The model also shows that even if emissions stop tomorrow, it could be a tenth that the planet could still be warmer than 3°C.
How hot a planet is dependent in part on the amount of CO2 and other greenhouse gases you enter into the atmosphere and how sensitive the climate is to increase greenhouse gases.
The amount of warming that occurs over a long period of time after doubling CO2 levels in the atmosphere is called equilibrium climate sensitivity. It depends on various fast feedback effects that occur when temperatures rise, such as increasing water vapor in the atmosphere and changing cloud behavior. Studies suggest that equilibrium climate sensitivity may be anywhere from 2°C to 5°C, but for models over 2100 it is assumed to be 3°C.
Ganopolsky and his team instead tweaked the model to investigate the impact of equilibrium climate sensitivity differing from this expectation.
Their models also include additional and long-term carbon feedback effects, such as the release of CO2 and methane from permafrost and marshlands. Previous models over 2100 did not include both CO2 and methane feedback, Ganopolski says.
Including both can have a huge impact, he says. This is because higher CO2 releases more methane and vice versa.
For the assumed equilibrium sensitivity of 3°C, team results are similar to those from other studies, leading to warming of approximately 3°C immediately after 2200 in a moderate emission scenario. For a higher assumed sensitivity, the model projects warming up to 7°C.
The 23rd century may seem like a long way to go, but it is not far from the human generation, says Ganopolsky. “I have a grandchild who is likely to live in the next century.”
The reason there is so much uncertainty about the climate sensitivity of equilibrium is that aerosol emissions counteract the warming effects of the greenhouse effect, says Ganopolsky. For example, reductions in shipping costs are believed to have contributed to recent rapid global warming.
The effects of aerosols are extremely complex, making it difficult to quantify the cooling effect. “We can’t rule out climate sensitivity right now,” he says.
“For example, the emission pa…
If equilibrium climate sensitivity is on the low side and future emissions are limited, 3°C warming can still be avoided, says Ganopolski, but he thinks it is already too late to limit warming to below 2°C.
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Source: www.newscientist.com