The melting of Greenland’s ice sheet has accelerated to unprecedented rates in the face of rising temperatures, analysis of ice cores has found.
Surface melting across the the mile-thick ice sheet increased in the 19th century as human activity started to warm the climate, but ramped up in the 20th and early 21st centuries and shows no signs of abating, scientists said.
Ice loss from Greenland, including runoff from melted snow and ice on the top of the ice sheet, is contributing to sea level rises.
If Greenland ice continues to melt at unprecedented rates as a result of warmer summers, it could accelerate the already fast pace of sea level rise, the researchers warned.
Analysis of ice core samples from sites 6,000ft (1,829m) above sea level has enabled the researchers to assess melting dating back 350 years.
The ice cores contain layers that show how ice melted and refroze on contact with the snow-pack underneath each year, revealing the intensity of melting.
The scientists combined the results from ice cores with satellite data and climate models to reconstruct melt-water runoff at lower elevations on the edge of the ice sheet that contributes to sea level rise.
They found that increases in melting closely follow the start of industrial-era warming in the Arctic in the mid-1800s but the magnitude of the melt has exceeded natural variability in the past few decades.
Study co-author Sarah Das, from Woods Hole Oceanographic Institution in the US, said: “From a historical perspective, today’s melt rates are off the charts, and this study provides the evidence to prove this.”
Total ice sheet melt-water runoff had increased 50% compared with the start of the industrial era, and had seen a 30% increase since the 20th century alone, she said.
Summer melting in 2012 was at levels that represented “exceptional highs” for the past 350 years.
And melt and runoff in the last decade are likely to be unprecedented in the past 6,800 to 7,800 years, a study published in the journal Nature showed.
The researchers also warned that Greenland, which locks up the equivalent of around 30ft (7m) of sea level rise, is becoming more sensitive to warming than it was in the past.
The world has already warmed by almost 1C since pre-industrial times, and there are concerns that the ice sheet could reach a tipping point at around 1.5C to 2C of warming, lead author Luke Trusel said.
At that point it could go into a state of irreversible retreat.
Dr Trusel said: “We see melting and runoff from Greenland start ticking up as warming initiated in the Arctic in the 1800s, but only in the last few decades has it really accelerated to levels we haven’t seen before in the last few centuries.”
He said melting had gone “into overdrive”, and as a result Greenland melt was adding to sea level more than any time during the last three-and-a-half
centuries, if not thousands of years.
As the ice sheet melts it becomes slightly darker, absorbing more sunlight and melting more, even if temperatures do not change, while increased melting can generate impermeable ice layers which exacerbate runoff.
Dr Trusel, from Rowan University’s School of Earth and Environment in New Jersey, US, said Greenland would melt more and more for every degree of warming.
“The melting and sea level rise we’ve observed already will be dwarfed by what may be expected in the future as climate continues to warm,” he said.