Greenland's ice is melting four times faster than just 15 years ago and major American coastal cities such as New York could be swamped, warns a new study.

The ice is melting quicker than scientists previously thought and they say it will likely lead to faster sea level rises, due to the continued, accelerating warming of the Earth's atmosphere.

Researchers concerned about global sea level rises have focused on Greenland's south east and north west regions, where large glaciers stream iceberg-sized chunks of ice into the Atlantic. The chunks float away, eventually melting.

But a new study found that the largest sustained ice loss from early 2003 to mid-2013 came from Greenland's south west region, which is mostly devoid of large glaciers.

Study lead author Professor Michael Bevis, of Ohio State University, said: 'Whatever this was, it couldn't be explained by glaciers, because there aren't many there.

'It had to be the surface mass - the ice was melting inland from the coastline.'

The melting, which Professor Bevis and his colleagues believe is largely caused by global warming, means that in the south western part of Greenland, growing rivers of water are streaming into the ocean during summer.

The key finding from their study is that South West Greenland, which previously had not been considered a serious threat, will likely become a 'major future contributor' to sea level rise.

Professor Bevis said: 'We knew we had one big problem with increasing rates of ice discharge by some large outlet glaciers.

'But now we recognise a second serious problem: increasingly, large amounts of ice mass are going to leave as meltwater, as rivers that flow into the sea.'

He said the findings could have 'serious implications' for coastal US cities, including New York and Miami, as well as island nations that are particularly vulnerable to rising sea levels.

And he warned there is no turning back.

Professor Bevis said: '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.'

Climate scientists and glaciologists have been monitoring the Greenland ice sheet as a whole since 2002, when NASA and Germany joined forces to launch the Gravity Recovery and Climate Experiment, known as GRACE.

It involves twin satellites that measure ice loss across Greenland. Data from the satellites showed that between 2002 and 2016, Greenland lost around 280 gigatons of ice per year, equivalent to an annual sea level rise of 0.03 inches.

But the rate of ice loss across the island was far from steady.

Professor Bevis' team used data from GRACE and from GPS stations scattered around Greenland's coast to identify changes in ice mass.

The patterns they found show an 'alarming' trend - by 2012, ice was being lost at nearly four times the rate that prevailed in 2003.

And the biggest surprise was that the acceleration was focused in south west Greenland, a part of the island that previously hadn't been known to be losing ice that rapidly.

Professor Bevis said a natural weather phenomenon - the North Atlantic Oscillation, which brings warmer air to West Greenland, as well as clearer skies and more solar radiation - was building on man-made climate change to cause 'unprecedented' levels of melting and runoff.

Global atmospheric warming enhances summertime melting, especially in the south west.

Professor Bevis said the North Atlantic Oscillation is a natural, if erratic, cycle that causes ice to melt under normal circumstances. When combined with man-made global warming, the effects are 'supercharged'.

He said: 'These oscillations have been happening forever.

'So why only now are they causing this massive melt? It's because the atmosphere is, at its baseline, warmer.

'The transient warming driven by the North Atlantic Oscillation was riding on top of more sustained, global warming.

'What's happening is sea surface temperature in the tropics is going up; shallow water gets warmer and the air gets warmer.

'The water temperature fluctuations driven by an El Niño are riding this global ocean warming.

'Because of climate change, the base temperature is already close to the critical temperature at which coral bleaches, so an El Niño pushes the temperature over the critical threshold value.

'And in the case of Greenland, global warming has brought summertime temperatures in a significant portion of Greenland close to the melting point, and the North Atlantic Oscillation has provided the extra push that caused large areas of ice to melt.'

Before the new study, scientists understood Greenland to be one of the Earth's major contributors to sea-level rise - mostly because of its glaciers.

But Professor Bevis said the new findings show that scientists need to be watching the island's snowpack and ice fields more closely.

He added: 'We're going to see faster and faster sea level rise for the foreseeable future.

'Once you hit that tipping point, the only question is: how severe does it get?'

The full findings of the study were published in the journal Proceedings of the National Academy of Sciences.

This article has been adapted from its original source.