Fast and slow adjustment of the climate system to abrupt changes in CO2 and solar forcing

A number of studies have shown that climate change can be characterized by two distinct responses: fast adjustment of the atmosphere and land that occurs before appreciable ocean response starts and slow response of the climate system that scales with the change in global mean surface temperature. Effects of the fast response can be included in estimates of 'regressed radiative forcing'. The aggregate effect of slow climate system feedbacks can be represented by a climate sensitivity parameter that can be used to estimate equilibrium warming from this regressed radiative forcing. To constrain equilibrium climate sensitivity, we need a better understanding of the evolution and mechanisms underlying both fast climate adjustment and slow climate change. Using Hadley Center Climate model, HadCM3L, we perform stepwise forcing experiment to investigate modeled climate change in response to an instantaneous quadrupling of atmospheric CO2 concentration or an instantaneous 4% increase in solar intensity. We analyze ensemble model results over time periods ranging from 1 day to 1000 years to examine climate response on a range of time scales. Both sets of simulations (i.e., step changes in CO2 concentration or solar intensity) produce about the same amount of equilibrium surface warming. However, averaged over the first day after the forcing is imposed, the net radiative imbalance at the top-of-atmosphere (positive into the atmosphere) is about 50% greater for the change in solar intensity than it is for the change in CO2 concentration. Nevertheless, the regressed radiative forcing is similar in these two cases. After several months, in both cases the top-of-atmosphere imbalances are consistent with what would be predicted based on regressed radiative forcing. We discuss this transition from the 'instantaneous' forcing to the 'regressed' radiative forcing. Regression approaches can be applied to estimate fast and slow responses of other fields (e.g., precipitation). The fast response scales with the amount of forcing whereas the slow response scales with global mean temperature. The fast response that is established in the first months continues to be reflected in the climate system as the Earth warms. For example, CO2 initially suppresses precipitation, but this suppression is more than offset by the increase in precipitation associated with CO2-induced warming. In contrast, increased solar insolation does not inhibit precipitation. As a result, equilibrium global-mean precipitation increases about twice as much in the solar simulations than in the CO2 simulations, despite having a similar equilibrium temperature change. The framework of fast climate adjustment and slow climate response can be used to investigate the time evolution and physical mechanisms underlying the adjustment of radiative forcing, climate change and climate feedbacks on time scales ranging from days to millennia.