“We find the 2001-2011 decade the warmest of the whole period of 1,000 years,” said Maria Hörhold, the study’s lead author and a scientist at the Alfred Wegener Institute in Bremerhaven, Germany.

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And since warming has only continued since that time, the finding is probably an underestimate of how much the climate in the high-altitude areas of northern and central Greenland has changed. That is bad news for the planet’s coastlines, because it suggests a long-term process of melting is being set in motion that could ultimately deliver some significant, if hard to quantify, fraction of Greenland’s total mass into the oceans. Overall, Greenland contains enough ice to raise sea levels by more than 20 feet.

The study stitched together temperature records revealed by ice cores drilled in 2011 and 2012 with records contained in older and longer cores that reflect temperatures over the ice sheet a millennium ago. The youngest ice contained in these older cores was from 1995, meaning they could not say much about temperatures in the present day.

The work also found that compared with the 20th century as a whole, this part of Greenland, the enormous north-central region, is now 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer, and that the rate of melting and water loss from the ice sheet — which raises sea levels — has increased in tandem with these changes.

The research was published in the journal Nature on Wednesday by Hörhold and a group of researchers at the Alfred Wegener Institute, the Neils Bohr Institute in Denmark, and the University of Bremen in Germany.

The new research “pushes back the instrument record 1,000 years using data from within Greenland that shows unprecedented warming in the recent period,” said Isabella Velicogna, a glaciologist at the University of California at Irvine who was not involved in the research.

“This is not changing what we already knew about the warming signal in Greenland, the increase in melt and accelerated flow of ice into the ocean, and that this will be challenging to slow down,” Velicogna said. “Still, it adds momentum to the seriousness of the situation. This is bad, bad news for Greenland and for all of us.”

Scientists have posited that if the air over Greenland becomes warm enough, a feedback loop would ensue: The ice sheet’s melting would cause it to slump to a lower altitude, which would naturally expose it to warmer air, which would cause more melting and slumping, and so forth.

That this north-central part of Greenland is now 1.5 degrees Celsius warmer than it was in the 1900s does not necessarily mean the ice sheet has reached this feared “tipping point,” however.

Recent research has suggested that Greenland’s dangerous threshold lies at about 1.5 degrees Celsius or higher of planetary warming — but that is a different figure than the ice sheet’s regional warming. When the globe reaches 1.5C of warming on average, which could happen as soon as the 2030s, Greenland’s warming will likely be even higher than that — and higher than it is now.

Researchers consulted by The Washington Post also highlighted that the northern region of Greenland, where these temperatures have been recorded, is known for other reasons to have the potential to trigger large sea-level rise.

“We should be concerned about north Greenland warming because that region has a dozen sleeping giants in the form of wide tidewater glaciers and an ice stream … that awakened will ramp up Greenland sea level contribution,” said Jason Box, a scientist with the Geological Survey of Denmark and Greenland.

Box published research last year suggesting that in the present climate, Greenland is already destined to lose an amount of ice equivalent to nearly a foot of sea-level rise. This committed sea-level rise will only get worse as temperatures continue to warm.

The concern is focused on the Northeast Greenland Ice Stream, which channels a major portion — 12 percent — of the ice sheet toward the sea. It’s essentially a massive slow-moving river that terminates in several very large glaciers that spill into the Greenland Sea. It is already getting thinner, and the glaciers at its endpoint have lost mass — one of them, the Zachariae Isstrom, has also lost its frozen shelf that once extended over the ocean.

Recent research has also demonstrated that in past warm periods within the relatively recent history of the Earth (i.e., the last 50,000 years or so), this part of Greenland has often held less ice than it does today. In other words, the ice stream might extend farther outward from the center of Greenland than can be sustained at current temperatures, and be strongly prone to moving backward and giving up a lot of ice.

“Paleoclimate and modeling studies suggest that northeast Greenland is especially vulnerable to climate warming,” said Beata Csatho, an ice sheet expert at the University at Buffalo.

In the same year when the researchers were drilling the ice cores on which the current work is based — 2012 — something striking happened in Greenland. That summer, in July, vast portions of the ice sheet saw surface-melt conditions, including in the cold and very high-elevation locations where the research took place.

“It was the first year it has been observed that you have melting in these elevations,” said Hörhold. “And now it continues.”