Atmospheric composition is changing today, and is known to have changed markedly over the last 160,000 years. The most important long-lived greenhouse gas is carbon dioxide, the concentration of which is determined by interactions with the terrestrial biosphere and the oceans, as well as by land clearing and the burning of fossil fuels. Both its sources and sinks are strongly influenced by human activity. Nitrous oxide, with a similar range of sources, and the purely anthropogenic chlorofluorocarbons are also greenhouse gases, although their greenhouse potency is offset partially by the stratospheric ozone they destroy.
Ozone is a key atmospheric chemical and protective ultraviolet shield.
It has a complex chemistry that is influenced by many other trace species. It is also toxic to humans and vegetation, and is an important greenhouse gas. Many other trace gases play key roles through their influence on the concentrations of ozone, the hydroxyl radical, and methane. Gaseous sulfur compounds, both natural and anthropogenic, are oxidized to form sulfate aerosols which have an important effect on albedo, counteracting to some extent the influence of greenhouse gases. Atmospheric chemistry is therefore closely linked to industrial activity, climate, and land-use through many complex environmental processes.