There is now almost no chance of limiting warming to 2 掳C higher than pre-industrial times.
Read more: 鈥�Climate change: What we do 鈥� and don鈥檛 鈥� know鈥�
How much warming will there be if we, say, double CO2 levels in our atmosphere? One way to get an idea of how complex feedbacks play out in Earth鈥檚 climate is to use computer models. The other, more trustworthy method is to look at how changes in CO2 have affected past climate, from the recent past to millions of years ago.
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Both methods suggest doubling CO2 warms the planet by 2掳C at least. Most studies point to a warming of about 3掳C being the most likely response, and this is the value for 鈥渃limate sensitivity鈥� assumed in recent IPCC predictions.
Some studies of past climate, however, point to climate sensitivities of 6掳C or more. One reason for this discrepancy is that , whereas palaeoclimate studies also include longer-term feedbacks such as changes to ice-sheet coverage. If these studies are closer to the true picture, models might be giving us accurate answers for warming over the next few decades, but underestimating warming over the next few centuries and more.
It now seems possible, though, that flaws in climate models are leading them to underestimate even shorter-term feedbacks. That might mean predictions for warming for as soon as 2050 or 2100 are on the low side. Some studies suggest that models assume that than occurs in reality. Others indicate there might be from cloud systems than in any current model. These issues cannot yet be resolved owing to the uncertainty over the extent of cooling caused by some aerosols, and hence the true size of the warming effect (see 鈥淐limate unknown: How great our cooling effects are鈥�).
The bulk of the evidence still points to a short-term climate sensitivity of around 3 掳C, as the IPCC鈥檚 models suggest. But while a figure much lower than that is unlikely, (see diagram).
Even assuming a sensitivity of 3 掳C, there is now almost no chance of limiting warming to 2 掳C higher than pre-industrial times. , to have a greater than 50 per cent chance of doing that we would need to slash emissions by 80 per cent by 2050.
The 2007 IPCC report, meanwhile, suggests we need to limit atmospheric CO2 levels to a maximum of 450 parts per million; they are currently at 380 ppm. It appears unlikely that we will manage this, given rising emissions from countries such as China and India. the temperature rise could exceed 4 掳C as early as the 2060s. If climate sensitivity is higher than we assume, or if CO2 emissions rise higher than in the worst-case IPCC scenario, even that could be an underestimate.
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