well-calibrated ozone trends over the past 30 years (Fusco and Logan, 2003), implying that fundamental problems remain in our understanding of tropospheric ozone chemistry. Uncertainty in quantifying the indirect radiative effect of ozone is related mainly to the complexity of factors controlling OH concentrations (Lawrence et al., 2001). Uncertainties in predicting the climatic response to changes in tropospheric ozone are also large and require further investigation using general circulation models (GCMs).

Stratospheric Ozone

Depletion of stratospheric ozone over the past 30 years has caused both a positive radiative forcing at the Earth’s surface (due to increased UV penetration) and a negative forcing (due to reduced IR emission from the stratosphere to the troposphere). The consensus from current radiative models constrained by observed ozone trends is that the net forcing is negative and of magnitude −0.10 ± 0.05 W m−2. Forster and Tourpali (2001) argue that about half of this forcing is due to an increase in tropopause heights and thus should not be considered a forcing but rather a feedback. The main indirect radiative effects of stratospheric ozone depletion are (1) increased UV penetration to the troposphere, increasing tropospheric OH concentrations and hence decreasing the lifetime of methane (IPCC, 2001), and (2) changes in stratospheric water vapor.

Several GCM studies have examined the climate response to changes in stratospheric ozone. Shindell et al. (1999) finds that changes in the upper stratosphere elicit far greater surface climate response than changes in the lower stratosphere. Stuber et al. (2001) find that changes in stratospheric ozone have a greater effect per unit forcing than changes in CO2, largely because of feedbacks associated with stratospheric water vapor.

The greatest uncertainty in quantifying radiative forcing from past changes in stratospheric ozone is the vertical distribution of the ozone trend in relation to temperature, since the magnitude of the forcing depends crucially on temperature (IPCC, 2001). Another critical issue is to better quantify indirect radiative forcings, particularly the effect on stratospheric water vapor, which could double the effective forcing according to Stuber et al. (2001).

DIRECT EFFECT OF AEROSOLS

Aerosol particles both scatter and absorb radiation, representing a direct radiative forcing; scattering generally dominates (except for black carbon particles) so that the net effect is of cooling. Global models have demonstrated the important role of sulfate aerosols in providing the cooling effect missing in past models of the atmospheric radiation balance (Kiehl et



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