The nightmare scenario of global warming predicted in Hollywood's latest blockbuster may not be quite as implausible as it first appears, writes Frank McDonald, Environment Editor

Hailstones the size of grapefruits kill people on the streets of Tokyo, tornadoes rip through Los Angeles, and New York City is engulfed by a massive tidal wave. These are among the cataclysmic events dramatised by The Day After Tomorrow, Hollywood's take on global warming.

If climate change is going to mean more "extreme weather events", it couldn't get more extreme than this. A background news broadcast reports that Europe is blanketed by 15 feet of snow and Dublin is among the cities being evacuated. But then, the film is about bringing the threat home to us.

A recent poll commissioned by the London-based Carbon Trust, which works to cut carbon dioxide emissions, found that nearly a third of adults in Britain wrongly believe climate change is reversible, another third see it as a minor or no threat, and a fifth are so confused that they blame plants for causing it. Missions to space, second-hand tobacco smoke, meteors and genetically modified organisms were also wrongly thought to cause climate change. And of the three in 10 who believe that global warming can be reversed, many were just hoping that "Mother Nature will somehow sort herself out".

The nightmare scenario of The Day After Tomorrow - depicting climate change happening in weeks rather than decades - is so wildly improbable that it may be dismissed as just another disaster movie, albeit more compelling than weather-related examples of the genre such as Twister or Dante's Peak. Its scenario is based on the notion that warm ocean currents - including the Gulf Stream, which gives us our temperate climate - will be diverted south by melting polar ice-caps, plunging the Northern Hemisphere into a new Ice Age "just like that". Go outside and you freeze to death - instantly.

But get this. The most recent US assessment of the risks posed by climate change said "significant uncertainties remain" about its regional impacts.

"It is likely that some aspects and impacts . . . will be totally unanticipated as complex systems respond to ongoing climate change in unforeseeable ways." And there's more. A secret report, commissioned by the US Department of Defence and published by Fortune magazine in March, suggested that an imminent scenario of catastrophic climate change is "plausible and would challenge United States national security in ways that should be considered immediately".

Average temperatures in Europe and North America would fall, creating a "Siberian" climate for all of us, while some major cities would be submerged by rising sea levels, according to the Pentagon report. As early as next year, widespread flooding by a rise in sea levels would create major upheaval for millions.

By 2020, "catastrophic" shortages of water and energy supplies will become increasingly harder to overcome, leading to the outbreak of "resource wars" on an already over-populated planet. "Disruption and conflict will be endemic features of life," it said. "Warfare may again come to define human life."

As the report gloomily put it: "History shows that whenever humans have faced a choice between starving or raiding, they raid. So we must assume that, after the great climate change, an ancient pattern re-emerges: the eruption of desperate, all-out wars over food, water and energy supplies."

Doug Randall, one of the report's authors, agreed that it was "depressing stuff". Climate change "could start tomorrow" and there was little we could do to prevent a disaster.

"It is a national security threat that is unique because there is no enemy to point your guns at," he told the Observer newspaper.

Prof Bob Watson, chief scientist for the World Bank and former chairman of the UN's Intergovernmental Panel on Climate Change, said the report could no longer be ignored by a president who had made national security his single highest priority. And the Pentagon was "no wacko liberal group".

Watson even suggested that the report may prove vital in the US elections in November. Democrat candidate John Kerry is on record as accepting that climate change is a real problem and scientists disillusioned with President Bush's stance have urged Kerry to cite its shocking scenarios in his campaign.

The US is the biggest emitter of greenhouse gases, accounting for 24 per cent of the global total - six times more than its share of the world's population. In 2001, however, the pro-oil Bush administration pulled out of the Kyoto Protocol on Climate Change, insisting that cuts would damage the US economy.

This position has been strongly criticised by Sir David King, chief scientific adviser to the British government. Writing in the journal Science last January, he said Bush was wrong to discount the reality of climate change, which he believed was "more serious even than the threat of terrorism".

Listing the evidence, Sir David noted that the 1990s were the hottest decade on record. Ice-caps were melting, sea levels rising and flooding had become more frequent. The Thames barrier was used once a year in the 1980s to protect London, but now it's used more than six times a year.

Last summer, 20,000 Europeans died in a suffocating heatwave - 10,000 in France alone. "As a consequence of continued warming," the British scientist said, "millions more people around the world may in future be exposed to the risk of hunger, drought, flooding and debilitating diseases."

In recent months a major study commissioned by the UN Environment Programme warned that more than a million species will become extinct over the next 50 years due to dramatic changes in their habitats as a result of global warming - unless we begin reducing our greenhouse gas emissions.

Sir David, who was upbraided by Downing Street for being so outspoken, said: "Delaying action for decades, or even just years, is not a serious option. I am firmly convinced that if we do not begin now, more substantial, more disruptive and more expensive change will be needed later on." Environmentalists have seized on The Day After Tomorrow to drive home the message they've been spelling out for years - that action must be taken. "The film is fiction, but climate change is real and humans are causing it," said Will St Leger, spokesman for StopEsso Ireland.

Greenpeace is rolling out a website www.thedayaftertomorrow.org to coincide with the film's release. Headed "The Day Is Today. What Will You Do?", suggestions include: get active; rewrite your own ending; don't buy ExxonMobil; say yes to wind; tell your friends and join Greenpeace.

If a Hollywood movie can't dramatise the dangers of climate change, however telescoped its timescale, nothing will. And as its digital imagery brings that tidal wave tumbling through the avenues and streets of New York City, you can only marvel at the breathtaking arrogance of the world we have built.

Those who continue to resist taking action to avert the worst impacts of global warming like to think that the ecosphere, our whole environment, is merely a division of the economy. In fact, as the Scots would say, it's quite the reverse. For our construct hangs by a slender thread that can so easily be broken.

The Day After Tomorrow opens nationwide on Friday