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

These include multiple ensemble members of twentieth-century climate with anthropogenic and natural forcings as well as single-forcing runs, sensitivity experiments with sulfate aerosol forcing, twenty-first-century representative concentration pathway (RCP) mitigation scenarios, and extensions for those scenarios beyond 2100-2300.

Equilibrium climate sensitivity of CCSM4 is 3.20 degrees C, and the transient climate response is 1.73 degrees C. Global surface temperatures averaged for the last 20 years of the twenty-first century compared to the 1986-2005 reference period for six-member ensembles from CCSM4 are +0.85 degrees, +1.64 degrees, +2.09 degrees, and +3.53 degrees 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 twentieth century in the model, nearly recovers to early twentieth-century values in RCP2.6, partially recovers in RCP4.5 and RCP6, and does not recover by 2100 in RCP8.5.