Stop me if you’ve heard this one before. A new study reports that a major glacier is shrinking at an accelerating rate and is likely to continue doing so into the future. In the last couple of years, we’ve covered studies reaching similar conclusions about large portions of West Antarctica and Greenland’s Jakobshavn Glacier. We’ve even looked at concerns about glaciers in comparatively stable East Antarctica that are vulnerable to this sort of situation in the future.

All these glaciers share a common destabilizing feature: a bed (valley) that drops farther below sea level as you move inland. Once one of these glaciers gets nudged down the hill, it can rapidly and irreversibly retreat until it reaches the relative safety of a point where the bed slopes upward again. The deeper the water, the greater the buoyant force counteracting the glacier’s weight and the smaller the friction that slows the glacier as it slides into the sea.

About 12 percent of the Greenland ice sheet flows toward the ocean through a pair of neighboring outlet glaciers on the northeastern coast—Zachariae Glacier and the alphabet soup that is Nioghalvfjerdsfjorden Glacier. A team of researchers led by University of California, Irvine’s Jeremie Mouginot has pulled together a wide variety of satellite datasets to evaluate the health of these two glaciers. One is feeling much worse than the other.

Let’s look at Nioghalvfjerdsfjorden first. Between 2002 and 2012, the front of the floating ice shelf at the glacier’s end retreated by a few kilometers, and the ice shelf thinned by roughly 30 percent. The glacier’s grounding line—the critical point where the ice is thin enough to begin floating and loses contact with its bed—retreated a kilometer between 1992 and 2011, but it hasn’t budged since.

The flow velocity of the glacier has picked up about eight percent since 1976, mostly in the last decade. But combine all the measurements (including snowfall farther inland) for an estimate of the glacier’s total mass, and it has stayed roughly constant over that period. That’s the good news.

Zachariae, on the other hand, is the bearer of bad news. Its floating ice shelf had been pretty stable until about 2003, when a big chunk cracked off. From there it fell back for about a decade, and then in 2012 one of the two main lobes of the shelf lost contact with the glacier, causing further retreat. An astounding 95 percent of the ice shelf has been lost since 2002—and ice shelves help to hold back the glacial ice behind them.

Zachariae’s grounding line has retreated 7km since 1992, and half of that happened after 2010 as the rate of retreat picked up greatly. The glacier has been thinning faster since then, too. A few kilometers inland from the grounding line, the surface is now dropping away by about 5m per year as the glacier deflates. That came with a 50-percent velocity increase between 2000 and 2014, and again, half that acceleration came in the last three years.

The mass estimate for Zachariae is less rosy than Nioghalvfjerdsfjorden’s. It had been more or less steady until about 2003, but it has been losing about 5 gigatons of ice per year since then. Why the difference between the two glaciers? The researchers built pictures of the topography beneath each of them and found that while Nioghalvfjerdsfjorden sits in a valley that rises as you head inland, Zachariae’s valley sports some of those destabilizing depressions. Back in 1996, its grounding line sat on a ridge that it has since retreated from, down into a deeper section that extends about 30km inland to the next area of higher ground.

The melting that caused Zachariae Glacier to shrink came from both warming air above and warming water below the ice shelf. Given the continued warming and the shape of the valley beneath it, the researchers write, “We project that [Zachariae Glacier] may continue retreating rapidly for another 20-30 years.” Its retreat probably won’t stop then, but it should slow. They note that two other major areas of Greenland with outlet glaciers in vulnerable valleys—including the Jakobshavn and Petermann Glaciers—are also shrinking rapidly. “The retreat of these marine-based sectors is likely to increase sea-level rise from Greenland for decades to come,” they write.

Science, 2015. DOI: 10.1126/science.aac7111 (About DOIs).