Melt of West Antarctic Ice Sheet Unstoppable

Biggest Loser: Thawing Greenland Competes With Collapsing Antarctic For Fastest Ice Loss

Several new studies underscore scientists’ concerns we’re headed toward a coastline at least this flooded (20 meters or 69 feet) over many hundreds of years:

Melt-of-West-Antarctic-Ice-Sheet-Unstoppable

The worst-case scenario for sea level rise has now become simply the “business-as-usual” scenario, recent studies from NASA make clear. NASA glaciologist Eric Rignot, co-author of a new Greenland study, says that, taken together, the new papers “suggest that the globe’s ice sheets will contribute far more to sea level rise than current projections show.”

That means if we don’t reverse carbon pollution emissions trends ASAP, sea level rise will likely be 4 to 5 feet or more by century’s end. Also, the rate of sea level rise in 2100 could be upwards of 1 inch per YEAR!

No one has any concept of how to adapt cities, ports, infrastructure and the like to such a rate of sea level rise. This underscores the New York Times reporting last month that we are risking “enough sea-level rise that many of the world’s coastal cities would eventually have to be abandoned.”

Last month two studies provided evidence that the West Antarctic Ice Sheet has begun an irreversible process of collapse, in part because its key glaciers are grounded below sea level and are melting from underneath.

Now, a team of researchers from NASA and UC Irvine reports that the Greenland ice sheet has a similar instability:

Greenland’s icy reaches are far more vulnerable to warm ocean waters from climate change than had been thought, according to new research by UC Irvine and NASA glaciologists. The work, published today in Nature Geoscience, shows previously uncharted deep valleys stretching for dozens of miles under the Greenland Ice Sheet.

The bedrock canyons sit well below sea level, meaning that as subtropical Atlantic waters hit the fronts of hundreds of glaciers, those edges will erode much further than had been assumed and release far greater amounts of water.

West-Antarctic-Ice-Sheet-Unstoppable
A glacier in southwest Greenland flows down a rocky canyon like those mapped in a new study. Hundreds of previously unknown coastal canyons that are buried under the ice “could contribute to far higher sea level rise than previously predicted.” Via NASA.

We’ve known for a while that the Greenland ice sheet has been melting at an “extraordinary” rate, “with nearly a five-fold increase since the mid-1990s,” as one 2012 study reported. The new study shatters the conventional wisdom that such an accelerating melt rate was not sustainable:

Ice melt from the subcontinent has already accelerated as warmer marine currents have migrated north, but older models predicted that once higher ground was reached in a few years, the ocean-induced melting would halt. Greenland’s frozen mass would stop shrinking, and its effect on higher sea waters would be curtailed.

“That turns out to be incorrect. The glaciers of Greenland are likely to retreat faster and farther inland than anticipated – and for much longer – according to this very different topography we’ve discovered beneath the ice,” said lead author Mathieu Morlighem, a UC Irvine associate project scientist. “This has major implications, because the glacier melt will contribute much more to rising seas around the globe.”

That 2012 study had also found that Antarctica’s rate of ice loss rose 50 percent in the decade of the 2000s. Now, a new paper out last month looks at “3 years of Cryosat-2 radar altimeter data to develop the first comprehensive assessment of Antarctic ice sheet elevation change.”

Based on measurements by the European Space Agency’s CryoSat-2 satellite, they find:

Three years of observations show that the Antarctic ice sheet is now losing 159 billion tonnes of ice each year – twice as much as when it was last surveyed….

Lead author Dr Malcolm McMillan from the University of Leeds said: “We find that ice losses continue to be most pronounced along the fast-flowing ice streams of the Amundsen Sea sector, with thinning rates of between 4 and 8 metres per year near to the grounding lines of the Pine Island, Thwaites and Smith Glaciers.”

So Antarctica’s ice loss is accelerating, and the greatest ice loss is exactly where last month studies said the risk of glacier collapse is the greatest.

Climatologist Richard Alley told me “The new Morlighem, Rignot et al study in Greenland is solid.” And since most sea level rise analyses use the older, incorrect topology for the ice sheet beds, they almost certainly underestimate the amount of sea level rise Greenland will contribute this century and beyond.

NASA Glaciologist — and coauthor of two of the recent studies — Eric Rignot told me:

All these studies clearly point toward an increasing contribution of ice sheets to sea level. What the layperson should get out of the recent news, however, is that ice sheet melting is a serious thing, there is no red button to stop it, we can slow it down or get it as fast as we can. Right now, we have chosen the latter.

So the “good” news is that it might take 1000 years (or longer) to raise sea levels several tens of feet, and the choices we make now can affect the rate of rise and whether we ultimately blow past 69 feet to beyond 200 feet.

Glaciologist Jason Box made this point in a 2013 interview, “Humans Have Already Set in Motion 69 Feet of Sea Level Rise“:

So what can we do? For Box, any bit of policy helps. “The more we can cool climate, the slower Greenland’s loss will be,” he explained. Cutting greenhouse gases slows the planet’s heating, and with it, the pace of ice sheet losses.

“It’s like a terrible competition between Greenland and Antarctica for Biggest Loser,” Box told me today. “Multiple factors have been combining to produce surprisingly rapid change. This is the hallmark of a system unable to catch up with an exponentially increasing forcing.”

This concern is underscored by the paleoclimate data. In 2012, the National Science Foundation reported on paleoclimate research that examined “rock and soil cores taken in Virginia, New Zealand and the Eniwetok Atoll in the north Pacific Ocean.” Lead author Kenneth Miller of Rutgers University said:

“The natural state of the Earth with present carbon dioxide levels is one with sea levels about 70 feet higher than now.”

And that was only slightly less worrisome than a 2009 paper in Science that found the last time CO2 levels were this high, it was 5° to 10°F warmer and seas were 75 to 120 feet higher.

See also: NASA Animation Shows Relentless Pace of 60 Years of Global Warming in 15 Seconds

The time to act was a quarter century ago, but now remains infinitely better than later. Otherwise, we’ll end up the biggest loser of all.

Source | ThinkProgress

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