Gases with an indirect greenhouse effect

I have so far described all the gases present in the atmosphere that have a direct greenhouse effect. There are also gases which through their chemical action on greenhouse gases, for instance on methane or on lower atmospheric ozone, have an influence on the overall size of greenhouse warming. Carbon monoxide (CO) and the nitrogen oxides (NO and NO2) emitted, for instance, by motor vehicles and aircraft are some of these. Carbon monoxide has no direct greenhouse effect of its own but, as a result of chemical reactions, it forms carbon dioxide. These reactions also affect the amount of the hydroxyl radical (OH) which in turn affects the concentration of methane. Emissions of nitrogen oxides, for instance, result in a small reduction in atmospheric methane which partially compensates for the increase in ozone due to aircraft mentioned in the last paragraph. Substantial research has been carried out on the chemical processes in the atmosphere that lead to these indirect effects on greenhouse gases. It is of course important to take them properly into account, but it is also important to recognise that their combined effect is much less than that of the major contributors to human-generated greenhouse warming, namely carbon dioxide and methane.

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