Greenland’s ice sheet may have shrunk past the point of return, with the ice likely to melt away no matter how quickly the world reduces climate-warming emissions, new research suggests.

Scientists studied 234 glaciers across the Arctic territory for 34 years till 2018, and found that annual snowfall was no longer enough to replenish glaciers of the snow and ice being lost to summertime melting.

That melting is already causing global seas to rise about a millimetre on average per year. If all of Greenland’s ice goes, the water released would push sea levels up by an average of 6 metres — enough to swamp many coastal cities around the world.

“Greenland is going to be the canary in the coal mine, and the canary is already pretty much dead at this point,” said glaciologist Ian Howat at Ohio State University. He and his colleagues published the study Thursday in the Nature Communications Earth & Environment journal.

The Arctic has been warming at least twice as fast as the rest of the world for the last 30 years, an observation referred to as Arctic amplification. The polar sea ice hit its lowest extent for July in 40 years. It has brought more water to the region, opening up routes for shipping traffic.