The levels of oxygen and carbon dioxide in our atmosphere are the result of biological processes that have taken place over geological time. The reservoir of free oxygen in the atmosphere is so large, that human activities have no effect on oxygen concentration. Chlorofluorocarbons, however, have caused significant declines in stratospheric ozone.
Stratospheric ozone is important for blocking ultraviolet radiation from space.
Humans have significantly increased the levels of carbon dioxide in the atmosphere, through burning of fossil fuels and through deforestation. Carbon dioxide is the major component of a natural greenhouse effect, which elevates the earth's temperature. Anthropogenic emissions of carbon dioxide and other greenhouse gases may increase the greenhouse effect and lead to global climate change. Describe the oxygen cycle and the carbon cycle. Describe how stratospheric ozone is produced.