Climate campaigners echoed Mr Reid's warning, and demanded that ministers redouble their efforts to curb carbon emissions.

Tony Blair will today host a crisis Downing Street summit to address what he called "the major long-term threat facing our planet", signalling alarm within Government at the political consequences of failing to deal with the spectre of global warming.

Activists are modelling their campaign on last year's Make Poverty History movement in the hope of creating immense popular pressure for action on climate change.

Mr Reid used a speech at Chatham House last night to deliver a stark assessment of the potential impact of rising temperatures on the political and human make-up of the world. He listed climate change alongside the major threats facing the world in future decades, including international terrorism, demographic changes and global energy demand.

Mr Reid signalled Britain's armed forces would have to be prepared to tackle conflicts over dwindling resources. Military planners have already started considering the potential impact of global warming for Britain's armed forces over the next 20 to 30 years. They accept some climate change is inevitable, and warn Britain must be prepared for humanitarian disaster relief, peacekeeping and warfare to deal with the dramatic social and political consequences of climate change.

Mr Reid warned of increasing uncertainty about the future of the countries least well equipped to deal with flooding, water shortages and valuable agricultural land turning to desert.

He said climate change was already a contributory factor in conflicts in Africa.

Mr Reid said: "As we look beyond the next decade, we see uncertainty growing; uncertainty about the geopolitical and human consequences of climate change.

"Impacts such as flooding, melting permafrost and desertification could lead to loss of agricultural land, poisoning of water supplies and destruction of economic infrastructure.

"More than 300 million people in Africa currently lack access to safe water; climate change will worsen this dire situation."

He added: "These changes are not just of interest to the geographer or the demographer; they will make scarce resources, clean water, viable agricultural land even scarcer.

"Such changes make the emergence of violent conflict more rather than less likely... The blunt truth is that the lack of water and agricultural land is a significant contributory factor to the tragic conflict we see unfolding in Darfur. We should see this as a warning sign."

Tony Juniper, the executive director of Friends of the Earth, said: "The science of global warming is becoming ever more certain about the scale of the problem we have, and now the implications of that for security and politics is beginning to emerge."

He said the problems could be most acute in the Middle East and North Africa.

Charlie Kornick, head of climate campaigning at the pressure group Greenpeace, said billions of people faced pressure on water supplies due to climate change across Africa, Asia and South America. He said: "If politicians realise how serious the problems could be, why are British CO2 emissions still going up?"

Tony Blair will be joined by the Chancellor Gordon Brown, the Environment Secretary, Margaret Beckett, and the International Development Secretary, Hilary Benn, at today's talks in Downing Street.

They will be meeting representatives of the recently created Stop Climate Chaos, an alliance of environmental groups including Greenpeace, Friends of the Earth and Oxfam. It will also meet opposition parties.

The alliance will call for the Government to commit itself to achieving a 3 per cent annual fall in carbon dioxide emissions.

The facts

* On our watery planet, 97.5 per cent of water is salt water, unfit for human use.

* Most of the fresh water is locked in the ice caps.

* The recommended basic water requirement per person per day is 50 litres. But people can get by with about 30 litres: 5 litres for food and drink and another 25 for hygiene.

* Some countries use less than 10 litres per person per day. Gambia uses 4.5, Mali 8, Somalia 8.9, and Mozambique 9.3.

* By contrast the average US citizen uses 500 litres per day, and the British average is 200.

* In the West, it takes about eight litres to brush our teeth, 10 to 35 litres to flush a lavatory, and 100 to 200 litres to take a shower.

* The litres of water needed to produce a kilo of:

Potatoes 1,000

Maize 1,400

Wheat 1,450

Chicken 4,600

Beef 42,500

The document 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 threat to global stability vastly eclipses that of terrorism, say the few experts privy to its contents.

'Disruption and conflict will be endemic features of life,' concludes the Pentagon analysis. 'Once again, warfare would define human life.'

