Today Greenland’s continental ice sheet is melting rapidly, and is a leading contributor to global sea level rise.
But in the past, ice caps in some areas of Greenland actually grew larger during periods of warming, a new study by Woods Hole Oceanographic Institution (WHOI) has shown.
The research is based on analysis of “ice cores”, which capture thousands of years of climate history, frozen in ice.
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Matthew Osman, postdoctoral research associate at the University of Arizona and a 2019 graduate of the MIT-WHOI Joint Program, said: "Currently, we know Greenland's ice caps are melting due to warming, further contributing to sea level rise.
“But we have yet to explore how these ice caps have changed in the past due to changes in climate.
"The findings of this study were a surprise because we see that there is an ongoing shift in the fundamental response of these ice caps to climate: today, they're disappearing, but in the past, within small degrees of warming, they actually tended to grow."
Greenland’s continental scale ice sheet that soars up to 3,000 metres above sea level – and its rapid melting is a leading contributor to global sea level rise.
The ice sheet covers 79% of the world's largest island.
When ice caps “grow” during warmer periods, this phenomenon happens because of a "tug-of-war" between what causes an ice cap to grow (increased precipitation) or recede (increased melting) during periods of warming.
This will not significantly slow today’s melting, due to melting rates that are outpacing the rate of annual snowfall atop ice caps.
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But in past centuries these ice caps would expand due to increased levels of precipitation brought about by warmer temperatures.
The difference between the past and present is the severity of modern warming caused by human beings.
The study, which began in 2015, centres around a core collected from the Nuussuaq Peninsula in Greenland.
This single core offers insight into how coastal climate conditions and ice cap changes varied during the last 2,000 years, due to tracked changes in its chemical composition and the amount of snowfall archived year after year in the core.
The researchers found that during periods of past warming, ice caps were growing rather than melting, contradicting what we see in the present day.
The team gathered this data by drilling through an ice cap on top of one of the higher peaks of the Nuussuaq Peninsula.
The entire core, about 460ft in length, took about a week to retrieve.
They then brought the metre-long pieces of core to the National Science Foundation Ice Core Facility in Denver, Colorado, and stored it at -20C.
By looking at different properties of the core's chemical content, such as parts per billion of lead and sulphur, investigators were able to accurately date the core by combining these measurements with a model of past glacier flow.
Sarah Das, associate scientist of Geology and Geophysics at WHOI, said: "These model estimates of ice cap flow, coupled with the actual ages that we have from this high precision chemistry, help us outline changes in ice cap growth over time. This method provides a new way of understanding past ice cap changes and how that is correlated with climate.
"Because we're collecting a climate record from the coast, we're able to document for the first time that there were these large shifts in temperature, snowfall and melt over the last 2,000 years, showing much more variability than is observed in records from the interior of Greenland.
"Our findings should urge researchers to return to these remaining ice caps and collect new climate records while they still exist.”
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