Greenland's ice is melting seven times faster than it was in the 1990s, and at much greater speeds than current predictions, according to a new study.
It came as another report warned that melting Arctic permafrost could be causing carbon stores to be released into the atmosphere, adding to warming.
The increase in Greenland ice melt could lead to global sea level rises of 7cm above current estimates by 2100, leaving 40 million more people at risk of flooding.
The study, which reexamined 11 individual satellite measurements from 1992 to 2018, showed that Greenland has already lost 3.8 trillion tonnes of ice since 1992, enough to push global sea levels up by more than 1cm.
It was a joint project between Nasa, the European Space Agency and Leeds University, which said it was the most comprehensive survey of Greenland ice loss so far.
The melt is a result of both surface runoff, caused by increased air temperatures, and warming oceans leading to melting from below.
The average annual land surface air temperature in the Arctic between October 2018 and August 2019 was the second warmest since 1900, according to US federal agency study published on Tuesday.
The Arctic Report Card also warned that thawing permafrost could mean the Arctic is a net emitter of greenhouse gases, as carbon stored in the remains of plants and animals from thousands of years ago is released into the atmosphere.
Sudden changes to the Greenland ice sheet has long been a particular cause for concern for polar scientists; between 1990 and 2008 they recorded a doubling in the pace of melting.
Melt from the ice has led to 25 per cent of global sea level rise, twice as much as that of the larger Antarctic ice sheet.
Professor Andrew Shepherd from the University of Leeds, who co-authored the study said: “On current trends, Greenland ice melting will cause 100 million people to be flooded each year by the end of the century, so 400 million in total due to all sea level rise.
“These are not unlikely events or small impacts; they are happening and will be devastating for coastal communities.”
Delegates are gathered at the COP 25 in Madrid this week to devise ways of putting the Paris plan in action, but key sticking points remain over emissions trading schemes and how the fight against climate change is funded.