Cinephiles may sneer at the climate disaster movie that is opening at the end of this month, The Day After Tomorrow, in which cities are inundated by tidal waves and snow drifts halfway up Manhattan's skyscrapers [see photo, "Iced"].

Leaked to Fortune magazine in February, the study is now being publicized in general-interest newspapers and magazines. It is widely seen as sounding an ironic counterpoint to the Bush administration's downplaying of concerns about climate change as overly alarmist.

Even by the standards of the worst-case analysis favored by Pentagon strategists, "An Abrupt Climate Change Scenario and Its Implications for United States National Security," delivered to the 83-year-old Marshall last October, goes over the top. Taking their cue from events believed by geoscientists to have occurred 8200 and 12 700 years ago, consultants Peter Schwartz and Doug Randall imagine a repeat scenario, in which what is known as the thermohaline circulation in the North Atlantic shuts down, preventing salty warm water in equatorial regions from flowing north.

That could happen before the end of this century, and it would take just a few years. The results:

Average annual temperatures will drop 2.8–3.3 °C in Asia, North America, and Europe.

Temperatures will increase 2.2 °C in Australia, South America, and southern Africa.

Persistent droughts will parch key agricultural areas and urban concentrations.

Winter storms and winds will intensify, amplifying the impacts of other changes.

In short, Earth's carrying capacity—its ability to support its populations—will suddenly and drastically be reduced, bringing the nightmarish prospect of ubiquitous warfare as people try to move into more habitable areas and struggle for resources—and ending what Schwartz and Randall see, a little bizarrely, as a relatively benign era in human affairs.

In ancient times, the authors say, states simply slaughtered all their enemies, while in more recent times, countries "merely kill enough to get a victory and then put the survivors to work in their newly expanded economy." They worry that "all of that progressive behavior could collapse if carrying capacities everywhere were suddenly lowered drastically by abrupt climate change."

The alarmist duo note in a boxed preface to their report, "Imagining the Unthinkable," that while the scientists they interviewed supported their project, the scientists cautioned that the scenario depicted is doubly extreme: the changes probably would not be so big, and they probably would affect only some regions. Despite those experts' reservations, the authors chose for the sake of argument to create "a climate scenario that although not the most likely, is plausible, and would challenge U.S. national security in ways that should be considered immediately."

Being successful consultants, they no doubt had the interests of their client squarely in mind. Marshall, a high-level Pentagon analyst for decades, is said to especially favor very long-term, very far-out scenarios. He has been associated with several hugely controversial causes, among them ballistic missile defenses and the so-called revolution in military affairs, which predicted that precision-guided weaponry and real-time intelligence would transform warfare (which they have, in fact, done).

Among his admirers, Marshall is sometimes called Yoda, after the wise old creature in the Star Wars films; members of his coterie are duly dubbed the Jedi. Among Marshall's critics, however, he is sometimes accused of "thinking outside the box for the sake of thinking outside the box," fused with a touch of the paranoid, as a writer put it in The American Prospect, a liberal monthly.

As it happens, Marshall is not the only Yoda around, and his followers not the only Jedi. The climate report he initiated was inspired, as is plain to anybody following developments in climate science, mainly by work done by Wallace S. Broecker of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. Perhaps the leading marine geochemist of his generation and an enormously influential figure in climate controversies for several decades, Broecker—himself well into his seventies—once outlined an event believed to have occurred 12 centuries ago during a cooling period known as the Younger Dryas.

The episode was triggered by a sudden infusion of ponded fresh water into the North Atlantic, which shut down the thermohaline conveyor, leading to a mini-Ice Age in the northern hemisphere. Broecker described that scenario, now widely accepted among experts, in a famous article, "The Biggest Chill," in Natural History magazine 17 years ago.

As it happens, Broecker is now a central figure in an ad hoc group assembled at Lamont-Doherty to evaluate abrupt climate-change scenarios. In a letter sent to Science magazine on the group's behalf, Broecker wrote: "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....Exaggerated scenarios serve only to intensify the existing polarization over global warming."

For a short guide to resources on abrupt climate change,including a recent National Research Council report and historical literature, go to here