An analysis of prehistoric “Heinrich events” that happened many thousands of years ago, creating mass discharges of icebergs into the North Atlantic Ocean, make it clear that very small amounts of subsurface warming of water can trigger a rapid collapse of ice shelves…If water were to warm by about 2 degrees under the ice shelves that are found along the edges of much of the West Antarctic Ice Sheet … it might greatly increase the rate of melting to more than 30 feet a year.
Greenland Ice Sheet Icebergs spilling out of Jakobshavn Fiord from the Greenland Ice Sheet, seen on the horizon. (Photo Credit: Oregon State University.)
We knew that one West Antarctic Glacier is disintegrating much faster than almost anybody imagined — see “Nothing in the natural world is lost at an accelerating exponential rate like this glacier” (8/09). And we knew deep ocean heat is rapidly melting Antarctic ice (12/10), which noted: “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.”
Now this new Proceedings of the National Academy of Sciences study, “Ice-shelf collapse from subsurface warming as a trigger for Heinrich events” (subs. req’d) provides paleoclimate evidence that very small temperature increases of the subsurface water at the poles can trigger rapid collapse of ice shelves. The study concludes that a subsurface warming of only 2 °C (3.6 °F) “increases basal melt rate under an ice shelf … by a factor of approximately 6.” And remember, we are headed for very large temperature increases at the poles on our current emissions path — see M.I.T. doubles its 2095 warming projection to 10 °F — with 866 ppm andArctic warming of 20 °F.
Here’s more from the news release:
The results are important, researchers say, due to concerns that warmer water could cause a comparatively fast collapse of ice shelves in Antarctica or Greenland, increasing the flow of ice into the ocean and raising sea levels. One of the most vulnerable areas, the West Antarctic Ice Sheet, would raise global sea level by about 11 feet if it were all to melt.“We don’t know whether or not water will warm enough to cause this type of phenomenon,” said Shaun Marcott, a postdoctoral researcher at Oregon State University and lead author of the report. “But it would be a serious concern if it did, and this demonstrates that melting of this type has occurred before.”If water were to warm by about 2 degrees under the ice shelves that are found along the edges of much of the West Antarctic Ice Sheet, Marcott said, it might greatly increase the rate of melting to more than 30 feet a year. This could cause many of the ice shelves to melt in less than a century, he said, and is probably the most likely mechanism that could create such rapid changes of the ice sheet….A present-day concern, Marcott said, is that ocean currents could shift and change direction even before overall ocean water had warmed a significant amount. If currents shifted and warmer water was directed toward ice shelves, more rapid melting might begin, he said.
Actually that appears to be happening already. In fact, just in June we learned the details of how ocean currents were carving out ice caves in Antarctic ice, as “seawater appear[s] to boil on the surface like a kettle on the stove”
Remember, 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.” And the ice loss is accelerating (see “JPL bombshell: Polar ice sheet mass loss is speeding up, on pace for 1 foot sea level rise by 2050“).
Not only is the West Antarctic ice sheet (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 Scienceexamined 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).
Buckle your seatbelts. It’s going to be a bumpy ride
Related posts:
- Science: CO2 levels haven’t been this high for 15 million years, when it was 5° to 10°F warmer and seas were 75 to 120 feet higher — “We have shown that this dramatic rise in sea level is associated with an increase in CO2 levels of about 100 ppm.”
- Science stunner: On our current emissions path, CO2 levels in 2100 will hit levels last seen when the Earth was 29°F (16°C) hotter
- Satellite data stunner: “Our data suggest that EAST Antarctica is losing mass” — Antarctica may soon be contributing significantly more to global sea-level rise.”
- Nature: “Dynamic thinning of Greenland and Antarctic ice-sheet ocean margins is more sensitive, pervasive, enduring and important than previously realized.”
- Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100
- Antarctica has warmed significantly over past 50 years
- West Antarctic ice sheet collapse even more catastrophic for U.S. coasts
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