Journal of Climate 2013 ; e-View
Gerald A. Meehl,*,1 Warren M. Washington,1 Julie M. Arblaster,1,2 Aixue Hu,1 Haiyan Teng,1 Jennifer E.Kay,1 Andrew Gettelman,1 David M. Lawrence,1 Benjamin M. Sanderson,1 and Warren G. Strand1
1 National Center for Atmospheric Research (+), PO Box 3000, Boulder CO 80307
Future climate change projections for the Coupled Model Intercomparison Project phase 5 (CMIP5) are presented for the Community Earth System Model version 1 that includes the Community Atmospheric Model version 5 (CESM1(CAM5)). These results are compared to CCSM4 and include simulations using the Representative Concentration Pathway (RCP) mitigation scenarios, and extensions for those scenarios beyond 2100 to 2300. Equilibrium climate sensitivity of CESM1(CAM5) is 4.10°C, which is higher than the CCSM4 value of 3.20°C. The transient climate response is 2.33°C, compared to the CCSM4 value of 1.73°C. Thus, even though CESM1(CAM5) includes both the direct and indirect effects of aerosols (CCSM4 had only the direct effect), the overall climate system response including forcing and feedbacks is greater in CESM1(CAM5) compared to CCSM4. The Atlantic Ocean meridional overturning circulation (AMOC) in CESM1(CAM5) weakens considerably in the 21st century in all the RCP scenarios, and recovers more slowly in the lower forcing scenarios. The total aerosol optical depth (AOD) changes from ~0.12 in 2006 to ~0.10 in 2100, compared to a preindustrial 1850 value of 0.08, so there is less negative forcing (a net positive forcing) from that source during the 21stcentury. Consequently, the change from 2006 to 2100 in aerosol direct forcing in CESM1(CAM5) contributes to greater 21st century warming relative to CCSM4. There is greater Arctic warming and sea ice loss in CESM1(CAM5), with an ice-free summer Arctic occurring by about 2060 in RCP8.5 (2040s in September) as opposed to about 2100 in CCSM4 (2060s in September).