Forget Hurricane Katrina. If you want to bring meteorologists out in a cold sweat, mention its near namesake Hurricane Catarina. In the third week of March 2004, Catarina made landfall in Brazil, destroying tens of thousands of houses. This was not supposed to happen. Rule one in Atlantic meteorology is that, whatever happens north of the equator, the South Atlantic doesn’t do hurricanes. But it did. And it could be the first of many. For Catarina blew up quite close to where Britain’s Met Office had previously predicted the first southern hurricanes might happen as the oceans warmed — in about 2075.

This story is one of the first in Mark Lynas’s saga of how, in the world as imagined by thousands of computer-modelling studies, global warming kicks in, degree by degree. Six Degrees, I tell you now, is terrifying.

One degree need not detain us long. Besides saying hello to more hurricanes, we say toodle-oo to Tuvalu and other lowlying Pacific island nations. Two degrees is bad for nature, but Lynas reckons that most of us will get by, although we will face regular repeats of the heat wave that struck Europe in 2003. It was rather nice here in Britain, then, as I remember, but on the Continent it was hell. As crops died and forest fires ripped through Portugal, some 25,000 people died of heatstroke.

Many people think of climate change as a kind of on-off switch. What Lynas shows rather well is that it is a continual process. Put more greenhouse gases into the atmosphere and you get more warming. One day, we will have to turn the thermostat down; the only question is when.

Should it, for instance, be before we reach three degrees of warming, by which time ice sheets such as Greenland will be doomed, with sea-level rises of possibly tens of metres to follow? Goodbye much of eastern England. At three degrees, we are also likely to kill off the Amazon rainforest and turn the planet’s oceans and soils from carbon “sinks” — soaking up as much as half of all the carbon dioxide we dump into the air — into carbon “sources”, releasing it all again. Beyond that point, we have lost control of events.

Incidentally, although Lynas doesn’t mention it, probably at about this time most of the trees being planted today by greens to offset their carbon emissions will reach the end of their natural lives. As they die, they will release the carbon again. Greens are engaged not so much in an offset as a time-shift. Future generations may not thank them.

Lynas’s narrative presupposes that the computer-climate models are broadly right. But are they? Lynas has been diligent in his research. He has probably read more papers on modelling studies than anyone on the planet. And, by three degrees, he emerges with some small sympathy with the climate sceptics. “Models don’t do anything magical,” he says. “All they do is solve physical equations. No one suggests they are perfect.” Garbage in; garbage out, in other words. We can check them, he says, by looking at the past. Climate change is a natural attribute of our planet. But if you think that means that we don’t need to worry about giving it a helping hand, then Lynas’s chapters on four, five and six degrees of warming should disabuse you. Abrupt and violent climate change seems to be the planetary rule, rather than the exception.

What is more, there is abundant evidence that carbon dioxide in the air acts as a global thermostat. One important reason why the world is still cooler today than tens of millions of years ago is that nature has secreted away below ground the carbon remains of dying forests. We are now digging up these “fossil fuels” and burning them. Every year we release into the air what it took nature a million years to lay down. No wonder the world is warming.

By five degrees, the giant West Antarctic ice sheet is toppling off its perch on a vast underwater archipelago. Another seven metres of sea-level rise, and mankind is heading for the hills. And the north. The Chinese move into a thawed Siberia, America invades Canada, and tens of millions of Africans head for Britain, where a shutdown of the Gulf Stream could have countered some of the warming.

Only the lucky will make it. “A drastic reduction in human population is unambiguously the most likely outcome of a rise in global temperatures towards five degrees,” says Lynas. “Billions will die.” Remember, five degrees could be only a century away on current trends.

On to six degrees. By now, Lynas is digging back hundreds of millions of years to find parallels. But he finds them. Above all, he finds methane. Billions of tonnes of the potent greenhouse gas will be belching out of Siberian perma-frost and the sea bed, triggering runaway warming. And he means belching. The book ends with “a methane fireball racing towards a city — London, say, or Tokyo — the blast wave spreading out from the explosive centre with the speed and force of an atomic bomb”. Hiroshima and Katrina rolled into one.

Hyperbole? Don’t you believe it. Lynas is in many ways conservative. In nature, climate change may not happen degree by degree. Our civilisation has grown during 10,000 years of remarkably stable climate. But the scene was set 10,000 years ago, when we lurched out of the last ice. At one point, temperatures rose by more than six degrees in a decade. Now that really is scary.

Bleak midwinters?

One key debate about climate change is whether global warming will lead to temperature drops in northwest Europe, as the warm Atlantic current diminishes in strength. Scientific opinion, says Lynas, had been hardening against the idea, until a 2004 ocean-sampling expedition found that Atlantic circulation had lessened by 30% since 1998. Opinion is still divided, and a decade’s worth of evidence will be needed before any trend can be confirmed, but Lynas points out that a 50% drop in circulation could lead to every British winter being as cold as the notorious one of 1962-63.

Fred Pearce’s When the Rivers Run Dry: What Happens When Our Water Runs Out? is published by Eden Books. Six Degrees is available at the Books First price of £11.69 (including p&p) on 0870 165 8585

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