The findings will prove humiliating to the Bush administration, which has repeatedly denied that climate change even exists. Experts said that they will also make unsettling reading for a President who has insisted national defence is a priority.

The report was commissioned by influential Pentagon defence adviser Andrew Marshall, who has held considerable sway on US military thinking over the past three decades. He was the man behind a sweeping recent review aimed at transforming the American military under Defence Secretary Donald Rumsfeld.

Climate change 'should be elevated beyond a scientific debate to a US national security concern', say the authors, Peter Schwartz, CIA consultant and former head of planning at Royal Dutch/Shell Group, and Doug Randall of the California-based Global Business Network.

An imminent scenario of catastrophic climate change is 'plausible and would challenge United States national security in ways that should be considered immediately', they conclude. As early as next year widespread flooding by a rise in sea levels will create major upheaval for millions.

Last week the Bush administration came under heavy fire from a large body of respected scientists who claimed that it cherry-picked science to suit its policy agenda and suppressed studies that it did not like. Jeremy Symons, a former whistleblower at the Environmental Protection Agency (EPA), said that suppression of the report for four months was a further example of the White House trying to bury the threat of climate change.

Senior climatologists, however, believe that their verdicts could prove the catalyst in forcing Bush to accept climate change as a real and happening phenomenon. They also hope it will convince the United States to sign up to global treaties to reduce the rate of climatic change.

A group of eminent UK scientists recently visited the White House to voice their fears over global warming, part of an intensifying drive to get the US to treat the issue seriously. Sources have told The Observer that American officials appeared extremely sensitive about the issue when faced with complaints that America's public stance appeared increasingly out of touch.

One even alleged that the White House had written to complain about some of the comments attributed to Professor Sir David King, Tony Blair's chief scientific adviser, after he branded the President's position on the issue as indefensible.

Among those scientists present at the White House talks were Professor John Schellnhuber, former chief environmental adviser to the German government and head of the UK's leading group of climate scientists at the Tyndall Centre for Climate Change Research. He said that the Pentagon's internal fears should prove the 'tipping point' in persuading Bush to accept climatic change.

Sir John Houghton, former chief executive of the Meteorological Office - and the first senior figure to liken the threat of climate change to that of terrorism - said: 'If the Pentagon is sending out that sort of message, then this is an important document indeed.'

Bob Watson, chief scientist for the World Bank and former chair of the Intergovernmental Panel on Climate Change, added that the Pentagon's dire warnings could no longer be ignored.

'Can Bush ignore the Pentagon? It's going be hard to blow off this sort of document. Its hugely embarrassing. After all, Bush's single highest priority is national defence. The Pentagon is no wacko, liberal group, generally speaking it is conservative. If climate change is a threat to national security and the economy, then he has to act. There are two groups the Bush Administration tend to listen to, the oil lobby and the Pentagon,' added Watson.

'You've got a President who says global warming is a hoax, and across the Potomac river you've got a Pentagon preparing for climate wars. It's pretty scary when Bush starts to ignore his own government on this issue,' said Rob Gueterbock of Greenpeace.

Already, according to Randall and Schwartz, the planet is carrying a higher population than it can sustain. By 2020 'catastrophic' shortages of water and energy supply will become increasingly harder to overcome, plunging the planet into war. They warn that 8,200 years ago climatic conditions brought widespread crop failure, famine, disease and mass migration of populations that could soon be repeated.

Randall told The Observer that the potential ramifications of rapid climate change would create global chaos. 'This is depressing stuff,' he 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.'

Randall added that it was already possibly too late to prevent a disaster happening. '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 said.

'The consequences for some nations of the climate change are unbelievable. It seems obvious that cutting the use of fossil fuels would be worthwhile.'

So dramatic are the report's scenarios, Watson said, that they may prove vital in the US elections. Democratic frontrunner John Kerry is known to accept climate change as a real problem. Scientists disillusioned with Bush's stance are threatening to make sure Kerry uses the Pentagon report in his campaign.

