During the cold war, every person on earth knew what the worst endgame would look like: the three-minute warning, the futile scrambling under desks, and universal incineration. With the just-as-real, just-as-dangerous threat of global warming, there is a vague sense of doom, but no clear mental picture of what meltdown would look like - until now.
Mark Lynas is, along with George Monbiot and Bill McKibben, the best writer about global warming working today. In Six Degrees, he does something so obvious and so necessary it is hard to believe nobody has done it before. He pores through the peer-reviewed scientific literature and describes, calmly and plainly, what scientists say will happen on earth as each degree of global warming occurs.
One of the last jeers of the dwindling band of climate change "sceptics" is that a world that is six degrees warmer sounds rather nice, thank you very much. John Redwood, a leading figure in David Cameron's fake-green New Tories, wheeled this canard out only last month. At first glance, they're right: a warming of 1°C to 6°C - which is what the authoritative Intergovernmental Panel on Climate Change (IPCC) predicts - doesn't sound like much.
It is. Lynas talks us through the six degrees of separation between us and a planet we do not recognise and cannot survive on. Some 18,000 years ago, the world was six degrees cooler. It was an ice age. Most of England was a freezing polar desert where winter temperatures went as low as -40°C. There were almost no animals, and the only plants to be found were a few species of lichens and mosses. It was possible to walk to France across the channel. No agriculture was possible, because the climate fluctuated too wildly. So what happens as we move in the opposite direction, up to six degrees warmer?
With just one degree of warming, here's what happens: the Great Barrier Reef bleaches and dies, the Greenland ice sheet melts, the Maldives and many islands in the South Pacific disappear beneath the waves, rockfalls from the Alps multiply as the mountains melt, the seasonal rainfalls in sub-Saharan Africa change leaving millions at risk of drought and famine, and hurricanes start to hit Brazil for the first time in millennia. One degree.
At three degrees, the Amazon rainforest - the planet's lungs - will die. Lynas explains: "The trees in the Amazon are used to constant humidity, and have no resistance to fire." Once the humidity dries out, so does the forest. They will burn and turn to ash.
And at six degrees - the IPCC's higher-end predictions for this century - humanity enters its endgame. "An entirely new planet comes into being - one unrecognisable from the Earth we know today," Lynas writes. The rainforests are gone, the world's ice supplies are only a memory, the seas are encroaching, and inland cities see temperatures ten degrees higher than today. In the world's major crop-growing areas - India, Australia, the inland United States - most crops are dying, and mass starvation is a perennial risk.
It becomes likely that the vast stores of methane lodged on sub-ocean shelves will bubble to the surface. Since methane is highly flammable, these could quickly be sparked - by lightning, or through human action - into vast fireballs tearing across the sky. The chemical engineer Gregory Ryskin calculates that this methane "could destroy terrestrial life almost entirely", with a major oceanic methane eruption having a force 10,000 times greater than the world's stockpiles of nuclear weapons.
The planet has been here before. Geologists have discovered that at the end of the Permian period, 251 million years ago, the world warmed rapidly by six degrees. It was the worst crisis ever endured by life on earth, "the closest this planet has come to losing its wonderful living biosphere entirely and ending up a dead and desolate rock in space". The earth was racked by "hypercanes" - hurricanes so strong they even left their mark on the ocean floor. Oxygen levels in the atmosphere plunged to 15 per cent - low enough to leave any fast-moving animal gasping for breath.
The only survivors were a few shelled creatures in the oceans, and a pig-like creature that had the land to itself for millions of years. (Whoever thought geological findings could give you nightmares?)
Six Degrees will make some readers want to sink into survivalism, but Lynas wisely warns: "Getting depressed about the situation now is like sitting inert in your living room and watching the kitchen catch fire, and then getting more and more miserable as the fire spreads throughout the house - rather than grabbing an extinguisher and dousing the flames."
Buy this book for everyone you know: if it makes them join the fight to stop the seemingly inexorable six degrees of warming and mass death, it might just save their lives.