The climate of our planet can change because of changes in incoming solar radiation, changes in the atmospheric greenhouse gases (e.g. CO2, CH4, N2O), aerosols (e.g. sulfates and black carbon) and land cover and land use change. Understanding the response of the climate system to the changes in these forcing agents is crucial for projecting future climate change. Earlier, it was assumed that all climate change agents are equal as they cause the same climate change as that of CO2 if the amount of radiative forcing introduced by them is the same. It was also assumed that the precipitation change per unit warming is the same irrespective of the forcing agents. However, climate modeling studies in the last two decades have shown that this is not always the case. To measure the effectiveness of various forcing agents in causing climate change, relative to CO2, the concept of efficacy is used. Efficacy is defined as the ratio of global temperature change due to a particular forcing agent to the temperature change caused by CO2 for the same imposed radiative forcing. Previous climate modeling studies have made estimates for some forcing agents such as solar irradiance, CH4, O3, and black carbon aerosols. At CAOS, we use idealized climate model simulations to understand the differing efficacy and differing hydrological cycle response to various forcing agents. We use the fast versus slow response framework to study the climate system adjustment on various timescales.
As an example, our estimate of efficacy for one of the most important forcing agents in the regional scale, black carbon aerosols, is discussed here. Black carbon (BC) aerosols absorb solar radiation and thus have a warming effect on the planet. Previous studies have found that they are less effective than CO2 in terms of global mean warming per unit radiative forcing. However, an explanation of the mechanisms responsible for the lower efficacy is lacking in the literature. Using idealized climate model simulations, we recently investigated the effectiveness of black carbon (BC) aerosols in warming the planet and the hydrological cycle response relative to CO2 forcing. We find that a sixty-fold increase in the BC aerosol mixing ratio from the present-day levels leads to the same equilibrium global mean surface warming (~ 4.1 K) as for a doubling of atmospheric CO2 concentration. However, the radiative forcing is larger (~ 5.5 Wm-2) in the BC case relative to the doubled CO2 case (~ 3.8 Wm-2) for the same surface warming (Figure) indicating that the efficacy of BC aerosols (0.69) is less than CO2. We found that the lower efficacy of BC aerosols is related to the differences in the shortwave (SW) cloud feedback: negative in the BC case while positive in the CO2 case. In the BC case, the negative SW cloud feedback is related to an increase in the low clouds over the tropics which is associated with a northward shift (~ 7o) of the Intertropical Convergence Zone (ITCZ). Our results indicate that understanding the response of the low clouds over the tropics is likely the key to reduce the uncertainty in climate sensitivity due to BC aerosols.