Stephen Leahy / National Geographic – 2019-01-25 00:12:15
https://www.nationalgeographic.com/environment/2019/01/greeland-ice-melting-four-times-faster-than-thought-raising-sea-level/
Greenland’s Ice Is Melting
Four Times Faster Than Thought — What It Means
Stephen Leahy / National Geographic
New science suggests Greenland may be
approaching a dangerous tipping point,
with implications for global sea-level rise.
(January 22, 2019) — A new study warns that Greenland’s ice is melting faster than scientists previously thought. But perhaps the biggest surprise is that most of this ice loss is from the land-fast ice sheet itself, not Greenland’s glaciers.
The new study, published January 21 in the Proceedings of the National Academy of Sciences, found that the largest sustained ice loss from early 2003 to mid-2013 came from Greenland’s southwest region, which is mostly devoid of large glaciers.
Greenland, the world’s biggest island, appears to have hit a tipping point around 2002-2003 when the ice loss rapidly accelerated, said lead author Michael Bevis, a geoscientist at Ohio State University. By 2012 the annual ice loss was “unprecedented” at nearly four times the rate in 2003, Bevis said in an interview.
Much of this new accelerated ice melt came from southwest Greenland, a part of the island that hadn’t been known to be losing ice that rapidly. Previously, the scientific focus was on Greenland’s southeast and northwest regions, where large glaciers stream iceberg-sized chunks of ice into the Atlantic Ocean.
“We knew we had one big problem with increasing rates of ice discharge by some large outlet glaciers,” Bevis said. “But now we recognize a second serious problem: Increasingly, large amounts of ice mass are going to leave as meltwater, as rivers that flow into the sea.
Data from NASA’s GRACE satellites and GPS stations scattered around Greenland’s coast showed that between 2002 and 2016, Greenland lost approximately 280 billion tons of ice per year. This average annual ice melt is enough to cover the entire states of Florida and New York hip deep in meltwater, as well as drowning Washington, DC and one or two other small states.
“This is going to cause additional sea-level rise. We are watching the ice sheet hit a tipping point,” said Bevis.
The Greenland ice sheet is 10,000 feet thick in places and contains enough ice to raise sea levels 23 feet (7 meters). In the 20th century, Greenland has lost around 9,000 billion tons of ice in total, accounting for 25 millimeters of sea-level rise. (It takes about 360 billion tons of ice to produce one millimeter of global sea-level rise.)
However, Greenland is dwarfed by the Antarctic ice sheet, which could raise sea level 57 meters if fully melted. Alarmingly, the Antarctic is also undergoing an accelerated melt down, losing six times as much ice as it was four decades ago, a January 14 study reported. Its ice loss averaged 252 billion tons a year over the past decade.
It’s the same story for western North America’s glaciers—ice loss quadrupled since the early 2000s to 12.3 billion tons annually, a recent study revealed.
What’s Causing the Melting?
Global warming of just 1 degree C is the main driver behind this massive meltdown of the world’s ice. In Greenland, researchers discovered that global warming, coupled with a negative phase of the North Atlantic Oscillation led to the rapid surface melt of the ice sheet during summers.
The North Atlantic Oscillation (NAO) is a natural, irregular change in atmospheric pressure and brings warm, sunny summer weather to the western side of Greenland when it’s in its negative phase. Prior to 2000, this did not lead to significant ice melt, said Bevis, but ever since then the negative phase of the NAO results in huge increases in ice melt.
This is analogous to the El Nino–Southern Oscillation (ENSO) cycle and coral bleaching, he said. In 1997-98 a strong El Nino event led to massive bleaching of most of the world’s tropical reefs. Previously, such events had little impact on reefs, but by 1997 climate change had warmed tropical ocean waters to the point where any additional warming due to an El Nino was too much for corals to withstand. Now every time there’s an El Nino corals suffer.
Bevis’ study shows southwest Greenland is where the ice sheet is most susceptible to El Nino-like atmospheric cycles that are superimposed on a warming trend, said Jason Box, a glaciologist at the Geological Survey of Denmark and Greenland. And it’s clear more of the overall ice loss is coming from the surface than marine terminating glaciers, Box said.
All it takes to melt Greenland’s ice sheet is a surface temperature of 1 C and sunlight. “It used to be rare to get temperatures above 0 degrees on the ice sheet, but no longer,” Bevis said. And each degree above 1 C doubles the amount of ice melt.
What Happens Next?
Without acting soon to dramatically reduce the burning of fossil fuels that is raising global temperatures, most or all of Greenland’s ice could melt, raising sea levels 23 feet, warns Richard Alley, a glaciologist at Penn State. This would occur on a time scale of centuries.
However, there is a warming threshold that could be crossed in a few decades or less and, if exceeded long enough, the meltdown of Greenland would be irreversible, said Alley.
Another major concern is that all this meltwater is slowing the Gulf Stream that brings warm water from the equator to the North Atlantic and cold water down into the deep ocean. The Gulf Stream, more properly known as the Atlantic meridional overturning circulation (AMOC), is why Western Europe has temperate weather. Last year, researchers reported in the journal Nature that the AMOC declined in strength by 15 percent since the mid-20th century.
Meteorologists now believe this slow-down is linked to recent summer heat waves in Europe. A co-author of the Nature study, Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, attributed the slow-down to the huge volumes of meltwater from Greenland. “I think it is happening . . . . And I think it’s bad news,” he told The Washington Post
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