[Wigley] Here is my text on the early 20th century warming ... "This small solar contribution applies equally to the early 20th century (191040) warming. The observed warming over this interval is about 0.5oC (see Figure 1 and Table 1), while the solar-induced change is either close to zero, perhaps even a cooling (assuming no secular TSI component), or a warming of about 0.02oC with a secular component (Figure 8). At most, therefore, the solar contribution to early 20th century warming is about 4% (even when one assumes a high value for the climate sensitivity). This minimal solar effect has been noted previously by Foukal et al. (2004). So, what caused the early 20th century warming? A possible explanation is that it is the result of a major increase in the rate of formation of NADW (Wigley and Raper, 1987), an idea that is supported by the pattern of warming which is a maximum in the North Atlantic (Schlesinger and Ramankutty, 1995). We noted earlier that the fact that this warming is similar for the land and the ocean (in fact, the 1910 to 1940 trend over the ocean is greater than over the land) suggests that it is not externally forced (since this would normally lead to warming over the land that was greater than over the ocean), and that it originates in the ocean. This also helps to explain why the land/ocean warming differential that one would expect as a consequence of external forcing has only become evident over the past three decades."
[Jones] The warming from 1915 to about 1940 is generally believed to be due to a slight increase in solar output and a reduction in volcanism. The recent warming from 1975 is due to the build-up of greenhouse gases, as they begin to dominate over sulphate aerosol releases. The slight cooling from 1940 to 1975 is thought to be due to industrial development (using lots of dirty coal) increasing aerosol emissions.