CMIP5 experiments with CCSM4 and CESM1

The CCSM4 has been run for the full suite of CMIP5 experiments including multiple ensemble members of 20th century climate, 21st century RCP scenarios, extensions beyond 2100 to 2300, and cloud and other feedback experiments. Equilibrium climate sensitivity of CCSM4 is 3.1°C, and the transient climate response is 1.5°C. The model does not include the indirect effect of sulfate aerosols, and this contributes to a 20th century climate simulation that is somewhat warmer than observed. Some possible consequences of not including the indirect effect are shown in a sensitivity experiment where the direct effect of sulfate aerosols is increased to compensate for the missing indirect effect. Globally averaged temperatures at the end of the 21st century show that by following the low mitigation scenario (RCP2.6), warming relative to pre-industrial is +1.9°C, which is below the widely cited desired target of 2°C above pre-industrial. To achieve this target RCP2.6 specifies negative CO2 emissions starting around the year 2075. Global surface temperatures averaged for the last 20 years of the century compared to the 1990-2009 reference period for five member ensembles from CCSM4 are +0.7°C, +1.5°C and +3.4°C for RCP2.6, RCP4.5, and RCP8.5 respectively. Changes in monsoon regimes in a future warmer climate in CCSM4 are also addressed, and some preliminary CMIP5 results from CESM1(CAM5) are shown.