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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.