Greenland is melting faster than previously believed, with its ice retracting four times more quickly than in 2003.
Researchers concerned about rising sea levels have studied melting glaciers in Greenland for many years, specifically from the southeast and northwest regions of the country.
Glaciers here are responsible for pushing enormous iceberg-sized chunks of ice into the Atlantic Ocean, which eventually float away and melt.
However, a new study into Greenland's southwest region - which is mostly devoid of large glaciers - found that the rate of ice loss was even higher than previously believed.
Here, instead of glaciers pushing icebergs into the sea, ice is actually melting inland and entering the ocean as melt-water.
"Whatever this was, it couldn't be explained by glaciers, because there aren't many there," said Professor Michael Bevis, lead author of the paper. "It had to be the surface mass - the ice was melting inland from the coastline."
The work from researchers at Ohio State University was published in the Proceedings of the National Academy of Sciences.
"We knew we had one big problem with increasing rates of ice discharge by some large outlet glaciers," said Professor Bevis.
"But now we recognise a second serious problem: increasingly, large amounts of ice mass are going to leave as melt-water, as rivers that flow into the sea."
The researchers warn that their findings could have serious implications for coastal cities in the US, as well as island nations such as the UK which are vulnerable to rising sea levels.
Professor Bevis added: "The only thing we can do is adapt and mitigate further global warming - it's too late for there to be no effect.
"This is going to cause additional sea level rise. We are watching the ice sheet hit a tipping point."
Scientists already understood Greenland to be one of the biggest contributors to sea-level rise due to its glaciers and ice sheets which cover 80% of its surface.
According to Professor Bevis, the new findings indicate that scientists may not have been watching the right parts of the island to detect where potential sea-level rise would come from.
There are GPS systems in place which monitor the movements of the ice sheet around most of Greenland's perimeter, but this has not had good coverage in the southwest region of the country.
The researchers say it is necessary to make the network there more dense, given these new findings, to monitor melt-water rates.
"We're going to see faster and faster sea level rise for the foreseeable future," Professor Bevis said.
"Once you hit that tipping point, the only question is: how severe does it get?"