The 21st century annual mean surface air temperature change and the 2℃ warming threshold over the globe and China as projected by the CMIP5 models

The paper examines the projected annual mean surface air temperature (ASAT) change characteristics and the occurrence time for 2℃ warming threshold over the globe and China in the 21st century under the three different Representative Concentration Pathways (RCPs) (RCP2.6, RCP4.5, RCP8.5) by the 29 coupled climate models participating the Coupled Model Intercomparison Project Phase 5 (CMIP5). The multi-model ensemble mean (MME) shows that the ASAT over the globe and China is projected to increase in the 21st century under the three RCPs, and the larger radiative forcing the higher the increment of ASAT by the late of the 21st century (2091-2100). MME projects an increase-peak-decrease process of ASAT over the globe (China) under RCP2.6, with the peak value in 2056 (2049) and 1.74℃ (2.12℃) warming by the late of the 21st century relative to 1871 to 1900 mean. Under RCP4.5, MME projects ASAT gradually increasing and then tending to slightly increase or to be stable with 2.60℃ (3.39℃) warming over the globe (China) by the late of 21st century. The ASAT is projected to fast increase under RCP8.5 with 4.75℃ (6.55℃) warming over the globe (China) by the late of 21st century. The global mean ASAT warming will unlikely reach 2℃ under RCP2.6, and will exceed 2℃ in 2047 and 2038 under RCP4.5 and RCP8.5 respectively by MME. The China-averaged mean ASAT warming will exceed 2℃ in 2032, 2033 and 2027 under RCP2.6, RCP4.5 and RCP8.5 respectively by MME, which are much earlier than those for the global means. For any RCP, there are obvious geographical differences in the occurrence time for 2℃ warming, and it is earlier in the Northern Hemisphere than in the Southern Hemisphere, earlier in higher latitudes than in the lower ones for the same hemisphere, and earlier over land than over ocean for the same latitude. It is reliable that the ASAT over the globe and China will increase in the 21st century under any RCP. There is much high consistence among the models for the occurrence of 2℃ warming during the 21st century under RCP4.5 and RCP8.5. There are also some uncertainties in ASAT change projection with the characteristics of the uncertainties increasing with the extension of the projected period, as well as larger for China than the global mean.