Greenhouse gas forcing increased by about 2.1 W m−2 since 1860 (Feichter et al., 2004), and the reported surface warming trend was about 0.6 K (IPCC, 2001). Some models that have been used to improve understanding of the relationship between the forcings and climate response have included only greenhouse gases, which lead to a warming. Others have included greenhouse gases, which lead to a warming, and aerosols, which lead to a cooling. As a thought experiment, consider two models that accurately simulate the observed temperature trend. Model A, which does include aerosols, estimates that it took the entire 2.1 W m−2 greenhouse gas forcing to produce the 0.6 K warming. On the other hand, Model B includes aerosols that offset 50 percent of the greenhouse gas warming with cooling. The net forcing increase in Model B is thus only 1.05 W m−2 (50 percent of 2.1 W m−2 contributed by the greenhouse warming). In Model B, it therefore takes only 1.05 W m−2 to produce the observed 0.6 K warming. If aerosol cooling were suddenly removed from Model B (e.g., aerosol emissions were significantly decreased in year 2005 due to their health impacts), it would lead to an additional warming during the next few decades without any further addition of greenhouse gases.
Methane reductions have been proposed as an effective double dividend for improving ozone air quality and mitigating climate change (Fiore et al., 2002). In the case of sulfate and other aerosols having a negative radiative forcing, reducing aerosol concentrations will likely create a more positive radiative imbalance and could actually accelerate increases in surface temperature. Because understanding of the effect of aerosols on the hydrological cycle and vegetation is still incomplete, it is difficult to predict the total effect on climate of reducing aerosol emissions.
Stricter regulation of scattering aerosols such as sulfate could also reveal potential problems with assumptions in climate models about the magnitude of climate sensitivity. Several modeling studies have attributed the surface temperature increases observed during recent decades to the addition of CO2 and other greenhouse gases (IPCC, 2001). More recent modeling studies that have incorporated aerosols show that aerosol direct and indirect forcing may have offset as much as 50 to 75 percent of the radiative forcing due to greenhouse gas increases since preindustrial times. Models that do not include cooling by aerosols will tend to underestimate climate sensitivity, which is adjusted to produce the best fit with the observed radiative forcing and temperature trends (see Box 5-1). This evidence implies a greater possibility that the climate sensitivity for the radiative effects of a doubling of CO2 is in the upper range of the 1.5 to 4.5 K global-averaged surface warming (Andronova and Schlesinger, 2001; Wigley and