The Pentagon prides itself on preparing for the worst - be it war, famine or other calamity.

So it may not seem surprising that the Pentagon last year asked two private consultants to consider the potential global impacts of an abrupt and severe change in the world's climate.

Which regions might be hurt the worst, they asked, and what would that mean for U.S. national security?

The scenario sketched out in the report, "Imagining the Unthinkable," may surprise some, though it seems to have been largely discounted by the official who ordered the report.

The report suggests global warming (search) already is approaching a threshold beyond which a sudden cooling will set in. The authors suggest a number of dire consequences in a scenario in which the current period of global warming ends in 2010, followed by a period of abrupt cooling.

— As temperatures rise during this decade, some regions experience severe storms and flooding. In 2007, surging seas break through levees in the Netherlands, making the Hague "unlivable."

— By 2020, after a decade of cooling, Europe's climate becomes "more like Siberia's."

— "Mega-droughts" hit southern China and northern Europe around 2010 and last 10 years.

— In the United States, agricultural areas suffer from soil loss due to higher winds and drier climate, but the country survives the economic disruption without catastrophic losses.

— Widespread famine in China triggers chaos, and "a cold and hungry China peers jealously" at Russia's energy resources. In the 2020-2030 period, civil war and border wars break out in China.

— In a "world of warring states," more countries develop nuclear weapons, including Japan, South Korea, Germany, Iran and Egypt.

— "Disruption and conflict will be endemic features of life."

Sounds pretty grim, and the authors of the report acknowledge in the introduction that the scientists with whom they consulted regard the gloomy scenario as extreme in scope and severity.

They said they were not predicting how climate change will happen but sought to "dramatize the impact climate change could have on society if we are unprepared for it." The scenario they sketched was patterned after a climate event - a sudden global cooling after an extended period of warming - that is believed to have happened 8,200 years ago and lasted for 100 years.

The Pentagon official who commissioned the study, Andrew W. Marshall, issued a brief statement saying it "reflects the limits of scientific models and information when it comes to predicting the effects of abrupt global warming. ... Much of what this study predicts is still speculation."

Marshall, head of the Pentagon's internal think tank, known as the Office of Net Assessments (search), said his intent was to explore the question of whether countries affected by rapid climate change would suffer or benefit, and whether the change would make them more or less stable.

"More pragmatically, what kinds of climate change might our worldwide forces encounter in the future?" Marshall said.

A spokesman for Marshall, Lt. Cmdr. Daniel Hetlage, said the report, which was commissioned last October and finished earlier this month, did not fully satisfy Marshall's needs. Hetlage said the report would not be passed along to Defense Secretary Donald H. Rumsfeld.

Still, the authors, Peter Schwartz and Doug Randall, said their scenario was "not implausible" and would challenge U.S. national security in ways that should be considered immediately. Schwartz is a co-founder of Global Business Network (search), based in Emeryville, Calif., which says it uses "out-of-the-box" thinking in its consulting services to business and government.

Hetlage said the Pentagon paid about $100,000 for the report.

Schwartz and Randall asserted the plausibility of severe and rapid climate change is higher than most scientists and nearly all politicians think. They also concluded it could happen sooner than generally believed.

"This report suggests that because of the potentially dire consequences, the risk of abrupt climate change - although uncertain and quite possibly small - should be elevated beyond a scientific debate to a U.S. national security concern," they wrote.