Human activities arc changing the composition of the atmosphere not only directly through the emission of trace gases and aerosols, hut also indirectly through perturbations in the physical, chemical, and ecological characteristics of the Earth System. These perturbations in turn influence the rates of production and loss of atmospheric constituents.

The impact of direct anthropogenic emissions oil the atmosphere is often relatively easy to assess, especially if they are tied to major industrial activities, where accurate and detailed records are kept for economic reasons. Classical examples are the release of chlorotluorocarbonsand the emission of OO2 I rom fossil fuel combustion. There are also cases, however, in which it is much more difficult to obtain accurate emission estimates. An example is biomass burning, for which no economic incentive for record keeping exists, and which takes on many forms, each with a different emission profile.

A more complex case exists in which human activities release a precursor compound, which is transformed in the atmosphere to a climatically active substance. This can be illustrated using the example of SO?, from w hich sulfate aerosol can be formed. The actual amount olradiatively active sulfate aerosol produced, however, is determined by a complex interplay of atmospheric transport processes, chemical processes in the gas phase, and interactions w ith other aerosol species.

Some of the most important anthropogenic modifications of the atmosphere, however, are the indirect results of human-caused changes in the functioning of the Earth System. For example, when land use and agricultural practices change, the emissions of trace gases such as N2O, NO, and CH4 change in highly complex ways, which are extremely difficult to assess at the scales of interest. An even higher level of complexity is encountered w hen human activities affect the atmospheric levels of some species, something that in turn changes the chemical functioning of the atmosphere and consequently the production rates and lifetimes of aerosol and greenhouse gases. An example of such a mechanism may be the large-scale change of trace gas inputs into the vast photochemical reactor of the tropical troposphere, where most of the photooxidation of long-lived trace gases takes place.

Finally, we must consider feedback loops, in which global change begets global change. Climate change, caused by upsetting the Earth's radiative balance, results in different circulation patterns, changes in water availability at the surface, changes in w ater vapor content of the atmosphere, and so on, all of which modify the atmospheric

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