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Campaign strategist James Carville coined the phrase “It’s the economy, stupid” to focus the attention of campaign workers on the one key issue that would get Bill Clinton elected president in the 1992 U.S. election.

Alas, the authors of the Fifth Assessment of the Intergovernmental Panel on Climate Change (IPCC), which will be published on Friday (September 27), have no such sage to guide them. They’ll have to make do with me.

The 800-odd authors of the report are selected by their fellow scientists in the various disciplines relevant to climate change as the acknowledged leaders in their field of study. Their job was to review all 14,000 scientific papers on climate change published in the past five years. And they are doing this work at the behest of the world’s governments, not as some random pressure group; it is the Intergovernmental Panel on Climate Change.

Scientists are very cautious people. They won’t go one millimetre beyond what the evidence makes indisputable, knowing that they will be attacked by rival scientists if they do. They are much more comfortable talking about probabilities rather than certainties. They are, in other words, a nightmare for journalists who have to transmit their findings to the world.

Of the nearly 100 scientists I have interviewed on climate change over the past five years, not one doubted that global warming is a big and frightening problem. Indeed, there was often an undercurrent of panic in their remarks. But when it comes to writing official reports, they retreat into science-speak.

So the Second Assessment of the IPCC, published in 1995, said that it was more than 50 percent likely that human emissions of greenhouse gases were contributing to global warming. The Third Assessment, in 2001, raised the likelihood to 66 percent. The Fourth, in 2007, upped the ante to 90 percent, and the Fifth, this week, says 95 percent.

But how do you make a headline out of that? How much warming? How fast? And with what effects on human beings? The latest report will run, in its final version, to 3,000 pages, and the answers are buried among the statistics. What would Jim (Carville) do? He’d say: it’s the feedbacks, stupid.

Without the feedbacks, we could go on burning fossil fuels and cutting down the forests, and the average global temperature would creep up gradually, but so slowly that most of the inhabited parts of the planet would stay livable for a long time. But if we trigger the feedbacks, the whole thing goes runaway.

The feedbacks are natural sources of warming that we activate by raising the average global temperature just a modest amount with our own greenhouse gas emissions. The consensus number used to be plus 2 degrees Celsius, but some scientists now argue that the real threshold may be as low as +1.5 degrees C. There are three main feedbacks.

As the highly reflective ice and snow that covers most of the polar regions melts, the rate at which the sun’s heat is absorbed goes up steeply over a large part of the planet. We are creating a new warming engine that will shift the planet’s heat balance, and once it has started we can’t turn it off again.

There is reason to believe that it’s already too late to avoid this one. The protective covering of floating ice that has shielded the Arctic Ocean from solar heating for so long is now going fast, and we will probably see an ice-free Arctic Ocean in the August-September period as early as the 2020s. Mercifully, this is the smallest of the three major feedbacks in terms of its impact— but it triggers a bigger one.

The warmer air and water in the Arctic then starts to melt the permanently frozen ground and coastal seabed (permafrost) that extends over more than 10 million square kilometres of territory, a considerably larger area than Australia. This melting releases a huge amount of methane that has been locked into the ground for millions of years. Methane is a far more effective warming agent than carbon dioxide, and so we spin closer to runaway.

Finally the oceans, as they warm, release some of the vast quantities of carbon dioxide they absorbed in the past, simply because warmer water can contain less dissolved gas. Most of the excess heat in the Earth system has been going into the oceans in the past few decades, which is why the rise in land temperatures seems to have slowed down. But that is no real consolation: it just means that the biggest feedback is also being activated.

Those are the killer feedbacks. Earth has lurched suddenly into a climate 5-6 degrees C higher than now a number of times in the past. The original warming usually came from massive, long-lasting volcanic eruptions that put a large amount of CO2 into the atmosphere—but in every case it was feedbacks like these that carried the planet up into a temperature regime where there was a massive dieback of animals and plants.

We are the volcanoes now. Our own emissions would take a long time to get us up to really high average temperatures worldwide, but all we have to do is pull the trigger on the feedbacks. The rest is automatic.