The West Greenland Ice Sheet is melting faster than at any point in the last 450 years, according to a new study. Scientists say the high melt rate is due to weather patterns combined with a trend of summer warming, which they suggest has been caused by human activity.
Greenland’s ice loss is a major contributor to global sea level rise, according to research referenced in the study. At nearly a millimetre each year, ice melt in Greenland contributes more to sea-level rise than melt in Antarctica. If Greenland’s ice completely melted, it could increase sea level by over six metres.
Scientists at Dartmouth College in New Hampshire led the research with support from three other U.S. universities: Boise State University, University of Alaska at Fairbanks and University of Maine. Their findings were published on Monday in the journal Geophysical Research Letters.
Researchers collected seven, roughly 30-metre-long cylinders of ice called ice cores from the West Greenland Ice Sheet’s “percolation zone.” This area experiences summer melting but is high enough in elevation that the meltwater refreezes in layers rather than entering the sea as runoff.
The area is “where we find the best record of Greenland melt going back through time in the form of the refrozen ice layers,” according to Karina Graeter, a graduate student in Dartmouth’s Department of Earth Sciences and the lead author of the study.
Using the ice cores, scientists measured the frequency and thickness of the ice layers to determine how much melt occurred over time. They also used chemical measurements to identify the age of the cores.
The researchers found that the ice layers began to thicken and become more frequent in the 1990s, indicating an increase in ice melt. Furthermore, their analysis showed that melt in recent years is higher than at any other time in the last 450 years, and possibly much longer.
“We see that west Greenland melt really started accelerating about twenty years ago,” said Erich Osterberg, the project’s lead scientist and an assistant professor of earth sciences at Dartmouth. “The ice core record ends about 450 years ago, so the modern melt rates in these cores are the highest of the whole record that we can see.”
The study also found evidence that large melt events observed in the ice cores were related to ocean temperatures in the North Atlantic and “summer blocking highs,” high-pressure systems that cause warm air to remain above Greenland’s ice sheet in the summer.
However, these phenomena alone were not enough to explain the increased melt seen in the last 20 years. The researchers concluded that additional summertime warming of more than two degrees Fahrenheit was needed to account for the extreme melting observed beginning in the 1990s.
“Our study shows that the rapid rise in west Greenland melt is a combination of specific weather patterns and an additional long-term warming trend over the last century,” said Osterberg.
The additional warming caused melt rates to nearly double in the period ranging from 1995 to 2015, compared to previous periods with the same atmospheric conditions and ocean temperatures.
The researchers said the additional warming was likely caused by climate change related to human greenhouse gas emissions. They warned that unless these emissions are significantly reduced, melting in Greenland will continue to increase.
“Cooler North Atlantic ocean temperatures and less summer blocking activity might slow down Greenland melt for a few years or even a couple decades, but it would not help us in the long run,” said Osterberg. “Beyond a few decades, Greenland melting will almost certainly increase and raise sea level as long as we continue to emit greenhouse gases.”
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