Whither the West Antarctic Ice Sheet?by Bill Chameides | March 24th, 2009
posted by Erica Rowell (Editor)
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Just partial melting of the Western Antarctic Ice Sheet spells trouble, even dire consequences, for many coastal populations. (NASA/GRACE team/DLR/Ben Holt Sr.)
Mention global warming, and you might think of rising sea levels inundating coastal cities and island nations. But how much could sea levels rise? Two new papers shed some light.
Sea level rise is caused by two processes:
- Rising temperatures. Just as the pressure inside a capped bottle of water increases as it warms, increasing the temperature of ocean water causes the water to push out or expand. Since there’s nowhere for sea water to go but up, sea levels rise. Study [pdf] of the period between 1993 and 2003 shows that about 60 percent of the sea level rise that occurred in that decade was due to thermal expansion.
- Melting of glaciers. As permanent ice sheets on land melt, their water flows into the sea causing sea level rise.
While global warming will continue to slowly and inexorably cause sea levels to rise from thermal expansion, the wildcard is the melting of the large ice sheets on Antarctica, which consists of the Eastern Antarctic Ice Sheet and the Western Antarctic Ice Sheet (WAIS), and Greenland. Loss of Antarctica’s two sheets would add 170 feet and 16 feet to sea levels, respectively. Loss of Greenland’s ice sheet would add about 20 feet to sea levels.
Such rises in sea levels would be large (some might say catastrophic) and so scientists have been working hard to determine if these ice sheets are indeed melting and how warm things need to get before we cross a tipping point and commit these glaciers to irrevocably falling into the ocean.
How Greenland’s Ice Sheets Look Then and Now
Thanks to a set of two satellites — the Gravity Recovery and Climate Experiment or GRACE — we can now accurately follow the mass of these ice sheets from measurements of the pull of gravity they exert on the satellites.
The less-than-encouraging results from studies of the Greenland Ice Sheet show pretty clearly that it is melting and at an accelerating rate. To assess how serious this is, scientists have looked back into the paleo (or historic) climate record to see what conditions were like at times when the ice sheet had completely melted.
From these studies many scientists have concluded that the point of no return for Greenland’s ice sheet is a 2-degree Centigrade (3.6-degree Fahrenheit) warming above pre-industrial temperatures, roughly corresponding to a carbon dioxide (CO2) concentration of 450 parts per million or ppm. (The current concentration is about 385 ppm; pre-industrial concentration was about 280 ppm.) As a result, many have suggested 450 ppm as the target the world community should adopt for emissions scenarios designed to avoid dangerous anthropogenic interference in the climate.
However, not all scientists agree on that target. Some suggest a more realistic target is 550 ppm. Others, including NASA scientist James Hansen, argue that we’ve already exceeded the point of no return and must endeavor to bring CO2 concentrations down from their current level to 350 ppm. (See here [pdf] and here).
Losing Greenland’s Ice Would Be a Big Deal — Losing Antarctica’s Would Be Bigger
A 20-foot increase in sea level from a Greenland thaw would be a big deal, but the loss of the Antarctic ice sheets with its combined sea level rise of 186 feet would be a considerably bigger deal. Fortunately, the GRACE data suggest that the Eastern Antarctic Ice Sheet is not melting, and, because of the unique meteorology over the continent, may even be growing slightly. However, the WAIS is melting — a study in 2008 found that the rate of loss there was equivalent to that of the Greenland Ice Sheet. (See here and here.)
As ice sheets go, WAIS is unusual. While the Greenland and East Antarctic Ice Sheets sit atop continents. WAIS is a marine-based sheet of ice that extends beyond the continent into the surrounding ocean. These ice-sheet extensions are referred to as ice shelves. The largest of WAIS’s ice shelves is the Ross shelf overlying the Ross Sea and named after the explorer who discovered them. These ice shelves do not in and of themselves affect sea level since they are already in the ocean, but they probably do help stabilize the part of the ice sheet lying on the continent and thus the fact that WAIS is melting should concern us.
To get a reading on whether a WAIS-melt is in our future, Tim Naish of the Antarctic Research Centre at the Victoria University of Wellington and co-workers analyzed sediment samples lying below the Ross Ice Shelf. They focused on sediments that had been laid down during the early Pliocene, an epoch from about 3–5 million years ago when global temperatures were generally warmer than what they’ve been in the last 3 million years, which have been characterized by ice ages. The researchers’ sediment analysis, published last week in Nature, shows that the size of WAIS waxed and waned on an approximate 40,000-year cycle corresponding to subtle changes in the tilt of the Earth’s rotational axis. (See my post on ice ages.)
A notable aspect of their finding is the inference that past losses of WAIS may have been triggered by global temperatures of only about 3 degrees Centigrade higher than today. A warmer threshold than that for the Greenland Ice Sheet but still within the temperature range we could hit this century.
In a companion paper in the same issue of Nature, David Pollard of Pennsylvania State University and Robert DeConto of the University of Massachusetts report on simulations using a coupled ice sheet/ice shelf model that find transitions from a glacial WAIS state to an ice-free WAIS state are “rapid.”
If you live near the coast, this can’t be welcome news. But it’s not quite that bad. You see “rapid” is a relative term. For Pollard and DeConto, who routinely study processes that literally progress at glacial speeds, rapid means “from one to several thousand years.”
Their findings also involve some melting of the EAIS leading to an overall rise of about 20 feet.
A thousand years is a long time so maybe you think this is not something to worry about. I wouldn’t be so sure. First there is a legacy issue — do we want to leave our heirs a world without cities like New York, London, and Hong Kong? And you know models are sometimes wrong. Those ice sheets could end up falling into the ocean a whole lot faster.filed under: Antarctica, climate change, faculty, global warming, Planetary Watch
and: climate, Greenland, ice sheets, Jim Hansen, sea level rise