Comparison of the Fast and Slow Climate Response to Three Radiation Management Geoengineering Schemes

Geoengineering has been proposed as a backup approach to rapidly cool the Earth and avoid damages associated with anthropogenic climate change. In this study, we use the NCAR Community Earth System Model to conduct a series of slab-ocean and prescribed sea surface temperature simulations to investigate the climate response to three proposed radiation management geoengineering schemes: stratospheric aerosol increase (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT). Our simulations show that different amounts of radiative forcing are needed for these three schemes to compensate global mean warming induced by a doubling of atmospheric CO2. With radiative forcing defined in terms of top-of-atmosphere energy imbalances in prescribed sea surface temperature simulations with land temperature adjustments, radiative forcing efficacy for SAI is about 15% smaller than that of CO2, and the efficacy for MCB and CCNCCT is about 10% larger than that of CO2. In our simulations, different forcing efficacies are associated with different feedback processes for these forcing agents. Also, these geoengineering schemes produce different land-ocean temperature change contrasts. The apparent hydrological sensitivity, that is, change in equilibrium global mean precipitation per degree of equilibrium temperature change, differs substantially between CO2, SAI, MCB, and CCNCCT forcings, which is mainly a result of different precipitation responses during fast adjustment. After removing the component of fast adjustment, the northward movement of the Intertropical Convergence Zone in response to these forcing agents is tightly related with changes in the interhemispheric energy exchange and hemispheric temperature gradient. Plain Language Summary To counteract the CO2-induced global warming effect, a number of geoengineering methods have been proposed. One proposed method (sulfate aerosol injection) is to inject sulfate aerosols or its precursors (SO2) into the stratosphere to deflect more sunlight back to space. Another method (marine cloud brightening) is to seed low-level marine stratocumulus clouds to reflect more sunlight. A third method, the intentional reduction of the coverage and optical thickness of high-level cirrus cloud (cirrus cloud thinning), could potentially reduce global warming by modifying the longwave radiative effect of cirrus clouds. In this study, we compare the climate response to these three geoengineering schemes that are designed to offset global mean surface warming caused by an abrupt doubling of atmospheric CO2. Our simulations show that to offset the same amount of CO2-induced global mean warming, different amounts of radiative forcing are needed, implying that the efficacy of climate forcing is different for different geoengineering schemes. Also, for the same amount of cooling achieved, cirrus cloud thinning produces a much smaller reduction in precipitation than does stratospheric aerosol injection or marine cloud brightening. Due to the different natures of imposed forcing, these different geoengineering schemes also produce different land-sea temperature contrasts