Abstract:The increases in atmospheric concentrations of CO₂ and associated global warming are of worldwide concerns. In the 2009 Copenhagen Accord, many nations agreed to limit the increase in global temperature since pre-industrial times to below 2 ℃ by initiating significant cuts in global emissions of greenhouse gases, assuming that the global warming of 2 ℃ would occur if greenhouse gas concentrations rose above 450 ppm CO₂ equivalent by volume. Climate sensitivity (the ratio of change in global mean surface temperature to that in CO₂- equivalent concentration) is the scientific fundamental for policies to reduce emissions of greenhouse gases. To support the nation's mitigation and adaptation to climate change, a group of projects within the Chinese Academy of Sciences Strategic Priority Research Program of ‘Climate Change: Carbon Budget and Relevant Issues' examine the key factors that influence climate sensitivity, including natural variability of climate, the feedback of clouds and water vapor, the cooling effect of atmospheric aerosols, and the uncertainties associated with the global climate simulations. During the past four and a half years, the CAS scientists have improved the understanding of warming in China and climate sensitivity in the following aspects: (1) New time series of temperature are obtained and analyzed to understand the amplitude, rate, periodicity, and abrupt change of temperature in China. Time series of temperature for different regions over the past 2000 years are reconstructed based on datasets from tree rings, lake sediments, ice cores, coral, etc., and the time series of observed temperature from meteorological stations over the past 100 years are compiled. For climate series over the past 100 years, a homogenization approach is used to remove systematic biases in the observation series because of relocation of meteorological stations or changes in observation instruments, rules, and methods. The homogenized temperatures show that the average temperature over China increased by 1.52 ℃(100 yr)^{- 1}, which is much higher than the global warming of 0.89 ℃ over 1901—2012. (2) Observational networks are launched to measure size-resolved speciated aerosol concentrations and optical properties of aerosols. The continuous measurements nationwide provide valuable datasets for studies of climatic effect of aerosols. Comparisons of simulated aerosol concentrations from climate models with measurements show that current aerosol-climate models worldwide generally underestimate aerosol concentrations in China, suggesting that the climate models might have underestimated the roles of aerosols in climate sensitivity. (3) The key parameterization schemes of cloud-aerosol-radiation and dynamic vegetation have been implemented into the CAS Earth System Model, which allow us to better quantify the feedbacks of clouds and water vapor in climate system. (4) The multi-model transient simulations of future climate indicate that, under the Intergovernmental Panel on Climate Change(IPCC)future emissions scenarios (the Representative Concentration Pathways, RCPs), the warming of 2 ℃ would not occur under RCP2.6 and would likely occur when CO₂ equivalent concentrations are approximately 550 ppm under RCP4.5, RCP6.0, and RCP8.5.