'It's the feedbacks, stupid'

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 US election.

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

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.

So the Second Assessment of the IPCC, published in 1995, said that it was more than 50% likely that human emissions of greenhouse gases were contributing to global warming.

The Third Assessment, in 2001, raised the likelihood to 66%. The Fourth, in 2007, upped the ante to 90%, and the Fifth says 95%.

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 three thousand 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 +2degC, but some scientists now argue that the real threshold may be as low as +1.5degC. 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.

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 sq km 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 5-6degC 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.

Gwynne Dyer is an independent London journalist.

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