Melting rate of mile-thick ice sheet ‘at fastest ever’

Sarah Das  WHOI		Large rivers form on the surface of the ice sheet in summer rapidly moving meltwater from the ice sheet to the ocean

Today, the ice is melting 50 percent faster than it did before industrialisation and 33 percent faster than during the 20th century. "As a result, Greenland melt is adding to sea level more than any time during the last three and a half centuries, if not thousands of years", said lead author Luke Trusel.

Trusel said the new research provides evidence rapid melting observed in recent decades is highly unusual when put in historical context.

From these numbers, the researchers estimated that ice sheet-wide levels of meltwater runoff have jumped 50 per cent in the past 20 years compared with pre-industrial times.

Low-lying tropical island states from the Maldives to Tuvalu view Greenland's ice sheet with foreboding since it contains enough ice to raise world sea levels by around 7 metres if it all melted. Greenland experiences seasonal melt during the warm summer days, and at low elevations, the melting is more intense.

Scientists tracking Greenland's ice by satellite and on the ground have seen increasingly dire ice loss.

Melting ice is the primary cause of rising water levels and researchers predict that over the next couple of decades there will be massive flooding in coastal towns round the world. Hence, the region could be the largest contributor to the rising sea level, more than any region across the globe. "Anything we can do to limit future warming, even by a little bit, is going to make a huge difference to keeping ice on Greenland and not in the ocean". Instead of escaping the ice sheet, the short-lived meltwater forms icy bands that stack up layers of densely packed ice over time. The team focused on these high elevations so they would be able to study records of the melting's intensity of the melting dating all the way back to the 17th century.

"We found a fifty percent increase in total ice sheet meltwater runoff versus the start of the industrial era and a thirty percent increase since the 20th century alone", Das said.

The team analysed these results in combination with the imaging data collected by various satellites and the data from sophisticated climate models, which enabled them to determine the rate of ice melting, not only at core site, but also broadly across Greenland.

"From a historical perspective, today's melt rates are off the charts".

Satellite methods to understand melting rates have only been around in recent decades, so the ability to go back further in time was important.

"The melting is not just increasing - it's accelerating", Trusel told Nature.

Experts concluded by saying that this is the ice sheet's response to the warming caused by humans and warming means more today that it did in the past.

The core samples were brought back to ice core labs at the U.S. National Science Foundation Ice Core Facility in Denver, Colo., WHOI in Woods Hole, Mass., Wheaton College in Norton, Mass., and the Desert Research Institute in Reno, Nev. where the scientists measured physical and chemical properties along the cores to determine the thickness and age of the melt layers.

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