Diverse Responses of Global-Mean Surface Temperature to External Forcings and Internal Climate Variability in Observations and CMIP6 Models

We investigate the impacts of external forcings and internal climate variability (ICV) on global-mean surface air temperature (GMST) variations (1850-2014) in observation and an ensemble of sixth Coupled Model Intercomparison Project (CMIP6) historical simulations. Using multiple regression models, we estimate that 83% of the observed GMST variance is explained by global-scale anthropogenic forcing. Most CMIP6 models underestimate this anthropogenic contribution (median explained variance is 67%), but compare better with observation when the North Atlantic regional contribution is included in the anthropogenic forcing (median 77%). The ICV contributions to the GMST variance are similar to 8% for observation and similar to 7% for the models (median). The models' GMST responses to the external forcings and ICV vary widely, with the two responses being oppositely related. The North Atlantic predominantly drives the free GMST variability in observations, whereas the Pacific drives this in most CMIP6 models due to stronger than observed simulated GMST-Pacific ICV relationships.