A Modeling Study Of The Climate Response To Geoengineering

Since the preindustrial times,anthropogenic CO2 emissions have caused rapid increases in the atmospheric CO2 concentration,which is mainly responsible for the observed global warming.Efforts to mitigate CO2 emissions remain challenging.To prevent risks of the anthropogenic climate change,scientists have proposed the concept of geoengineering that aims to rapidly cool the climate by perturbing the global energy balance at large scale.In this study,we investigate the climate response to a series of geoengineering scenarios based on results simulated by the Earth System Models.Using the Community Earth System Model(CESM),we first simulate the climate response to four geoengineering schemes,including solar constant reduction,increase in stratospheric sulfate aerosol,marine cloud brightening,and cirrus cloud thinning.The first three schemes cool the climate by reducing the incoming shortwave radiation,whereas cirrus cloud thinning scheme increases outgoing longwave radiation.Our results show that all these geoengineering schemes can offset the global mean temperature increase induced by a doubling of atmospheric CO2,but the radiative forcing is different.Relative to 1×CO2 case,global mean precipitation increases in the cirrus cloud thinning case by-4.1%,and decreases in other cases by-2.3%.We further decompose the total precipitation change to fast adjustments that represent the climate response before substantial change in ocean temperature occurs,and slow feedbacks that are associated with the ocean temperature change.Our results indicate that different precipitation changes are mainly a result of different fast adjustment processes,due to changes in atmospheric energy balance and vertical motion.Our study,for the first time,investigate the combined impact of stratospheric sulfate aerosol and cirrus cloud thinning geoengineering schemes.By altering the sensitivity of global mean precipitation response to temperature change,our results indicate that it is possible to offset CO2-induced global mean temperature change and precipitation change at the same time using two geoengineering schemes(0.0 K and-0.4%relative to the 1 ×CO2 case,respectively).CO2 emitted to the atmosphere will have a long-term impact on the climate system,raising needs for persistent deployments of the geoengineering approaches.Most modeling studies of geoengineering consider time scale only ranging from decades to a century.In this study,solar constant reduction scheme is used to offset CO2-induced global mean warming that lasts for hundreds of years.Compared to the high-CO2 case,our results indicate that geoengineering is able to prevent the long-term increase in temperature at both surface and deep ocean,reduce the melting of sea ice,and maintain large-scale ocean circulations.Volcanic eruptions have been considered as natural analogous to the stratospheric aerosol geoengineering.In this study,the climate impact of a short-term stratospheric aerosol forcing representative of volcanic eruptions(sulfate aerosols are added at the beginning of simulations and decline with time)is compared with a long-term aerosol forcing representative of the geoengineering(additional sulfate aerosols persist for the whole simulation).For similar global mean cooling(?0.7 K),decreases in temperature,precipitation,and runoff over land are much larger in volcanic than that in the geoengineering case.For example,decreases in runoff in the short-term case is about twice greater than that in the long-term aerosol forcing case.Spatial pattern responses also differ substantially between these two cases.Our results indicate that direct extrapolations from volcanic eruption records provide limited insight into impacts of potential stratospheric aerosol geoengineering.By studying the climate response under different geoengineering scenarios,our study helps to further our understandings of the underlying mechanisms of the climate response to potential geoengineering deployments.It is also expected that our results will shed new lights on the understandings of climate feedbacks to various external forcings.