The fact that Marshall is behind its scathing findings will aid Kerry's cause. Marshall, 82, is a Pentagon legend who heads a secretive think-tank dedicated to weighing risks to national security called the Office of Net Assessment. Dubbed 'Yoda' by Pentagon insiders who respect his vast experience, he is credited with being behind the Department of Defence's push on ballistic-missile defence.

Symons, who left the EPA in protest at political interference, said that the suppression of the report was a further instance of the White House trying to bury evidence of climate change. 'It is yet another example of why this government should stop burying its head in the sand on this issue.'

Symons said the Bush administration's close links to high-powered energy and oil companies was vital in understanding why climate change was received sceptically in the Oval Office. 'This administration is ignoring the evidence in order to placate a handful of large energy and oil companies,' he added.

WIRED: Until recently, defense planners talked about a "revolution in military affairs." Now the buzzword is "transformation." Why the change?

MARSHALL: Transformation is more of an imperative: We've got to transform the force. I personally don't like the term. It tends to push people in the direction of changing the whole force. You need to be thinking about changing some small part of the force more radically, as a way of exploring what new technologies can really do for you.

What is the next radical change the US will reveal on the battlefield?

"One future intelligence problem: knowing what drugs the other guys are on." One that's still under way is the emergence of a variety of precision weapons, and also coupling them with sensors. Another is the ability to coordinate the activities of separate elements of the forces to a level that has never been possible before. That's promising, but less far along than precision weapons. A third is robotic devices: unmanned vehicles, of which the UAVs are the furthest along, but also similar kinds of devices undersea, and smaller devices that might change urban warfare by being able to crawl through buildings.

Are there revolutionary developments that don't involve combat?

There are ways of psychologically influencing the leadership of another state. I don't mean information warfare, but some demonstration of awesome effects, like being able to set off impressive explosions in the sky. Like, let us show you what we could do to you. Just visually impressing the person.

Did 9/11 change your mind about anything?

Not much. It was obvious that we were wide open to attack.

Has anything happened that surprised you?

The rapidity of the collapse of the Soviet Union surprised me. I thought they were in trouble, but the rapidity and completeness of the withdrawal were really striking.

Is there a precedent for one country staying on top through a series of military revolutions? Or does one country always leapfrog another?

Through most of the 19th century, the British Navy exhibited that kind of thing. But it was quite interesting the way they did it. They tended to let other countries, mainly France, do the early experiments and come out with new kinds of ships. If something looked like a good idea, they could come in and quickly overtake the innovator. They seemed to do that as a way of capitalizing on their advantage and saving resources.

Isn't the United States in a similar position now?

That's probably the case. But some of the countries that would be candidates to make innovations aren't doing it. The Japanese and West Europeans aren't really making big changes. The Swedes are an interesting case. For 200 years their basic problem was the possibility of a large-scale land invasion by the Russians. They've decided that that has gone away. If anything could happen, it would happen across the Baltic. So they're rethinking, given modern technology, how to create a defense largely on sea frontiers. It's possible that they will make some innovations that we'll pick up and capitalize on.

For instance?

They've designed three new naval vessels. One is an air-independent submarine [running on fuel cells rather than nuclear power, which allows it to travel almost silently and remain submerged for extended periods]. They have a surface ship that's a bit more conventional. And then a radically new naval vessel called the Visby, which has practically no metal in it other than the engine. It's constructed to be very stealthy.

You're known for following technology outside the traditional realm of national security. Pharmaceuticals, for instance.

People who are connected with neural pharmacology tell me that new classes of drugs will be available relatively shortly, certainly within the decade. These drugs are just like natural chemicals inside people, only with behavior-modifying and performance-enhancing characteristics. One of the people I talk to jokes that a future intelligence problem is going to be knowing what drugs the other guys are on.

In an era of terrorism and peacekeeping, are Cold War ideas based on striking a big enemy from afar and defending against missile attack still relevant?

Yes, if we want to stay in the business of long-range power projection. And if we play the role of intervening in messy disputes, some of this weaponry is still useful, as it was in Afghanistan. However, we need ground forces to go in and keep the peace.

