If you have ever wondered why there is so much attention paid to the touchstone idea of a 2 degree Celsius limit on the temperature rise above pre-industrial times, the Economist has the answer:
It was born in the 1970s, in papers written by William Nordhaus, now an economics professor at Yale. Back then, few had heard of the idea of global warming, and fewer cared. Mr Nordhaus, who had the foresight to realise something important might be happening, suggested that a reasonable precaution would be to stop temperatures exceeding their upper bound during the past 100,000 years — the period for which ice-core data are available and for which the correlation between temperatures and other environmental effects can thus be seen reasonably clearly. The cores suggested this upper bound was 2°C above pre-industrial levels.
Mr Nordhaus himself agreed this estimate was “deeply unsatisfactory”. It was based merely on the observation that it did not push the climate into unknown territory, whose safety could not be assessed. So little was known about the impacts of warming at that stage, he wrote later, that his target was “a substitute” for balancing costs and damages.
The Economist says that, now, the idea of a 2 degree limit is more a political target than a scientific one, and as usual, the paper's coverage of this issue is nuanced. It ends:
Despite its questionable past, the 2°C limit does have merits. By boiling the vast complexities of the climate system down into a single, comprehensible number it gives politicians something simple to aim at, and against which they can measure the success of their endeavours.
But some worry that it is not simple enough, for taking the world’s temperature is not as easy as it sounds. Different parts of the planet warm at different rates, as do different layers of the atmosphere, so all sorts of corrections have to be applied to arrive at a single number. A truly simple, and arguably better, approach would be to use concentrations of greenhouse gases — the cause of the warming—as putative maxima. These gases mix rapidly into the atmosphere, so are easily sampled in ways that brook little dissent.
Others think the idea of a single-number limit is itself flawed. They would try to create some sort of index out of greenhouse-gas concentrations, measures of soot (which absorbs heat), sulphate pollution (which reflects it) and the heat content of the oceans. Such arguments, however, rather miss the point. To quote Bismarck again, “politics is not an exact science”. The 2°C limit is certainly not perfect, and will almost certainly be breached. But its existence focuses minds. And, when the disparate interests of 190-odd countries have to be reconciled, a little mind-focusing is, perhaps, not such a bad thing.
If you have to teach about such things (and nowhere near enough people do this), then the Economist's coverage seems essential reading. If you go to the 'coverage' link you'll also find coverage of carbon pricing, adaptation, and related issues.
Post-COP21 script ... If the idea of a 2 degree limit is more political than scientific, then the target emerging from Paris of a 1.5 limit must be even more political. And of course it is: no matter how improbably in outcome, it is surely in place to give those island states where high tides are getting higher, something of a victory to shout about when they go home.