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FIGURE 2.2 Estimated climate forcings during the three recent decades of the twentieth-century owing to measured changes in greenhouse gases (solid line), net anthropogenic forcing from greenhouse gases, aerosols, clouds and ozone changes (dotted line), and solar irradiance variations associated with the 11-year solar activity cycle alone (small squares). Combined greenhouse plus solar (dashed line) and net anthropogenic plus solar (dash-dot line) forcings are also shown. In each case, the thin lines are projections. The solar forcing is from the empirical model of Foukal and Lean (1990), which accounts for irradiance changes during the 11-year cycle caused by dark sunspots and bright faculae, but does not include additional variability sources acting on longer time scales. Zero point of solar forcing is the 1978–1989 mean. Adapted from Hansen and Lacis (1990) and Hansen et al. (1993). Reprinted with permission from Nature, Copyright 1990, Macmillan Magazines Limited.