Does new technology ultimately make us more or less vulnerable?

A friend of mine, Yale economist Martin Shubik, says an important way to think about the world is to draw a curve of the number of people 10 determined men can kill before they are put down themselves, and how that has varied over time. His claim is that it wasn't very many for a long time, and now it's going up. In that sense, it's not just the US. All the world is getting less safe.

Disclosing that he had commissioned a team of scientists and engineers to find ways of reducing the severe damage the UK faces from climate change, he says the potential damage to property runs into "tens of billions of pounds per annum".

Britain is doing its bit to reduce emissions, but acting alone is not enough, he says. "We and the rest of the world are now looking to the USA to play its leading part."

As an example of what his team is discussing, he says Britain's coastal defences will be subject to attack from both increased sea-level rises and greater storm surges.

"These combined efforts have the potential to increase risk of floods in 2080 by up to 30 times present levels. In the highest emission scenario, by 2080 flood levels that are now expected once in 100 years could be recur ring every three years. People at high risk of flooding in Britain will double to nearly 3.5 million."

If no work is done coastal erosion in Britain will increase nine-fold, he adds.

Urging action to reduce carbon dioxide emissions at once Sir David comments: "Delaying action for decades, or even just years, is not a serious option. I am firmly convinced that if we do not stop now, more substantial, m ore disruptive, and more expensive change will be needed later on." [...]

Yesterday a major study published in Nature magazine showed that climate change over the next 50 years is expected to drive a quarter of land animals and plants into extinction.

Chris Thomas, professor of conservation biology at Leeds University, who was lead author of the research from four continents into the effect of higher temperatures, called the results "terrifying", estimating that more than 1 million species will be lost by 2050.

Much of that loss - more than one in 10 of all plants and animals - is irreversible because of the extra global warming gases already discharged into the atmosphere. However the scientists who conducted the research believe action to curb greenhouse gases now could save others from the same fate.

It's in that process that he's acquired the data, including one of the most comprehensive and accurate temperature records for the entire globe, that makes him so unsafe-data he's repeatedly tried to spread to the world, but always against resistance, mostly from politicians but also from scientists.

The latest dust-up came last week, when The New York Times reported that the public affairs staff at NASA was trying to censor Hansen's contacts with journalists-not to mention postings on his website, his lectures, and his future papers-after he told the American Geophysical Union, in a speech on Dec. 6, that 2005 had been the warmest year on record. Not that they acted out of any untoward motive, NASA officials insist, just to make sure that he wasn't misquoted.

Hansen has had to deliver unpopular news before, and he's always persisted-and this time, as usual, he managed to turn the gag order into a megaphone. In fact, if you follow the thread of the controversies that have marked Hansen's career, you can understand how the idea of global warming first came to light, how it's been resisted, and why we seem now to be entering into the most dangerous era of all, when theory turns ever more quickly into reality.

. . .

In the late 1970s, global warming was something that very few scientists thought about and almost no politicians took seriously. Hansen, however, then a newly minted PhD arriving in New York from the Midwest, had begun to build his first global climate model, an immensely complex computer simulation of the planet's climate that allows its user to, say, add a layer of carbon dioxide to the atmosphere and see what happens. What happens, as he put it in a paper in Science, is that the temperature goes up, a lot: He predicted that ''the continuing increase in fossil fuel use would lead to about 4.5-degree Fahrenheit global warming by the end of the twenty-first century."

The incoming Reagan administration, unfortunately, did not want to hear that-and so they cut his funding to the bone, forcing him to lay off most of his NASA staff. Still, he'd raised the issue, gotten it on the front pages of the papers for the first time, and-since he had the best model of the world's climate anywhere-he eventually got a new round of funding.

