BETWEEN FOUR AND FIVE DEGREES OF WARMING

We are looking now at an entirely different planet. Ice sheets have vanished from both poles; rainforests have burnt up and turned to desert; the dry and lifeless Alps resemble the High Atlas; rising seas are scouring deep into continental interiors. One temptation may be to shift populations from dry areas to the newly thawed regions of the far north, in Canada and Siberia. Even here, though, summers may be too hot for crops to be grown away from the coasts; and there is no guarantee that northern governments will admit southern refugees. Lynas recalls James Lovelock’s suspicion that Siberia and Canada would be invaded by China and the US, each hammering another nail into humanity’s coffin. Any armed conflict, particularly involving nuclear weapons, would of course further increase the planetary surface area uninhabitable for humans.

When temperatures were at a similar level 55m years ago, following a very sudden burst of global warming in the early Eocene, alligators and other subtropical species were living high in the Arctic. What had caused the climate to flip? Suspicion rests on methane hydrate – “an ice-like combination of methane and water that forms under the intense cold and pressure of the deep sea”, and which escapes with explosive force when tapped. Evidence of a submarine landslide off Florida, and of huge volcanic eruptions under the North Atlantic, raises the possibility of trapped methane – a greenhouse gas 20 times more potent than carbon dioxide – being released in a giant belch that pushed global temperatures through the roof.

Summer heatwaves scorched the vegetation out of continental Spain, leaving a desert terrain which was heavily eroded by winter rainstorms. Palm mangroves grew as far north as England and Belgium, and the Arctic Ocean was so warm that Mediterranean algae thrived. In short, it was a world much like the one we are heading into this century. Although the total amount of carbon in the atmosphere during the Paleocene-Eocene thermal maximum, or PETM, as scientists call it, was more than today’s, the rate of increase in the 21st century may be 30 times faster. It may well be the fastest increase the world has ever seen – faster even than the episodes that caused catastrophic mass extinctions.