Your Planet: The storm approaches

Does climate change really threaten the human race with imminent catastrophe? Almost certainly, says Mark Lynas. And if you don't believe it, just consider all the hard evidence that currently surrounds us

Thinking of buying a house? Better check the Environment Agency online map first. That desirable riverfront property may look tempting in the brochure, but increasing numbers of floodplain-sited developments are soon going to be uninsurable because of flood risk. And once a property is uninsurable, its value - not surprisingly - goes through the floor.

Remember the "negative equity" misery of the 1980s? Well, millions of homeowners with big mortgages - who didn't bother to check back in 2005 whether their desired area was coloured blue on the Agency map before buying - are going to suffer "climate change negative equity" in future decades, potentially leaving them bankrupt and homeless. That's assuming the experience of being evacuated in their pyjamas in the middle of the night as the waters rise hasn't already soured the appeal of what was once a dream home.

Currently, 11 per cent of new homes are built in flood risk areas in England, and hundreds of new developments are still planned in flood-prone areas against Environment Agency advice. Five million people already face the uncertainty of living in flood risk areas. Flood damage costs the UK about £1bn every year, and it's going to get much, much worse.

The reason is not just that new homes are poorly situated, but that the climate is changing - and by the second half of this century will have changed almost beyond recognition. According to the UK Climate Impacts Programme, winter rainfall will be up to a third heavier by the 2080s, drastically increasing flood risk. Summers will get drier overall, but when rain does come it is likely to fall in heavier bursts, triggering more frequent flash floods. Our countryside will also change: a Royal Horticultural Society conference in June heard that familiar tree species such as beech and silver birch will increasingly give way to less familiar invaders such as Corsican pine and walnut.

The British climate may be about to get stormier, but at least we don't yet need to worry about real tropical hurricanes striking our shores. Vulnerable areas in the tropical Atlantic include the Caribbean, Central America and low-lying coastal areas of the United States such as the Mississippi delta. New Orleans has just suffered a deadly strike from Hurricane Katrina, and the city may need to be abandoned completely in the longer term as sea level rise combines with stronger storms to overwhelm flood defences.

Although an acrimonious scientific debate has recently flared up about whether the impacts of global warming are already visible in the incidence and strength of tropical cyclones, few experts doubt that hurricanes will get stronger in a warmer future. Tropical storms derive their energy from the oceans, and with higher sea temperatures more energy is available to drive the swirling vortex at a hurricane's core.

Hurricane modellers Tom Knutson and Bob Tuleya concluded in a major study last year that the potential energy available to hurricanes might increase by 20 per cent in a high-greenhouse gas world. This means, in the authors' words, that even "if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas-induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-five storms". Bad news for anyone living close to tropical seashores.

So if mid-latitudinal areas like Britain are likely to see an increase in flooding while tropical areas are likely to experience fiercer hurricanes, what about continental interiors such as North America and Africa? The news is bad here too. A paper published in the science journal Nature in June by Oxford University's David Thomas and colleagues suggested that much of southern Africa will have turned to desert by the end of the century. Areas of old stabilised sand dunes, covered by vegetation and currently supporting millions of subsistence farmers and animal herders, in a huge area from southern Zambia to Botswana and South Africa, could remobilise as windspeeds increase and rainfall totals diminish over coming decades.

A similar process could also begin in the High Plains of the US, where major agricultural states such as Nebraska could also find themselves suddenly overrun with shifting sands.

The implications for global food production can only be guessed at, but according to Lester Brown of the Earth Policy Institute, humanity will be experiencing net food deficit sooner than anyone currently expects. Higher temperatures and falling water supplies are expected to lead to a reduction in overall harvests, even as new areas are opened up to farming in once-cold places like Canada and Siberia.

The heatwave of 2003 reduced the grain harvest in continental Europe by 30 million tonnes, helping push world grain stocks to their lowest level in three decades. Several scientific studies have identified the 2003 heatwave as a harbinger for the future - one paper published last year in the journal Global and Planetary Change predicted that summer heatwaves like 2003 - which killed an estimated 35,000 people across Europe - will be the norm by the latter part of this century.

Global agriculture is also threatened by the loss of mountain glaciers, which supply water to arid lowland regions in South America and Asia. In Peru, for example, the glaciers which keep desert cities like Lima alive are receding rapidly, and may be gone completely within as little as 20 or 30 years.

