8 January 2008Andrew C. Revkin
The melting edge of the Greenland ice sheet near Kangerlussuaq. (Credit: Andrew C. Revkin)Most forecasting is easier and more reliable in the short run than over the long haul. Think of weather prediction. (And history is full of failed long-term forecasts of everything from oil prices to human population trends.)
But for scientists studying the fate of the vast ice sheets of Greenland and West Antarctica, the situation seems reversed. Their views of sea trends through this century still vary widely, while they agree, almost to a person, that centuries of eroding ice and rising seas are nearly a sure thing in a warming world. The great shifts of sea level and temperature through cycles of ice ages and warm intervals make that clear. I wrote about that consensus last year in covering the reports released by the Intergovernmental Panel on Climate Change, but also wrote about scientists’ frustrations over trying to convey the importance of a slow-motion disaster.
Many researchers are working hard to try to clarify whether more melting, both on the ice surface and along the coasts, could greatly speed things. I wrote about some of that work for Science Times this week. This post offers a bit more depth than could fit on the printed page.
And it offers a more vivid view of the work in this video report, which takes you on a ride into the depths of the Greenland ice:
Last summer, scientists from the University of Colorado, NASA, and elsewhere tried to probe Greenland’s internal plumbing, which can carry water from the melting surface down to the base, potentially lubricating where ice grinds over rock and speeding its movement toward the sea. The melting and gushing is dynamic and startling, and has produced a flood of media coverage in recent months. But many scientists doubt, for all the drama, that this process will end up moving meaningful quantities of ice into the sea. The same goes for the snouts of “outlet” glaciers, where ice from the interior funnels through gaps in coastal mountain ranges, and where warming seawater has broken up clots of ice that can hold things up, like a logjam in a river.
Some scientists assessing the recent acceleration of ice flows propose that the rates of increase can’t be sustained long enough to get a truly disastrous rise in seas by 2100 from a warming Greenland. Tad Pfeffer, of the University of Colorado, and Joel T. Harper of the University of Montana laid out their argument for caution at the American Geophysical Union meeting in San Francisco in December, and quite a few glaciologists seem to agree with them.
Of course, there is another wild card in the deck, called the West Antarctic Ice Sheet.
Seasoned experts differ sharply on which storehouse of ice poses the bigger threat to coasts. My emails and calls to more than a dozen experienced ice scientists produced about a 50/50 split on whether Greenland or Antarctica was the biggest short-term risk.
But there was little disagreement that playing what amounts to two games of high-stakes poker at the same time by driving up greenhouse-gas concentrations is a bad idea, particularly as ever more people concentrate on coastlines in both rich and poor countries.
James E. Hansen, the prominent NASA climatologist who has become an outspoken advocate for sharp cuts in greenhouse gases, complained last year about the “reticence” of many of his peers when considering the risk of runaway ice loss within the lives of today’s children. He has co-written several papers recently positing how sustained warming could lead to coastal calamity by 2100.
While the breakup and slipping of ice sheets is a small part of sea rise now, he wrote last year, it could easily accelerate under the heating from a “business as usual” path for emissions. “The broader picture gives a strong indication that ice sheets will, and are already beginning to, respond in a nonlinear fashion to global warming,” he wrote last May in the online journal Environmental Research Letters, adding there was “near certainty” that unabated emissions “would lead to a disastrous multi-meter sea level rise on the century timescale.”
Many experts on polar and climate science push back, saying there is scant evidence to support that level of certainty.
Waleed Abdalati, a NASA scientist focused on the ice sheets, said, “Ice sheets are continually responding to their changing boundary conditions in ways that might mitigate these changes.”
There is a real risk of bigger ice losses and sea-level shifts, but much more work would be needed to clarify the odds, Dr. Abdalati said. “I think that close to a meter is a real possibility in the coming century, and the adverse effects of that should be enough to get people’s attention.”
He concluded by saying scientists were in a real bind in trying to figure out how to discuss climate-related threats of this sort without causing the public and policymakers to glaze over.
In an email, he said: “It is always a challenge to convey scientific uncertainty (and there is a lot in this case) to the general public. People want ‘the answer,’ and when you start to explain why ‘the answer’ is not as obvious as they would like, it is easy to lose them. Plus, there is so much hype made of uncertainty by skeptics, that it gets spun into the idea that scientists don’t really know what they are talking about and don’t have the answers.
“At the end of the day, you can be 90% confident of something, and all people will hear is that you aren’t certain about what you are saying. This is why the debate is often cast in extremes, rather than an honest consideration of the data. It is really too bad, because an honest consideration of the data is still quite compelling.”
http://dotearth.blogs.nytimes.com/2008/01/08/melting-ice-rising-seas-easy-how-fast-hard/#more-129
