Can a combination of internal climate variability and anthropogenic forcing account for the observed global warming of the early 20th century?

The observed global warming of the past century largely occurred in two distinct 20 year periods (1925-1944 and 1978-1998). While the warming in the latter period is often attributed to anthropogenic increases of greenhouse gas concentrations in the atmosphere, the causes of the earlier warming are less clear. Results from a set of six integrations of a coupled ocean-atmosphere climate model are used to explore whether the warming of the early 20th century could have resulted from a combination of anthropogenic forcing and internal multidecadal climate variability.

The climate model used is an R30 version of the GFDL coupled climate model. The six integrations include a 900 year control run with no change of external forcing, and an ensemble of five runs forced by an estimate of past concentrations of greenhouse gases and sulfate aerosols (with the latter parameterized in terms of surface albedo changes). One of the five ensemble members shows a remarkable similarity to the observed global mean temperature record. In addition, the observed early 20th century warming was characterized by a pronounced maximum at higher latitudes of the Northern Hemisphere. The model also captures this feature, thereby lending credibility to the possibility that the simulated warming and observed warming arise from similar physical processes.

In summary, the results suggest that the observed early 20th century warming could have arisen from a combination of anthropogenic forcing and internal multidecadal climate variability involving the North Atlantic and Arctic, provided that the model's climate sensitivity and internal variability are realistic. This study has not evaluated the potential roles of varying solar and volcanic forcing on the early 20th century warming.