The kilometer-thick ice sheet that covers Greenland saw an almost record imbalance last year between new snowfall and discharges of meltwater and ice into the sea, researchers have reported.
A net loss of 600 billion tonnes was enough to raise the global watermark 1.5 millimeters, about 40 percent of the total sea level rising in 2019.
The Greenlandic ice sheet – which until the end of the 20th century accumulated as much mass as it threw – holds enough frozen water to lift the world’s sea by seven meters.
Almost as alarming, though, as the faster resolution of the ice sheet is the force that drives it, the authors reported this week in cryosphere, a peer-reviewed journal published by the European Geosciences Union.
More than half of the dramatic loss in 2019 was not due to warmer air temperatures than the average, but rather unusual high-pressure weather systems linked to global warming.
These anticyclone conditions blocked the formation of clouds over southern Greenland, which caused unfiltered sunlight to melt the surface of the ice sheet. Fewer clouds also meant less snow – 100 billion tonnes below the 1980-1999 average.
In addition, the lack of snowfall has been exposed to darker, sooty ice that absorbs heat rather than reflects it, as untouched white snow does.
Conditions were different, but not better in the northern and western parts of Greenland, due to hot and humid air being pulled up from lower latitudes, the study showed.
All of these factors led to faster melting and runoff, which created currents that cut through the ice towards the sea.
“These atmospheric conditions are becoming more common in recent decades,” said lead author Marco Tedesco, a researcher at Columbia University’s Lamont-Doherty Earth Observatory.
“This is very likely due to the ‘wave’ in the jet stream,” a powerful high altitude wind band that moves from west to east across the polar region, he said.
Twice the global average
The disruption of the jet stream’s normal pattern has been linked to the disappearance of the sea ice, the faster proportion of atmospheric warming in the Arctic and the disappearance of the snow cover in Siberia – all the consequences of global warming.
Average temperatures in the Arctic have risen by two degrees Celsius since the mid-1800s, twice the global average.
“In other words, climate change can make the destructive atmospheric conditions in high pressure more common over Greenland,” Tedesco said.
In fact, 2019 is not the first time such anomalies have occurred, with more than half of the century having similar, if less pronounced, patterns.
The effects of these high-pressure systems are not taken into account in climate models used by the United Nations Climate Change Panel on Climate Change (IPCC) to project the effects of global warming on the green ice sheet, the study warns.
“It is likely that we underestimate the future melting by a factor of two,” co-author Xavier Fettweis, a research assistant at the climate lab at the University of Liège in Belgium, told AFP.
The new study attributes nearly 70 percent of the meltwater and iceberg wastewater to the high-pressure systems last year, and the rest to direct warming of the atmosphere during climate change.
The discharge last year was comparable to the record year 2012, but the air temperatures in 2019 were considerably lower.
During the 1990s, Greenland’s ice sheet was roughly in balance, but the annual mass loss has increased since then.
In total, Greenland has thrown about four trillion tons of ice between 1992 and 2018, which has led to an average sea level rise of 11 millimeters, according to a December 2019 study in Nature.
The IPCC has predicted that the global sea level may rise one meter by 2100, mainly due to emissions from the Arctic ice sheets in Greenland and Western Antarctica.
Unusually clear skies drove record loss of the Greenland Ice 2019
Marco Tedesco et al. Unmatched atmospheric conditions (1948–2019) drive the exceptional melting season 2019 over Greenland’s ice sheet. cryosphere (2020). DOI: 10.5194 / tc-14-1209-2020
© 2020 AFP
Alarms ring when Greenland’s ice loss causes 40% of sea level 2019 (2020, April 16)
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