ABSTRACT
In this chapter the procedures used to calculate the major anthropogenic forcings, and the associated uncertainties, are explained. These are the forcings due to changes in the concentration of well-mixed GHGs, the decrease in the amount of ozone in the stratosphere, the increase in the amount of ozone in the troposphere, and increases in the atmospheric loading of various aerosols. There are a number of reasons why it is useful to be able to compute present and future radiative forcings. First, to the extent that the climate sensitivity is the same for all forcings, the relative magnitudes of the different forcings indicate the relative importance of different causes of climatic change. Second, although forcings do not need to be explicitly calculated when running AGCMs (since the models respond on their own to the internally calculated spatial variation in the radiative perturbation), the ability to diagnose what the forcings are in different AGCMs is useful in understanding the reasons for different responses by different models to a given change in the atmospheric composition. Third, when simpler models - such as the 1-D upwelling-diffusion model - are used for the analysis of emission scenarios or to test the impact of alternative assumptions concerning climate sensitivity or ocean mixing, the various radiative forcings need to be prescribed for the model. Finally, a knowledge of the forcings associated with different GHGs and aerosols is one piece of information that is needed - along with information on atmospheric lifespans - in assessing the relative benefits of decreasing the emissions of different GHGs or aerosol precursors.
