THE MYTH OF EARLY 20TH CENTURY SOLAR FORCING

Changes in global-mean temperature over the past 100 years have a step-like character: a rapid warming until around 1940, little change to the mid 1970s, warming again over the next two decades, and little change since then. It is often assumed that natural forcing from changes in Total Solar Irradiance (TSI) can explain the early 20th century warming, while subsequent changes (at least to the late 1990s) have been dominated by anthropogenic effects. The observed warming over 1910-1940 is about 0.4^oC, while the solar-induced change is a warming of only about 0.02^oC, virtually independent of the assumed value of the climate sensitivity. Anthropogenic warming is also small – between 0.03^oC and 0.09^oC. A possible explanation for the residual warming is that it is the result of an increase in the rate of formation of North Atlantic Deep Water (NADW), an idea that is supported by the pattern of warming which is a maximum in the North Atlantic (cf, Wigley and Raper, 1987; Schlesinger and Ramankutty, 1994). This does not mean that solar forcing is always unimportant. For example, over the past decade solar forcing has had an important cooling effect, helping to explain the anomalously small warming observed over this period. These different solar and internal variability effects will be described and discussed.