A Modeling Study Of The Climate Change And Carbon Cycle Response To Geoengineering

Since the preindustrial period,atmospheric CO₂ concentration has rapidly increased mainly as a result of anthropogenic CO₂ emissions including fossil fuel combustion and land use change,causing global warming through the greenhouse effect.Currently,mitigation of CO₂ emissions is still challenging.Therefore,solar geoengineering(also termed as solar radiation modification or solar radiation management)has been proposed as a potential means to cool the Earth by reducing solar radiation reaching atmosphere and the Earth surface.In this study,we conduct a series of simulations using the Earth system model to analyze the response of climate system and the global carbon cycle under different solar geoengineering scenarios.First,we conduct a series of simulations using the University of Victoria Earth System Climate Model(UVic ESCM)with the geoengineering scheme of solar constant reduction.Under a high CO₂ emission scenario without emission mitigation policy(A2),geoengineering is implemented in the model at year 2020 by reducing solar constant,aiming at offsetting radiative forcing caused by increasing atmospheric CO₂.Simulation results show that decrease in solar irradiance can mitigate many aspects of CO₂-induced climate changes,including temperature,sea ice and the thermohaline circulation.Meanwhile,geoengineering has significant effect on the key carbon cycle processes through the cooling effect.Relative to the high atmospheric CO₂ scenario,solar geoengineering can increase terrestrial vegetation productivity,suppress soil heterotrophic respiration,and increase CO₂ solubility,and consequently affect land and ocean CO₂ uptake,reducing atmospheric CO₂ concentration.From year 1800 to 2100,compared to the high-CO₂ simulation without geoengineering,land and ocean cumulative carbon storage increases by 234 GtC(24%)under solar geoengineering case,causing a reduction of 110 ppm(12%)in the atmospheric CO₂ concentration.Furthermore,we design simulations to investigate the effect of carbon cycle feedback on the geoengineering cooling efficiency.We design a solar geoengineering scenario in which solar irradiance is reduced to prevent the global mean warming from exceeding 1.5?above preindustrial level along with CO₂ emission mitigation.Under such scenario,we conduct geoengineering simulations with and without carbon cycle feedback.Our results demonstrate that under a certain temperature control target,the inclusion of the global carbon cycle feedback would reduce the required strength of geoengineering.Relative to the geoengineering scenario without carbon cycle feedback,the strength of required solar geoengineering(as measured by the change in solar radiation)can reduce by at most 13%with carbon cycle feedback.We further analyze a set of large ensemble simulations simulated by the Community Earth System Model(CESM)to examine the changes in climate and land carbon cycle in response to stratospheric sulfate aerosol geoengineering.Under RCP8.5 CO₂ scenario,stratospheric aerosol geoengineering simulated in this ensemble can maintain the annual and global mean temperature,annual mean interhemispheric and equator-to-pole temperature gradients at the current climate state.Our analysis show that such geoengineering scheme can substantially affect the high-latitude climate seasonal cycle.Relative to the current climate state,geoengineering causes significant overcooling in summers and undercooling in winters over high-latitude areas,and consequently sea ice extent extends in summers and shrinks in winters.Also,stratospheric sulfate aerosol geoengineering can significantly influence the ability of terrestrial CO₂ uptake.The results show that compared to the RCP8.5 high-CO₂ world without geoengineering,land productivity decreases under geoengineering scenario mainly as a result of low temperature and nitrogen limitation effect,whereas soil heterotrophic respiration is suppressed by geoengineered cooling.As a net result,terrestrial carbon uptake is enhanced.By the end of this century,relative to RCP8.5 simulations without geoengineering,cumulative land carbon storage increases by 37%in the stratospheric sulfate aerosol geoengineering scenario.In this study,we have explored the mechanisms of interactions between climate change,the global carbon cycle,and solar geoengineering.Through Earth system modeling,our study demonstrates that solar geoengineering has a profound effect on the climate change and the global carbon cycle,and the climate-carbon cycle feedback in turn play an important role in solar geoengineering cooling efficiency.This study helps to advance our understanding of the effect of solar geoengineering on the climate system,and contributes to a full assessment of climate effect of solar geoengineering.