an oceanographer.

Other Greenland experts agree that Jakobshavn's recent stagnation is not optimistic news.

"This study does not mean we are out of the water," said Luke Trusel, a geologist at Rowan University who had no role in the study. "In fact, I’d say it says the opposite by demonstrating just how sensitive this major glacier is to changes in the ocean." Trusel recently visited and published research on Greenland's accelerating melt.

Jakobshavn's stark retreat since the 1850s. Image: Nasa

Jakobshavn is a big actor in the planet's future and Greenland's stability — a land that holds enough ice to raise sea levels by 23 feet. That's because Jakobshavn isn't a normal glacier.

The abysmal ice river fills a canyon that penetrates deep into the heart of Greenland. Today, Jakobshavn acts like a plug or cork keeping much of Greenland's colossal ice masses at bay and locked into the landmass. But as scientists found, the ocean may progressively dissolve Jakobshavn's cliff-like face and start unleashing this ice in the coming decades.

"When I say it's like the cork on a champagne bottle, this really is a channel that potentially could tap the ice on the rest of the ice sheet," said NASA's Willis.

Understanding how quickly glaciers like Jakobshavn will melt as Earth continues its accelerated warming trend is the trillion dollar question, noted Willis. The same can be said of glaciers in Antarctica. So top research agencies like NASA, the National Oceanic and Atmospheric Administration (NOAA), and a number of university and research institutes around the world have undertaken missions to these remote polar worlds, often flying planes over the hard-to-reach lands.

Willis and his team determined Jakobshavn had slowed by combining radar from airborne expeditions with satellite images of the great ice river. They found that over the last couple years, temperatures in Disko Bay, which sloshes against the great glacier, dropped by 2 degrees Celsius (3.6 degrees Fahrenheit).

But while detailed projections of Greenland's melting are under heavy investigation, the greater picture in Greenland — continued, accelerated melting — is already clear.

The Jakobshavn glacier. Image: google Earth

"This tipping point has started and there’s no going back," said Michael Bevis, a professor of geodynamics at Ohio State University who had no role in the study.

Greenland's glaciers have reached a point of no return, he emphasized, because the colossal ice sheets are getting hit from both above (the air) and below (the oceans). While the sea has dislodged massive of chunk's of ice from Jakobshavn's cliff-like face, warmer air melts the ice atop Greenland, creating increasingly vigorous blue rivers that pour into the ocean. After a half-century of steady glacial runoff, things changed dramatically in 2003, said Bevis.

"This tipping point has started and there’s no going back."

"The amount of melting in the summertime just took off," said Bevis, who recently published Greenland research of his own.

What's more, he emphasized, Greenland's melting will become all the more exacerbated when widespread, decades-long temperature shifts in the Atlantic Ocean, called the Atlantic Multi-decadal Oscillation, or AMO, bring back warmer waters to the region in the coming decades. This well-understood shift in the AMO, combined with the ocean's continued absorption of Earth's accumulating heat, will bring a "double whammy" of melt to Greenland, Bevis said.

Overall, though, it's important to remember that Greenland's melting — and that of its major glaciers — will have some ups and downs within the greater accelerating melting trend.

"Think of this variability as dips and crests in a road that is taking you up a mountain," explained Trusel. "It would be incorrect to think that because you are going down a slight dip in this upward path that you are no longer climbing the mountain."

Jakobshavn fills a channel that leads to the ice-filled center of Greenland. Image: University of CAlifornia Irvine Ice Sheet Modeling Group

"The longer term trends cannot be ignored here," emphasized Trusel. "As emissions of greenhouse gases continue, the atmosphere and ocean will warm."

But this latest research means that the warming oceans — which wash right onto and beneath the Jakobshavn and other Greenland glaciers — will likely have an outsized role in adding to the planet's overall sea level rise. Our best current estimates for the globe's total sea level rise by century's end show the ocean is on track to rise by over two feet by 2100. But it could be as much as six feet.

"The short answer is we'll have to revise the projections upward," said Willis, referencing the current projection of over two feet of sea level rise.

While the planet's carbon emissions continue to rise — and may not even peak for over a decade — Jakobshavn will continue to shed great masses of ice, even if that melting has temporarily slowed.

"Imagine an iceberg spanning the length of several New York city blocks," said Trusel, who witnessed Jakobshavn's icebergs floating out to sea. "They’re awe-inspiring and illuminate the magnitude of changes happening to the Greenland ice sheet."