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Even with moderate emissions of greenhouse gases, a scenario currently expected to warm the planet by about 3掳C, there is a 1-in-10 chance of reaching 7掳C of warming in around 200 years. That is according to a climate model that has explored what could happen over the next thousand years.

鈥淭hree degrees is already very bad, of course,鈥� says at the Potsdam Institute for Climate Impact Research in Germany. 鈥淪even is not twice as bad as 3, or even three times as bad. It鈥檚 probably 10 times worse, because many things are non-linear.鈥�

The model also shows that even if all CO2 emissions stop this century, there is around a 1-in-10 chance that the planet could still warm by more than 3掳C.

How much hotter the planet gets depends partly on how much CO2 and other greenhouse gases we pump into the atmosphere, and partly on how sensitive the climate is to increases in greenhouse gases.

The amount of warming that would occur in the long run after a doubling of CO2 levels in the atmosphere is called the equilibrium climate sensitivity. It depends on various fast feedback effects that occur when temperatures rise, such as increases in water vapour in the atmosphere and changes in cloud behaviour. Studies suggest that the equilibrium climate sensitivity could be anywhere from 2掳C to 5掳C, but for models run beyond 2100 it has been assumed to be 3掳C.

Ganopolski and his team instead tweaked their model to explore what the impact would be if the equilibrium climate sensitivity differs from this expectation.

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Their model also includes additional and longer-term carbon feedback effects such as the release of CO2 and methane from permafrost and wetlands. Previous models looking beyond 2100 haven鈥檛 included both CO2 and methane feedbacks, says Ganopolski.

Including both can have a big impact, he says, because higher CO2 leads to more methane being released and vice versa.

For an assumed equilibrium sensitivity of 3掳C, the team鈥檚 results are similar to those of other studies, leading to warming of around 3掳C just after 2200 for a moderate-emissions scenario. For higher assumed sensitivities, the model projects warming of up to 7掳C.

While the 23rd century might seem a long way off, it鈥檚 not that far off in terms of human generations, says Ganopolski. 鈥淚 have grandchildren who will likely live in the next century.鈥�

The reason there is so much uncertainty about the equilibrium climate sensitivity is that aerosol emissions have been counteracting the warming effect of greenhouse gases, says Ganopolski. A reduction in shipping emissions is thought to have contributed to the recent rapid warming, for instance.

The effects of aerosols are extremely complex, making it hard to quantify their cooling effect. 鈥淎t the present, we cannot rule out high climate sensitivities,鈥� he says.

鈥淎n emission pathway that we think will get to, say, 2掳C in 2100 could be much higher or much lower depending on these fundamental uncertainties,鈥� says at the Met Office, the UK鈥檚 national weather service.

This was well known already, says Wiltshire, but is usually expressed in terms of the percentage likelihood of meeting a specific target. This paper shows it in a different way. 鈥淲hat I like about the paper is the way the information is presented,鈥� he says.

If equilibrium climate sensitivity is on the low side and future emissions are limited, warming of 3掳C can still be avoided, says Ganopolski 鈥� but he thinks it is already too late to limit warming to under 2掳C.