Buried within the newly released IPCC report is an apocalyptic warning: if greenhouse gas emissions continue to rise at current rates, global warming by the end of the century could total 6.4C. The scientists don't say so explicitly, but a rise in temperatures of this magnitude would catapult the planet into an extreme greenhouse state not seen for nearly 100 million years, when dinosaurs grazed on polar rainforests and deserts reached into the heart of Europe. It would cause a mass extinction of almost all life and probably reduce humanity to a few struggling groups of embattled survivors clinging to life near the poles.
An eco-alarmist fantasy? Unfortunately not - having spent the past three years combing the scientific literature for clues to how life will change as the planet heats up, I know that life on a 6C-warmer globe would be almost unimaginably hellish. A clue to just how unpleasant things can get is contained within a narrow layer of strata recently exposed at a rock quarry in China, dating from the end of the Permian period, 251 million years ago. For reasons that are still not properly understood, temperatures rose by 6C over just a few thousand years, dramatically changing the climate and wiping out up to 95 per cent of species alive at the time. The end-Permian mass extinction was the worst ever: the closest that this planet has ever come to becoming just another lifeless rock orbiting the sun. Only one large land animal survived the bottleneck: the pig-like Lystrosaurus, which for millions of years after the disaster had the globe pretty much to itself.
Clues as to how the world looks in a long-term extreme greenhouse state also come from the Cretaceous period, 144 to 65 million years ago, when there was no ice on either pole and much of Europe and North America was flooded by the higher seas. Tropical crocodiles swam in the Canadian high Arctic, whilst breadfruit trees grew in Greenland. The oceans were incredibly hot: in the tropical Atlantic they may have reached 42C, whilst at the North Pole itself, the oceans were as warm as the Mediterranean is today. The tropics and sub-tropics were so hot that no forests grew, and desert belts probably extended into the heart of modern-day Europe.
During the Cretaceous, of course, species evolved over millions of years to be able to survive on a much hotter planet. Nowadays very few species could survive such a sudden transition. Cold-adapted species like polar bears would obviously be an early casualty, and coral reefs will also disappear from the tropics. The Met Office's Hadley Centre has predicted that the Amazonian rainforest could start to burn as early as 2050, gradually transforming towards desert as temperatures soar in the interior of South America. Ash and smoke would blanket much of the southern hemisphere, and nearly half of the world's terrestrial biodiversity would be wiped out at a stroke.
How people might fare is anyone's guess. With the tropics too hot to grow crops, and the sub-tropics too dry, billions of people would find themselves in areas of the planet which are essentially uninhabitable. This would probably even include southern Europe, as the Sahara desert crosses the Mediterranean. As the ice-caps melt, hundreds of millions will also be forced to move inland due to rapidly-rising seas. As world food supplies crash, the higher mid-latitude and sub-polar regions would become fiercely-contested refuges. The British Isles, indeed, might become one of the most desirable pieces of real estate on the planet. But with a couple of billion people knocking on our door, things might quickly turn rather ugly.
Six Degrees: Our Future on a Hotter Planet by Mark Lynas will be published by Fourth Estate on 19 March.