FIGURE 10-5 Comparison of the average response to volcanic eruptions in an energy balance model and the Briffa et al. (2001) reconstruction for the year of the eruption (year 1) to the next major eruption. Dotted lines represent 9–95 percent uncertainty ranges for the observed response (note the sample size decreases with time). SOURCE: Hegerl et al. (2003). Reproduced by permission of American Geophysical Union; copyright 2003.

stronger cooling than the models that prescribe the volcanic forcing in terms of a reduction in the Sun’s emission.

Based on current estimates, variations in natural climate forcings over the last 2,000 years were much less than the increase in current greenhouse forcing due to human activities. Over the 27-year period in which it has been monitored with satellite-borne instruments, the solar radiative forcing has varied only by 0.1–0.2 W·m^–2 (Foukal et al. 2004). Volcanic activity has not been anomalous as compared to the last 1,000 years and cannot be used to explain the late 20th century warmth. Concentrations of the greenhouse gases in the atmosphere increased substantially over the 20th century and are appreciably above their preindustrial levels. The radiative forcing due to the increases of long-lived greenhouse gases from 1750 to 2000 is about 2.5 W·m^–2 (Ramaswamy et al. 2001). Tropospheric aerosols and land use have also changed due to human activities, but the magnitudes of these forcings are not as well