WILL OCEANS SURGE 59 CENTIMETRES THIS CENTURY - OR 25 METRES?
By Zoe Cormier
The Globe & Mail
August 25, 2007
Original LinkLONDON -- When Al Gore predicted that climate change could lead to a 20-foot rise in sea levels, critics called him alarmist. After all, the International Panel on Climate Change, which receives input from top scientists, estimates surges of only 18 to 59 centimetres in the next century.
But a study led by James Hansen, the head of the climate science program at the National Aeronautics and Space Administration's Goddard Institute for Space Studies in New York and a professor at Columbia University, suggests that current estimates for how high the seas could rise are way off the mark -- and that in the next 100 years melting ice could sink cities in the United States to Bangladesh.
"If we follow 'business-as-usual' growth of greenhouse gas emissions," he writes in an e-mail interview, "I think that we will lock in a guaranteed sea-level rise of several metres, which, frankly, means that all hell is going to break loose."
The scientific basis for this idea -- which Prof. Hansen and five co-authors gleaned from geological records, ice core samples and analysis of the sea floor -- is outlined in a recent paper published by the British journal Philosophical Transactions of the Royal Society.
In stark contrast to estimates put forward by the IPCC, Prof. Hansen and his colleagues argue that rapidly melting ice caps in Antarctica and Greenland could cause oceans to swell several metres by 2100 -- or maybe even as much as 25 metres, which is how much higher the oceans sat about three million years ago.
Their argument goes like this: As the atmosphere warms and the ice caps melt, they will not melt in a consistent, gradual fashion. Rather, they will start to melt faster and faster as the century progresses, quickly reaching a point where they could disappear altogether. This is because of "positive feedback" effects -- factors that create a loop of exacerbated melting and global warming.
For example, snow and ice reflect sunlight and reduce global warming. But as the temperature of the planet increases and the polar caps melt (as scientists are already observing at both poles), there is less ice to reflect sunlight and more water to absorb it, thus making the planet warmer and increasing ice cap melting further.
Likewise, the mass release of methane from thawing permafrost (happening now in the Canadian Arctic) means that natural greenhouse-gas emissions could be added to man-made emissions -- potentially speeding up climate change. And as meltwater from polar caps lubricates the contact points between the ice and the bedrock below it (evidenced by an increase in "ice quakes" in Greenland), ice sheets could be further destabilized and result in increased melting.
So why the radical discrepancies between Prof. Hansen's predictions and those of the IPCC? Certain positive feedback effects, as well as recent data on the melting of the Greenland ice sheet, were not included in the IPCC's report. "Because of the cumbersome IPCC review process, they exclude recent information," Prof. Hansen says, "so they are very handicapped."
Richard Peltier agrees. A University of Toronto physicist and the director of the Centre for Global Change Science, he works on mathematical models to explain the melting and freezing dynamics of the Greenland ice sheet and has contributed to the IPCC publications - but even he agrees that their assumptions tend to be "extremely conservative."
"[Prof. Hansen's] basic thesis is undeniable, because the mathematical models, which we have developed to describe the evolution of ice sheets, do not include certain processes that control how quickly an ice sheet could respond to climate warming," he says. "You need a model that incorporates all physical processes -- and no such model exists."
However, Prof. Peltier does not think that the ice caps are likely to melt as quickly as Prof. Hansen suggests. "We really don't know what the future has in store. I am incapable of predicting how fast the ice sheets will melt, and so is he. But I don't think we are going to hell in a handbasket."
Others are even less convinced of the catastrophic predictions put forward by Prof. Hansen. Andrew Weaver, a physicist at the University of Victoria who works on the dynamics of the polar ice caps and also contributes to the IPCC reports, says he thinks the "upper bound for sea-level rise this century is a metre.
"I don't disagree with the seriousness of the issue or the importance of these positive feedback effects," he says, "but runaway feedbacks have extraordinarily low probabilities, which is why they are not given much attention by the IPCC."
He adds that the Greenland ice sheet will almost certainly melt away completely, but the IPCC predicts that this will take 1,700 years -- not a century. "The complete disintegration of the ice sheets cannot happen in 100 years," he says.
Moreover, although he calls Prof. Hansen his "hero" for speaking out about global warming in the 1980s "when nobody was listening," he criticizes the tone of his recent paper and the use of words such as "cataclysm," which he believes move "dangerously away from scientific discourse to advocacy."
And at any rate, Prof. Weaver says, we have enough to worry about, regardless of what the future holds: The disappearance of the Himalayan glaciers threatens the source of fresh water for about one billion people and climate change is causing severe weather ranging from the droughts in Darfur to the flooding seen in Britain this summer.
Still, Prof. Hansen insists that his predictions are on target - and that the conservative take on climate change put forward by the IPCC and others could result in catastrophe.
"I believe there is pressure on scientists to be conservative. Caveats are essential to science. They are born in skepticism, and skepticism is at the heart of the scientific method and discovery," he wrote in New Scientist magazine last month. "However, in a case such as ice-sheet instability and sea-level rise, excessive caution also holds dangers. 'Scientific reticence' can hinder communication with the public about the dangers of global warming. We may rue reticence if it means no action is taken until it is too late to prevent future disasters."
Instead, Prof. Hansen urges a swift curb on greenhouse-gas emissions. The last time sea levels rose by almost 25 metres, he points out, was when the greenhouse-gas levels in the atmosphere were on par with what may happen if fossil-fuel emissions continue unchecked. He believes we should keep the concentration of carbon dioxide in the atmosphere below 450 parts per million. Right now, it stands at about 385 ppm.
"The first step should be a moratorium on new coal-fired power plants until technology is available to capture and store the carbon dioxide," he says, "and a gradually rising tax on carbon emissions."
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