Greenland is losing ice seven times faster than in the 1990s, according to the most complete study of the polar island yet, which said continued melting may lead to 40 million more people exposed to coastal flooding by 2100. The researchers from 50 international organisations, including those from the University of Leeds in the UK, combined 26 separate surveys to estimate changes in the mass of Greenland's ice sheet between 1992 and 2018.
They used data from 11 different satellite missions, including measurements of the ice sheet's changing volume, flow, and gravity.
The findings of the study, published in the journal Nature on Tuesday, showed that Greenland had lost 3.8 trillion tonnes of ice since 1992 -- enough to push global sea levels up by 10.6 millimetres.
The researchers said the rate of ice loss has risen from 33 billion tonnes per year in the 1990s to 254 billion tonnes per year in the last decade -- a seven-fold increase within three decades.
The study noted that in 2013, the Intergovernmental Panel on Climate Change (IPCC) predicted global sea levels to rise by 60 centimetres by 2100, putting 360 million people at risk of annual coastal flooding.
But based on the current study, the researchers said sea levels are rising faster than expected, and are tracking the IPCC's high-end climate warming scenario, which predicts 7 centimetres more.
"As a rule of thumb, for every centimetre rise in global sea level another six million people are exposed to coastal flooding around the planet," said study co-author Andrew Shepherd from the University of Leeds.
"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," Shepherd said.
The researchers said the predictions are not unlikely events or small impacts. They said these are currently happening, and will be devastating for coastal communities.
According to the researchers, ice losses peaked at 335 billion tonnes per year in 2011 - ten times the rate of the 1990s - during a period of intense surface melting.
Since then, they said, the rate of ice loss dropped to an average 238 billion tonnes per year, but has remained seven times higher.
The polar scientists cautioned that the study results do not include all of 2019, which could set a new high due to widespread summer melting.
"Satellite observations of polar ice are essential for monitoring and predicting how climate change could affect ice losses and sea level rise," said study co-author Erik Ivins from NASA's Jet Propulsion Laboratory in the US.
"While computer simulation allows us to make projections from climate change scenarios, the satellite measurements provide prima facie, rather irrefutable, evidence," Ivins said.
The new estimate of Greenland ice loss is timely for the IPCC, said lead author of the IPCC's sixth assessment report Guofinna Aoalgeirsdottir, who was not involved in the study.
"Their satellite observations show that both melting and ice discharge from Greenland have increased since observations started. The ice caps in Iceland had similar reduction in ice loss in the last two years of their record, but this last summer was very warm here and resulted in higher loss. I would expect a similar increase in Greenland mass loss for 2019," he added.