https://www.gfdl.noaa.gov/early-20th-century-global-warming/ The observed global warming of the past century occurred primarily in two distinct 20 year periods, from 1925 to 1944 and from 1978 to the present. While the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear since this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. Kalvin says, " C'mon everone knows that. Several climate models accurately simulate the global warming of the late 20th century when the radiative effects of increasing levels of human-induced greenhouse gases (GHG) and sulfate aerosols are taken into account. However, the warming in the early part of the century has not been well simulated using these two climate forcings alone. Factors which could contribute to the early 20th century warming include increasing greenhouse gas concentrations, changing solar and volcanic activity, and internal variability of the coupled ocean-atmosphere system. The relative importance of each of these factors is not well known. Pshaw, really, just guessing are we. That makes sense, like the Earth always says, Bi thar dun thet.