Greenland’s mountain glaciers and floating ice shelves are melting faster than they were just a few decades ago and becoming destabilized, according to two separate studies published this week.

The island’s peripheral glaciers, located mostly in coastal mountains and not directly connected to the larger Greenland ice sheet, retreated twice as fast between 2000 and 2021 as they did before the turn of the century, according to a study published on Thursday.

“It got a lot harder to be a glacier in Greenland in the 21st century than it had been even in the 1990s,” said Yarrow Axford, a professor of geological sciences at Northwestern University and a co-author of the paper, published in the journal Nature Climate Change.

Dr. Axford’s team found that glaciers in southern Greenland have become shorter by 18 percent on average since 2000, and glaciers elsewhere on the island have become shorter by 5 to 10 percent.

“These glaciers react really quickly to changes in climate,” Laura Larocca, the lead author and a NOAA postdoctoral fellow at Northern Arizona University, said. Since 2000, temperatures in the Arctic have risen twice as fast as the global average temperature.

The researchers compared historical photos dating back to the 1930s with modern satellite measurements of more than 1,000 glaciers. By converting both types of images into digital maps, the researchers could measure how far the glaciers’ end points had retreated over the years.

Ice melting into the ocean from Greenland is one of the biggest contributors to global sea level rise. Glaciers there are also important locally for maintaining natural ecosystems and biodiversity, and for providing electricity through hydropower. Some Greenlanders even depend on glaciers’ meltwater for drinking water. As the ice melts, more water will temporarily become available, but if warming is left unchecked the ice and its meltwater will eventually run out.

Peripheral glaciers can be an “early warning system” for the rest of Greenland’s snow and ice, Dr. Axford said. The individual glaciers are small compared with the vast ice sheet covering the country’s interior, although some could still dwarf entire cities, and respond more directly to the warming atmosphere. These glaciers only represent about 4 percent of Greenland’s total ice cover, but account for about 14 percent of the island’s ice loss. As a result, they’re contributing disproportionately to sea level rise.

That could change as the ice sheet itself becomes less stable. Greenland’s northern coast is buttressed by floating ice shelves that prevent inland glaciers, which are part of the ice sheet, from flowing freely into the Arctic Ocean.

These ice shelves have shrunk in volume by more than 35 percent since 1978, according to a separate study published Tuesday in Nature Communications. The ice is mostly melting from the bottom, as the ocean underneath the floating shelves warms. Three ice shelves in northern Greenland have already collapsed almost entirely, all within the past 20 years. After one of these collapses, ice loss from the glacier behind the shelf more than doubled.

“It’s like removing bricks from the ice dam,” said Romain Millan, a glaciologist at the Université Grenoble Alpes in France and the lead author of the paper.

The research published this week builds on work by Anders Bjork, an assistant professor of geography at the University of Copenhagen who was a co-author of both papers. He has been traveling to Greenland since 2006 and said evidence of shrinking glaciers was clearly visible all over the landscape.

“The glaciers are retreating, but they are also thinning,” he said. “It’s not just that you have to walk longer to get to the ice, but it’s also that mountains are suddenly poking through.” As ice retreats from Greenland’s coastline, new islands are also popping up in the fjords.

Despite these anecdotal signs, scientists have only recently been able to measure just how quickly ice is disappearing, thanks to more comprehensive bird’s-eye views afforded by satellites and recently discovered archival photos. About 15 years ago, Dr. Bjork and his colleagues found a stockpile of about 200,000 historical photos taken by Danish pilots during early 20th century mapping expeditions in Greenland.

“People had forgotten that they existed,” he said. The pictures provide invaluable data from decades before satellite observations started, allowing researchers to track changes in ice cover further back in time. Other glaciologists have done similar studies recently using historical photos from Svalbard and Switzerland.

The consistent trend across so many glaciers studied in this new research shows strong evidence that global climate change is causing their demise, as opposed to shorter term and more local natural variability, said Ginny Catania, a professor of Earth and planetary sciences at the University of Texas at Austin who was not involved in either study.

“There’s no other way to explain the coincident retreat of hundreds of glaciers,” she said. “When they’re all doing that, that’s notable.”

Understanding these past changes can help climate scientists better predict the future.

Both of this week’s new papers have great promise in improving people’s understanding of future sea level rise, said Twila Moon, deputy lead scientist at the National Snow and Ice Data Center at the University of Colorado Boulder, who was not involved in either study. Researchers would still need to take some intermediate steps to incorporate these findings into climate models and sea level rise budgets, Dr. Moon said, so she expects the process to take time.

The latest Intergovernmental Panel on Climate Change report projects that, by 2050, global average sea levels will likely rise by 7 to 9 inches, or 18 to 23 centimeters. By 2100, they will likely rise by more than a foot or even more than two feet. The behavior of ice shelves is one of the biggest uncertainties currently complicating these estimates.

These two studies together highlight that even the world’s northernmost glaciers are melting rapidly, and that ice shelves right next to them are vulnerable to collapse, Dr. Bjork said. “It’s very clear now that glaciers both big and small, from the smallest glacier to the ice sheet, have gone into a new phase.”