Deep ocean heat is rapidly melting Antarctic ice
Oceanographer at AGU: Western Antarctic Peninsula is seeing "the highest increase in temperatures of anywhere on Earth."
“Warm waters carried by the Antarctic Circumpolar Current are brushing the ice front in the western part of the continent, in the area of the Bellingshausen Sea.” [Click to enlarge.]
Antarctica is disintegrating much faster than almost anybody imagined — see “Nothing in the natural world is lost at an accelerating exponential rate like this glacier.” In 2001, the IPCC “consensus” said neither Greenland nor Antarctica would lose significant mass by 2100. They both already are. As Penn State climatologist Richard Alley said in March 2006, the ice sheets appear to be shrinking “100 years ahead of schedule.”
A presentation Monday at the fall meeting of the American Geophysical Union sheds some light on the underlying cause of this rapid melt — the ice is being attacked from the bottom. Discovery News has the story:
Global warming is sneaky. For more than a century it has been hiding large amounts of excess heat in the world’s deep seas. Now that heat is coming to the surface again in one of the worst possible places: Antarctica.
New analyses of the heat content of the waters off Western Antarctic Peninsula are now showing a clear and exponential increase in warming waters undermining the sea ice, raising air temperatures, melting glaciers and wiping out entire penguin colonies.
“In the area I work there is the highest increase in temperatures of anywhere on Earth,” said physical oceanographer Doug Martinson of the Lamont-Doherty Earth Observatory. Martinson has been collecting ocean water heat content data for more than 18 years at Palmer Island, on the western side of the Antarctic Peninsula.
“Eighty-seven percent of the alpine glaciers are in retreat,” said Martinson of the Western Antarctic Peninsula. “Some of the Adele penguin colonies have already gone extinct.”
Martinson and his colleagues looked not only at their very detailed and mapped water heat data from the last two decades, but compared them with sketchier data from the past and deep ocean heat content measurements worldwide. All show the same rising trend that is being seen in Antarctica.
“When I saw that my jaw just dropped,” said Martinson. The most dramatic rise has happened since 1960, he said.
The figure comes from Lamont-Doherty Earth Observatory (via Columbia University’s Earth Institute blog), which quotes Martinson explaining, “This is like a huge freight of hot coals – fresh, hot water being delivered right to the the front door.”
So while global warming has continued its fitful warming of the temperature on Earth’s surface, the planet is warming from human-cause greenhouse gases just where climate science said it would — the oceans, which is where more than 90% of the warming was projected to end up (as we learned in two key 2009 papers, see “Skeptical Science explains how we know global warming is happening.“). The key findings in the second study are summed up in these two figures:
Total Earth Heat Content from 1950 to 2003 (Murphy 2009).
Time series of global mean heat storage (0–2000 m), measured in 108 Jm-2
This new finding makes action now to reduce greenhouse gas emissions all the more important because we’re already stuck with more melting to come:
What the rising water heat means, he said, is that even if humanity got organized and soon stopped emitting greenhouse gases, there is already too much heat in the oceans to stop a lot of impacts — like the melting of a huge amount of Antarctic ice.
“There’s the potential that we’re locked into long-term sea level rise for a long time,” Martinson told Discovery News…
As for how fast the ice will melt and in what locations, that depends largely on whether the upwelling warm water comes in contact with the thick ice shelf that crowds the coast and blocks the glaciers from reaching the sea.
That, in turn, depends on the winds which drive away the surface waters and make it possible for the deeper waters to rise to the surface, said senior researcher Robert Bindschadler of NASA’s Goddard Earth Science and Technology Center and the University of Maryland-Baltimore County.
“It can destroy the ice shelf if that heat can get to it,” said Bindschadler, who at the same meeting presented his work from the melting Pine Island Ice Shelf in Antarctica.
Now that the upwelling deep sea water is the clear cause of the melting ice shelf, rather than summer melt water, as had been thought in the past, it’s a question of how winds will change in a warming world and whether they will drive more warm water into the ice shelves.
The warming of West Antarctica is most worrisome (at least for this century) because it’s going to disintegrate long before the East Antarctic Ice Sheet does. Not only is the WAIS melting from underneath, it is, as I wrote in the “high water” part of my book, inherently less stable:
Perhaps the most important, and worrisome, fact about the WAIS is that it is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. The WAIS rests on bedrock as deep as two kilometers underwater. One 2004 NASA-led study found that most of the glaciers they were studying “flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is under way.” A 2002 study in Science examined the underwater grounding lines – the points where the ice starts floating. Using satellites, the researchers determined that “bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed.” And that melt rate is positively correlated with ocean temperature.
The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more-and increasingly warmer-water underneath it, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS (see, for instance, here).
The time to act was a while ago, but now is far better than later.
No comments:
Post a Comment