Simulation of early 20th century global warming

The observed global warming of the past century occurred primarily in two d istinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Al though the Latter warming is often attributed to a human-induced increase o f greenhouse gases, causes of the earlier warming are less clear because th is period precedes the time of strongest increases in human-induced greenho use gas (radiative) forcing. Results from a set of six integrations of a co upled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiat ive forcing and an unusually Large realization of internal multidecadal var iability of the coupled ocean-atmosphere system. This conclusion is depende nt on the model's climate sensitivity, internal variability, and the specif ication of the time-varying human-induced radiative forcing.