Climate sensitivity – Could we be lucky?

One of potentially the most important climate change scientific papers for a long time has just been published in Science Express, the top science journal’s rapid-publication service. Unlike most, this does not deepen the general global warming gloom by suggesting things are ‘worse than we thought’ – instead it suggests that very high climate sensitivities (the kind that make it already ‘too late’, or turn us into Venus) are vanishingly unlikely. And more, that the most likely climate sensitivity could be slightly less than previous studies concluded.

This is good news because there is no sign of carbon emissions being curtailed any time soon, and indeed the current global picture is that we are above the worst-case A1F1 emissions trajectory traced by the IPCC in its original ’emissions scenarios’ projections. Even the IPCC, in all its wisdom, couldn’t imagine we would be as hopeless at cutting CO2 as has so far proven to be the case. Just as well then that the planet could be somewhat less sensitive to the warming effect of carbon dioxide and other greenhouse gases than has long been thought.

For those that have ‘sub req’d’ access to Science, the paper and its supporting online material are worth perusing in detail. For those that don’t, read on – I have no doubt that many will use these latest results to support their pre-existing political agendas, whatever these might happen to be. Ignore that stuff – because the science itself is what is interesting, and what should inform any policy implications that arise from this work.

So what is new? Well, what authors Andreas Schmittner et al have done is to use improved reconstructions of the planet’s ice-age temperature to drive a set of model runs using different climate sensitivities to see which can best reproduce what we think we know about the conditions of the ‘last glacial maximum’ around 20,000 years ago. This was a time when the Earth was in relative energy equilibrium (unlike now, when it has a lot of catching up to do with increased GHGs and other forcings) and when CO2 was about 100ppm lower than pre-industrial (about 185ppm).

And the conclusions? That the most climate-sensitive models, those which project a warming of over 6 degrees for doubled CO2, cannot reproduce the conditions of the last ice age – in fact, because they are so sensitive to climate change feedbacks, they experience runaway cooling and produce a near-completely ice-covered ‘snowball Earth’. This suggests that the models with the highest climate sensitivities (remember the ‘fat tail’ that could not be ruled out?) are not representing the physical properties of the climate system accurately and should, in the words of Schmittner et al, “be assigned near-zero probability”.

But what of the most-likely climate sensitivity, commonly put at or around 3C (plus or minus 1.5 degrees) for doubled CO2? The paper estimates a slightly lower median of 2.3C, but also usefully reduces the uncertainty on either side, giving a 66% probability of a 1.7-2.6C climate sensitivity. Since it seems rather likely at the moment that we will see a doubling of CO2 – and in the not too distant future either – this is somewhat reassuring.

There are caveats, as always: this is just one paper, based on one model, and previous work has come to quite different conclusions (but using less accurate data on LGM temperatures), as an accompanying commentary piece by Gabriele Hegerl and Tom Russon points out. Also, the actual ‘data’ on the last ice age temperatures is itself dependent on various proxies and therefore somewhat uncertain. As always, it would be sensible to wait for follow-up studies which may confirm or undermine these conclusions before reading too much into them.

Having said all that, this is still a very important piece of work because it suggests already-unlikely worst-case scenarios to be even less likely – and that we therefore should not be blinded by pessimistic paralysis about the situation we are in, especially with the ongoing rise in global emissions. That our situation may be slightly less hopeless than we thought yesterday should not lead to any complacency, but it should lead at least to renewed optimism and hope as the Durban climate talks get underway.

© Mark Lynas
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