Climate system response to external forcings and climate change projections in CCSM4

Abstract Results are presented from experiments performed with the Community Climate System Model version 4 (CCSM4) for the Coupled Model Intercomparison Project phase 5 (CMIP5). These include multiple ensemble members of 20th century climate with anthropogenic and natural forcings as well as single forcing runs, sensitivity experiments with sulfate aerosol forcing, 21st century Representative Concentration Pathway (RCP) mitigation scenarios, and extensions for those scenarios beyond 2100 to 2300. Equilibrium climate sensitivity of CCSM4 is 3.20°C, and the transient climate response is 1.73°C. Global surface temperatures averaged for the last 20 years of the 21st century compared to the 1986-2005 reference period for six member ensembles from CCSM4 are +0.85°C, +1.64°C, +2.09°C and +3.53°C for RCP2.6, RCP4.5, RCP6.0 and RCP8.5 respectively. The ocean meridional overturning circulation (MOC) in the Atlantic, which weakens during the 20th century in the model, nearly recovers to early 20th century values in RCP2.6, partially recovers in RCP4.5 and RCP6, and does not recover by 2100 in RCP8.5. Heat wave intensity is projected to increase almost everywhere in CCSM4 in a future warmer climate, with the magnitude of the increase proportional to the forcing. Precipitation intensity is also projected to increase, with dry days increasing in most subtropical areas. For future climate, there is almost no sea ice left in the Arctic in the high RCP8.5 scenario by 2100, but in the low RCP2.6 scenario there is substantial sea ice remaining in late summer at the end of the century.