What happens if we stop emitting greenhouse gases all at once, given that the Earth's climate is currently out of equilibrium? Will the climate continue to warm by inertia? If so, for how long? How long will it be before we return to pre-industrial temperatures and levels? These are the questions that motivated the ZECMIP (Zero Emission Commitment Model Intercompaison Project) intercomparison exercise [MacDougall et al., 2020].
According to the results of this project, as soon as emissions stop, the concentration decreases exponentially (figure 19a). The anthropogenic atmospheric is absorbed by vegetation and the ocean, reaching its pre-industrial value over a period of 200 to 1 000 years (figure 19c). Temperature response varies widely from model to model (figures 19b,d), with either a slowdown in warming or a cooling back to a pre-industrial climate after 200 to 1 000 years.
At present, SimClimat simulates a reduction in concentration that is far too slow (figures 19e,g), and the persistence of excessive global warming despite the halting of emissions (figures 19f,h). SimClimat does manage to return to its pre-industrial equilibrium state, but only after 100,000 years. This excessive climate inertia therefore limits SimClimat's applicability for this type of simulation. This problem will be addressed in future maintenance releases.
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