The next time he had something to say, he didn't choose a scientific journal. It was June 1988, and the country was unusually hot-the Mississippi River was so dry that barges were unable to navigate the shallow waters. A congressional committee asked him to testify, and he did. In his allotted 15 minutes, in his usual mild voice, he predicted that 1988 would set a new global temperature record, and indeed that he was ''99 percent confident" that it was due to the greenhouse effect. As he left the hearing, he told reporters: ''It's time to stop waffling so much and say that the greenhouse effect is here and is affecting our climate now."

That sound bite rankled the Republican administration (which tried to edit his further testimony, with about as much success as NASA had last week) but it also angered his scientific peers. He'd broken the scientific code, talked in real-world terms, ''gone beyond the data." Daggers were drawn, and they were used. Mark Bowen, in his fine recent history of climate science, ''Thin Ice" (2005), describes the next meeting of the world's climatologists as a ''get-Hansen session." Scientists, accustomed to publishing in peer-reviewed journals, speak in nuances and caveats. To say ''I'm 99 percent certain" goes against the academic grain.

Hansen, of course, understood this. He's spent his whole career in academic conferences and scientific meetings. But he also knew that the rest of the world-the audience that needed to understand our predicament-doesn't speak that way.

The professional criticism stung him, but it had clearly been worth it. Every newspaper and every newsmagazine published long reports on the subject-within a week, ''greenhouse effect" had gone from a scientific term to a media buzzword. By year's end Time had named ''our endangered earth" its ''planet of the year," and the elder George Bush, running for president, was promising to ''fight the greenhouse effect with the White House effect"-an applause line, unfortunately, which didn't lead to much in the way of policy. More importantly, the fear Hansen sparked led to almost bottomless federal research budgets for his fellow scientists, and by 1995 they had concluded he was correct: The planet, said the world's foremost body of climatologists, the Intergovernmental Panel on Climate Change, was heating up and people were the cause.

In the rest of the developed world, that was enough to get policy makers working on projects like the Kyoto Treaty. But here, where all the crucial science had been done, the prospect of grappling with our fossil fuel addiction was simply too daunting for either the Clinton administration, which at least recognized the problem, or the George W. Bush White House, which turned its back. When one Environmental Protection Agency document dared to mention the possibility of a warming planet, Bush dismissed it as a product of ''the bureaucrats."

And so we go on burning ever more fossil fuel, and the earth keeps getting warmer-as Hansen's monthly monitoring of 10,000 temperature gauges around the planet makes depressingly clear.

But the new high temperature record isn't the real reason Hansen is so agitated right now, nor the reason the Bush administration would like to silence him. Instead, it's the messages about future change that his computer climate models keep spitting out.

Those models reveal a miserable situation at present, but a dire one in the years ahead. In his December speech to the Geophysical Union, he noted that carbon dioxide emissions are ''now surging well above" the point where damage to the planet might be limited. Speaking to a reporter from The Washington Post, he put it bluntly: Having raised the earth's temperature 1 degree Fahrenheit in the last three decades, we're facing another increase of 4 degrees over the next century. That would ''imply changes that constitute practically a different planet." The technical terms for those changes include drought, famine, pestilence, and flood.

''It's not something we can adapt to," he continued. ''We can't let it go on another 10 years like this."

And that's what makes him so dangerous now. He's not just saying that the world is warming. He's not just saying we're the cause. He's saying: We have to stop it now. Not wait a few decades while Exxon Mobil keeps making record profits. Not wait a few decades until there's some painless new technology like hydrogen cars that lets us drive blithely into the future. Not even wait a few years until the current administration can cut and run from Washington.

The president, just this week, said that we've become ''addicted to oil," which is a little as though Abe Lincoln suddenly noticed the South had slaves. Bush's package of fixes-a little money for nuclear, for clean coal, for wind power-goes in the right direction, but so slowly as to be a gesture, not a policy. If we want to keep a semblance of the planet we were born on to, we have to act decisively, expensively, quickly, and now.

You can argue with Hansen if you want. But you better bring a pretty big data set with you. He's been right so far.

Bill McKibben, a scholar in residence at Middlebury College, is the author of ''The End of Nature" and eight other books on environmental topics.