Millions of people will have to find new supplies of water or become environmental refugees. Crisis on an even larger scale will affect the Indian sub-continent once Himalayan glaciers become severely depleted by warming. The Indus river in particular, on which the overwhelming majority of Pakistan's food production depends, will lose much of its flow once the glaciers in the Karakoram recede. In this case, hundreds of millions of people could be affected throughout Asia, from northern China to Vietnam.

Glaciers are already disappearing from mountain ranges across the world. According to the Switzerland-based World Glacier Monitoring Service, 96 per cent of glaciers surveyed - from Kazakhstan to Bolivia - are currently retreating. Whilst this will cause water deficits in years to come, increased runoff due to the melting also holds dangers. Retreating mountain glaciers often leave lakes behind, which can overflow catastrophically when hit by landslides or avalanches. Nepal is already dotted with dangerous glacial lakes - the last count identified 26. A glacial lake at the base of Annapurna II burst in 2003, killing five people in the ensuing flash flood.

No part of the globe will remain untouched as global warming accelerates, pushing temperatures to heights not experienced on Earth for millions of years. Already scientists are defining the rise of the human species as the start of a new geological era, the "anthropocene", which unfortunately looks set to be characterised by a rapidly-changing climate and mass extinctions throughout the natural world.

Plant and animal behavior is already changing in response to rising temperatures - the virtual disappearance of sand eels from the North Sea ecosystem and resulting crash in seabird breeding numbers is a striking example. Species can only survive in a particular climate envelope, and once temperatures shift beyond their adapted range, they must either move or die out. A landmark paper in Nature last year estimated this extinction risk, and concluded that up to a third of species would be "committed to extinction" by 2050 because of global warming.

There are precedents for this kind of grim scenario: extreme greenhouse episodes in the distant geological past are clearly associated with major extinction pulses. The end-Permian mass extinction - the worst die-off of all time, when life came close to disappearing from the planet 250 million years ago - is now thought to have been caused by an deadly interval of global warming, which heated the oceans to such an extent that they became stagnant, killing up to 95 per cent of marine organisms and devastating terrestrial life too.

A repeat of a catastrophe like this would certainly lead to the collapse of human civilisation and the deaths of billions of people. But how likely is it? There are two key sources of uncertainty about how hot it is likely to get in the future. The first comes from the Earth's system itself - scientists are unable to say precisely how sensitive the atmosphere is to elevated levels of greenhouse gases.

Estimates for how much temperatures will rise following a doubling of carbon dioxide from pre-industrial levels vary from a survivable 1.9°C to a definitely-not-survivable 11.5°C, according to the first results from Oxford University's ClimatePrediction.net experiment. Most estimates come up with a figure of around 3-4°C - high enough to have a catastrophic impact on both natural life and human societies.

A second and bigger source of uncertainty is how many more billions of tons of greenhouse gases humans will pour into the atmosphere by burning fossil fuels over the coming decades. This uncertainty is particularly difficult to quantify because it depends on future rates of economic growth - particularly in rapidly-industrialising countries like India and China - but also on political questions such as whether future governments take seriously the need to reduce the threat of climate change.

Most difficult of all to predict are the countless millions of decisions that individual citizens - you and I - will take about our own lifestyles. Should the family fly to Barbados or take a holiday closer to home? Should we buy imported food in Tesco's or shop at the local farmers' market? Should we oppose or support the nearby wind farm? It is these everyday choices which will ultimately determine the fate of the Earth's climate, and thereby the fates of the planet's natural and human inhabitants in this century and beyond.

No sensible person can any longer be in doubt that man-made global warming is a reality, and that its consequences are going to be increasingly catastrophic. Yet in reality most of us remain in stony denial about the need to change our lifestyles to reduce the severity of climate change.

As long as it's Portugal or Boscastle, Tuvalu or New Orleans that bears the brunt, the rest of us will go on driving and flying indefinitely. The government makes a few half-hearted exhortations about kettles and cavity wall insulation, but continues to allow the rapid expansion of airports and roads.

Consumerism and car culture tighten their grip with every new off-roader advert that hits our screens. In a bizarre twist of the knife, wind farms are now more controversial than motorways. But with our own and our childrens' futures now in question, things are beginning to change - if still at a glacial pace.

From signing up to green electricity to growing their own veg, increasing numbers of ordinary people are prepared to do their bit. These are the trend-setters, the visionaries who will pave the way for the collective national and international action which must inevitably follow if the human species is confront the greatest threat we have ever faced. Climate change isn't an issue of conscience or choice, it's an issue of survival.

Mark Lynas is author of "High Tide: News from a Warming World" (HarperPerennial, £7.99). His website is at www.marklynas.org