â€œIf this were to go, sea levels worldwide would go up 20 feet,â€ says former Vice-President Al Gore in his global warming documentary slide show, An Inconvenient Truth (2006). Gore was talking about what would happen if the West Antarctic ice sheet or the Greenland ice sheet were to melt entirely away.
As recently as his April 27, 2009 testimony before the Senate Subcommittee on Energy and the Environment, Gore again noted, â€œWere the Greenland ice sheet to melt, crack up and slip into the North Atlantic, sea level would rise almost 20 feet.â€ In a presentation made at the Copenhagen climate change conference earlier this month, Gore claimed, â€œThere is a 75 percent chance that the entire north polar ice cap during some of the summer months will be completely ice-free within the next five to seven years.â€
How do these statements square with a report, Melting Snow and Ice: A Call for Action, commissioned by Gore and by Norwayâ€™s Foreign Minister Jonas Gahr StÃ¸re and released at the Copenhagen conference? Among other things, the report finds that the mean monthly snow cover in the Northern Hemisphere has been declining by about 1.5 percent per decade. In addition, rivers and lakes are freezing over later and breaking up earlier. â€œOver the past 150 years, river and lake ice cover duration has been decreasing at a rate of about 1.4 days and 1.7 days per decade,â€ according to the report. But the chief reason anyone cares about global snow and ice trends is that melting ice sheets and glaciers contribute to rising sea levels.
First, a bit of background. In 2007, the United Nations Intergovernmental Panel on Climate Changeâ€™s Fourth Assessment Report (4AR) noted that during the 20th century sea levels rose at about 1.7 millimeters per year (almost 7 inches over the 20th century), but that the rate had increased to about 3 millimeters per year between 1993 and 2003 (about 12 inches per century). The 4AR projected future sea level rise of between 18 and 59 centimeters (7 to 23 inches) by 2100. However, this projection did not take ice sheet dynamics into account because it was thought that they were inadequately modeled at the time the 4AR was released.
So what did the Norwegian Polar Institute report find? Arctic sea ice is now declining at a rate of 11.2 percent per decade, relative to the 1979 to 2000 average. Melting sea ice does not affect sea levels, but does affect temperatures near land-based ice sheets. On the other hand, the sea ice around Antarctica has been increasing at a rate of about 1 percent per decade. Parts of the Antarctic Ice Sheet are melting away, and there are now indications that the Antarctic continent is experiencing net loss of ice.
The report also notes that the Greenland ice sheet is losing volume and the mass loss has increased significantly over the last 10 years. And while the speed with which Greenlandâ€™s glaciers have been flowing into the sea accelerated over the last decade, a recent report at the American Geophysical Unionâ€™s annual meeting in January 2009 by Swansea University glaciologist Tavi Murray found that the acceleration has stopped and that outlet glacier flows in southern Greenland have returned to the levels of 2000. One concern has been that meltwater at the base of glaciers may be speeding up their flow into the sea, but another recent study in Nature Geoscience found that this is not the case. The researchers concluded that â€œrecent rates of mass loss in Greenland's outlet glaciers are transient and should not be extrapolated into the future.â€ But before one breathes a sigh of relief about the effects of Greenlandâ€™s glaciers on future sea levels, a new report published in November in Science finds that Greenlandâ€™s ice cap is melting faster than ever.
So how much sea level rise is the planet likely to experience over the next century? The Melting Snow and Ice report notes, â€œThe near-future contribution of ice sheets to sea level is highly uncertain but potentially large.â€ The report does cite a recent study from Geophysical Research Letters that finds that â€œif the climate continues to warm along current trends, a minimum of 373 Â± 21 millimeters (about 15 inches) of sea-level rise over the next 100 years is expected from glaciers and ice caps.â€
Taking into account future thermal expansionâ€"sea water expanding as the globe warmsâ€"University of Copenhagen glaciologist Dorthe Dahl Jensen, speaking at the session in which the report was released, projected that global sea levels could rise by 1 meter (39 inches) by 2100, plus or minus half a meter (20 inches).
The lower bound of this new projection overlaps with the upper bound of the IPCC 4AR projection. In contrast, a recent article in Energy & Environment (a journal, it is fair to say, that is editorially skeptical of catastrophic global warming projections) suggests that the â€œbest guessâ€ for sea level rise over the next century is 23 centimeters (about 9 inches).
So sea levels are very likely going to rise over the coming century, but not by 20 feet. Admittedly, Gore does not say that sea levels will rise by 20 feet this century, just that that much increase would occur if Greenlandâ€™s ice cap melted away. Fortunately, researchers generally project that the complete melting away of the Greenland ice sheet caused by man-made global warming would take between 500 and 1000 years. This is not great, but it certainly still gives humanity time to figure out how to adapt to such an increaseâ€"or figure out how to stop it. And it would be a good idea for builders and insurance companies to keep the projected rise in sea levels in mind.
What about Goreâ€™s Copenhagen prediction of an ice-free Arctic Ocean by 2014? The researcher whose work Gore said he was citing, Wieslav Maslowski of the Naval Postgraduate School in Monterey, California, repudiated Goreâ€™s assertion the next day. The Melting Snow and Ice report cites modeling studies that find that the Arctic Ocean in September could be ice-free by 2100, or perhaps as early as 2037.
Ultimately, the Melting Snow and Ice report offers a lot of good evidence that the planet has been warming up. Of course, its predictions of rising sea levels as a result of melting ice sheets depend upon projections of future man-made global warming being accurate. But thatâ€™s a topic for another time.
Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is available from Prometheus Books.