Scientists stirred to ridicule ice age claims
The New Scientist, April 15, 2004
Climate scientists have been stirred to ridicule claims in an upcoming Hollywood blockbuster that global warming could trigger a new ice age, a scenario also put forward in a controversial report to the US military.
The $125-million epic, The Day After Tomorrow, opens worldwide in May. It will show Manhattan frozen solid after the warm ocean current known as the Gulf Stream shuts down.
The movie's release will come soon after a report to the US Department of Defense (DoD) in February predicting that such a shutdown could put the northern hemisphere into a deep freeze and trigger global famine within 15 years.
But in the journal Science on Thursday, Andrew Weaver of the University of Victoria in British Columbia, Canada, surveys the current research and concludes "it is safe to say that global warming will not lead to the onset of a new ice age".
The DoD's doomsday scenario, which is very similar to that in the film, was drawn up by Peter Schwartz and Doug Randall of the San Francisco-based Global Business Network. Neither is a climate scientist.
The scenario suggests that as global warming melts Arctic ice packs, the North Atlantic will become less salty. This would shut down a global ocean circulation system that is driven by dense, salty water falling to the bottom of the north Atlantic and that ultimately produces the Gulf Stream.
This much is respectable scientific theory, and some researchers believe it could happen for real in 100 years or so. But the film-makers and DoD authors go further.
They say it could happen very soon. And that if it did, the northern hemisphere would cool so much that that ice sheets would start to grow, creating a catastrophic new ice age.
This is too much even for sympathetic climatologists. Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, whose own models say the Gulf Stream could shut down within a century, told New Scientist: "The DoD scenario is extreme and highly unlikely."
And Wallace Broecker of Columbia University,New York, US, who has warned for two decades that the Atlantic circulation is "the Achilles heel of our climate system", seriously questions both the speed and severity of the changes proposed.
In a letter to Science, (reprinted below) he accuses the DoD authors of making exaggerated claims that "only intensify the existing polarisation over global warming". He adds: "What is needed is not more words but rather a means to shut down CO2 emissions." Such action could avert any Gulf Stream shutdown in the next 100 years.
Schwartz defends his scenario, saying that while it is "not the most likely scenario, it is plausible, and would challenge US national security in ways that should be considered immediately".
Weaver notes that the movie's budget "would fund my entire research group for my entire life, 10 times over". That might even allow him to discover which scenarios are most plausible.
But there are no sour grapes. "I will be one of the first to see the movie.," he says. "It'll be the Towering Inferno of climate extremely entertaining." It will not confuse the public, he thinks, but it will not help them understand climate science either.
Future Global Warming Scenarios
Science, Vol 304, Issue 5669, 388 , April 16, 2004
In a study commissioned by the Pentagon, Peter Schwartz and Doug Randall (1) present a very alarming scenario regarding the short-term consequences of global warming. This scenario, which predicts a shutdown of the Atlantic Ocean's conveyor circulation in the next 10 to 15 years, is based on analogies to two large and abrupt climate changes, which occurred 12,700 and 8200 years ago. Both are thought to have been triggered by catastrophic releases of meltwater stored in lakes that formed along the southern margin of the retreating Canadian ice sheet.
These floods appear to have squelched deep water formation in the North Atlantic and, by as yet unknown mechanisms, caused Earth's climate to plunge back toward its glacial condition. Clearly, if global warming were to cause a repeat of such an abrupt change, the consequences would be akin to those alluded to in the warning to the Pentagon, namely, a large cooling of northern Europe. But there is no reason to believe that the impacts could occur in a mere decade, nor would they be so awesome.
As the one who first pointed out the link between the Atlantic's conveyor circulation and abrupt climate changes, I take serious issue with both the timing and the severity of changes proposed in the Pentagon scenario. Computer simulations do suggest that a greenhouse-induced warming would increase the delivery of precipitation and river runoff to the North Atlantic and, further, that given a large enough warming, this excess fresh water could cause the conveyor to sag and, in the extreme, shut down. However, the time required for this to happen is more likely a century, not a decade. Further, no full-fledged global model has yet reproduced the immense impacts coincident with the two meltwater floods. We suspect that the required amplifier involves sea ice formation in the North Atlantic. If indeed this is the case, then as the globe warms, amplification by this mechanism becomes ever less likely.
Exaggerated scenarios serve only to intensify the existing polarization over global warming. What is needed is not more words but rather a means to shut down CO2 emissions to the atmosphere. Although we are powerless to accomplish this by 2015, we certainly have the wherewithal to do it by 2075.
Wallace S. Broecker
Lamont-Doherty Earth Observatory of Columbia University,
Post Office Box 1000,
Palisades, NY 10964-8000,
1. P. Schwartz, D. Randall, "Abrupt climate change," report prepared by
Global Business Network (GBN) for the Department of Defense, available at