Simulated Response Of The Climate System To Carbon Dioxide Forcing And Solar Radiation Forcing Over Timescales Ranging From One Day To One Thousand Of Years

Concentrations of carbon dioxide?CO₂?in the atmosphere due to human beings have been increasing,cuasing global warming.Other external forcings,such as solar radiation and aerosol,also play important roles in climate change.In this study,we used the UK Met Office Hadley Centre global climate model,HadCM3L,to investigate climate change in response to C02 and solar forcing.Increasing atmospheric CO₂ exerts a forcing on the climate system not only through its greenhouse effect?CO₂-radiative forcing?on atmospheric radiation,but also through its physiological effect on the opening of plant stomata?CO₂-physiological forcing?.To investigate the effect of CO₂ and solar forcing on climate system,we conduct a series of idealized abrupt forcing experiments:1)4xCO₂.In this experiment,atmospheric CO₂ incresaes abruptly from its pre-indutrial value of 280 ppm by four times to 1120 ppm,and both the CO₂-radiative and CO₂-physiological forcing are considered;2)4×CO₂ atm.In this experiment,atmospheric CO₂ incresaes abruptly from its pre-indutrial value of 280 ppm by four times to 1120 ppm,but only the CO₂-radiative forcing is considered;3)4%Solar.In this experiment,solar irradiance increases 4%abruptly from 1365.0 W m-2 to 1419.6 W m-2.Climate response to changes in external forcings can be divided into two parts:fast response and slow feedback.Fast climate adjustment refers to rapid changes in climate variables such as precipitation and cloud due to changes in atmospheric thermal structure that occurs before substantial change in sea surface temperature.Slow feedback refers to climate response associated with gradual changes in surface temperature.From the perspective of fast response and slow feedback,this study investigates the evolution of the climate system from one day to one thousand years in response to CO₂ forcing and solar forcing.We also examine the similarities and differences between the climate effect of CO₂ forcing and solar forcing from the perspective of fast response and slow feedback.The simulation results show that,?1?Averaged over day one after CO₂ and solar forcing is imposed,net downward flux at the top of atmosphere in response to solar forcing experiment is 54%greater thanthat in 4×CO₂ experiment.The near-equilibrium global mean surface temperature increases by 5.71 K and 5.70 K for 4×CO₂ and 4%solar experiments,respectively.It indicates that a larger solar forcing is required than CO₂ forcing to cause the same amount of global mean surface temperature change.In other words,the efficacy of solar forcing is smaller than that of CO₂ forcing.Fast response is mainly related to the change of outgoing clear sky longwave radiation to space in 4%Solar experimentis more.?2?On the first day after the forcing is imposed,land surface temperature increases by 0.67 K and 0.25 K in the 4×CO₂ simulation and 4×CO₂ atm simulation,respectively.After a month,the land surface temperature in the 4×CO₂ experiment is almost twice as large as that in the 4×CO₂ atm experiment.It indicates that the physiological effect of CO₂ on the land surface temperature plays an important role during the first month.During the first month,increased CO₂ concentration warms the land surface through its physiological effect that reduces evaporative cooling and increases shortwave radiation reaching the surface as a result of decrease in low cloudiness.So the land surface temperature in the 4×CO₂ experiment is larger than that in the 4×CO₂ atm experiment.When the climate system reaches quasi-equilibrium after 1000 years,mean surface temperature of the 4×CO₂ experiment and the 4×CO₂ atm experiment increases by 5.71 K and 5.22 K,respectively.This indicates that the CO₂ radiation effect in the 4×CO₂experiment play a leading role in long-term surface temperature changes.?3?In the 4×CO₂ experiment,the quasi-equilibrium global mean surface temperature change is almost the same as that in the 4%solar experiment,but the change of precipitation is distinctly different:the increase in precipitation in the 4%solar experiment is nearly twice as much as the 4×CO₂ experiment.Using the method of linear regression,the change of quasi-equilibrium precipitation can be expressed as the sum of the fast response of precipitation to forcing and the slow feedback of precipitation to temperature change.The difference of precipitation is mainly in the fast response period.In the fast response period,the effect of CO₂ radiation leads to the enhancement of atmospheric longwave absorption,while the specific heat of ocean is larger than that of atmospheric,so surface temperature of the ocean increases slower than the temperature of atmospheric.Because of this,the stability of the lower atmosphere increases.Increased vertical stability in response to CO₂ forcing tends to suppress convective activity and decrease precipitation over ocean.Besides,the stomatal effect of CO₂ will reduce the transpiration of plants.So evaporation and precipitation reduces over land,too.This study can help us understand the effect of CO₂ forcing and solar forcing on climate system.The study of the response of the climate system to CO₂ forcing and solar radiation foring will also help to improve our understanding of the climatic effects of solar geoengineering that aims to conteract anthropogenic climate change by intentionally reduce the amount of solar radiation reaching the Earth System.