The report was ordered by an influential US Pentagon advisor but was covered up by "US defense chiefs" for four months, until it was "obtained" by the British weekly The Observer.
The leak promises to draw angry attention to US environmental and military policies, following Washington's rejection of the Kyoto Protocol (news - web sites) on climate change and President George W. Bush (news - web sites)'s skepticism about global warning -- a stance that has stunned scientists worldwide.
The Pentagon report, commissioned by Andrew Marshall, predicts that "abrupt climate change could bring the planet to the edge of anarchy as countries develop a nuclear threat to defend and secure dwindling food, water and energy supplies," The Observer reported.
The report, quoted in the paper, concluded: "Disruption and conflict will be endemic features of life.... Once again, warfare would define human life."
Its authors -- Peter Schwartz, a CIA (news - web sites) consultant and former head of planning at Royal Dutch/Shell Group, and Doug Randall of Global Business Network based in California -- said climate change should be considered "immediately" as a top political and military issue.
It "should be elevated beyond a scientific debate to a US national security concern", they were quoted as saying.
Some examples given of probable scenarios in the dramatic report include:
-- Britain will have winters similar to those in current-day Siberia as European temperatures drop off radically by 2020.
-- by 2007 violent storms will make large parts of the Netherlands uninhabitable and lead to a breach in the acqueduct system in California that supplies all water to densely populated southern California
-- Europe and the United States become "virtual fortresses" trying to keep out millions of migrants whose homelands have been wiped out by rising sea levels or made unfarmable by drought.
-- "catastrophic" shortages of potable water and energy will lead to widespread war by 2020.
Randall, one of the authors, called his findings "depressing stuff" and warned that it might even be too late to prevent future disasters.
"We don't know exactly where we are in the process. It could start tomorrow and we would not know for another five years," he told the paper.
Experts familiar with the report told the newspaper that the threat to global stability "vastly eclipses that of terrorism".
Taking environmental pollution and climate change into account in political and military strategy is a new, complicated and necessary challenge for leaders, Randall said.
"It is a national security threat that is unique because there is no enemy to point your guns at and we have no control over the threat," he said.
Coming from the Pentagon, normally a bastion of conservative politics, the report is expected to bring environmental issues to the fore in the US presidential race.
Last week the Union of Concerned Scientists, an influential and non-partisan group that includes 20 Nobel laureates, accused the Bush administration of having deliberately distorted scientific fact to serve its policy agenda and having "misled the public".
Its 38-page report, which it said took over a year to prepare and was not time to coincide with the campaign season, details how Washington "systematically" skewed government scientific studies, suppressed others, stacked panels with political and unqualified appointees and often refused to seek independent expertise on issues.
Critics of the report quoted by the New York Times denied there was deliberate misrepresentation and called it politically motivated.
The person behind the leaked Pentagon report, Andrew Marsall, cannot be accused of the same partisan politicking.
Marsall, 82, has been an advisor for the defense department for decades, and was described by The Observer as the author of Defense Secretary Donald Rumsfeld's plans for a major transformation of the US military.