The role of Atlantic Multi-decadal Oscillation in the global mean temperature variability

The global mean 1900-2015 warming simulated by 42 Coupled Models Inter-comparison Project, phase 5 (CMIP5) climate models varies between 0.58 and 1.70 A degrees C. The observed warming according to the NASA GISS temperature analysis is 0.95 A degrees C with a 1200 km smoothing radius, or 0.86 A degrees C with a 250 km smoothing radius. The projection of the future 2015-2100 global warming under a moderate increase of anthropogenic radiative forcing (RCP4.5 scenario) by individual models is between 0.7 and 2.3 A degrees C. The CMIP5 climate models agree that the future climate will be warmer; however, there is little consensus as to how large the warming will be (reflected by an uncertainty of over a factor of three). A parsimonious statistical regression model with just three explanatory variables [anthropogenic radiative forcing due to greenhouse gases and aerosols (GHGA), solar variability, and the Atlantic Multi-decadal Oscillation (AMO) index] accounts for over 95 % of the observed 1900-2015 temperature variance. This statistical regression model reproduces very accurately the past warming (0.96 A degrees C compared to the observed 0.95 A degrees C) and projects the future 2015-2100 warming to be around 0.95 A degrees C (with the IPCC 2013 suggested RCP4.5 radiative forcing and an assumed cyclic AMO behavior). The AMO contribution to the 1970-2005 warming was between 0.13 and 0.20 A degrees C (depending on which AMO index is used) compared to the GHGA contribution of 0.49-0.58 A degrees C. During the twenty-first century AMO cycle the AMO contribution is projected to remain the same (0.13-0.20 A degrees C), while the GHGA contribution is expected to decrease to 0.21-0.25 A degrees C due to the levelling off of the GHGA radiative forcing that is assumed according to the RCP4.5 scenario. Thus the anthropogenic contribution and natural variability are expected to contribute about equally to the anticipated global warming during the second half of the twenty-first century for the RCP4.5 trajectory.