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FIGURE 2.5 Climate forcings determined for the past 140 years (upper bar chart) and a scenario for future climate forcing (lower) if anthropogenic forcing continues to increase at its current rate of 1 W/m 2 per 140 years but is partly offset by a solar Maunder Minimum-type event commencing in 2000, taking 200 years to develop. Courtesy of J. Hansen, after Wigley and Kelly (1990).

(1982). While statistical relationships have in some cases been significant, thescientific community as a whole has strongly resisted accepting the findingsas proof of a causal relationship, primarily because the mechanisms providingthe linkage have not been apparent.

The subject has received new impetus in the past decade, due both to the observation of total and ultraviolet irradiance variations associated with the 11-year solar activity cycle and to observations of a distinct 10-to-12-year oscillation (TTO) in various atmospheric parameters that appear to be in phase with the solar cycle (e.g., Labitzke and van Loon, 1990;