<P> Before this time, any oxygen produced by photosynthesis was consumed by oxidation of reduced materials, notably iron . Molecules of free oxygen did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reducing materials that removed oxygen . This point signifies a shift from a reducing atmosphere to an oxidizing atmosphere . O showed major variations until reaching a steady state of more than 15% by the end of the Precambrian . The following time span from 541 million years ago to the present day is the Phanerozoic Eon, during the earliest period of which, the Cambrian, oxygen - requiring metazoan life forms began to appear . </P> <P> The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 30% around 280 million years ago, significantly higher than today's 21% . Two main processes govern changes in the atmosphere: Plants use carbon dioxide from the atmosphere, releasing oxygen . Breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which oxidizes and hence reduces the amount of oxygen in the atmosphere . However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen . The exact cause of the variation of the amount of oxygen in the atmosphere is not known . Periods with much oxygen in the atmosphere are associated with rapid development of animals . Today's atmosphere contains 21% oxygen, which is great enough for this rapid development of animals . </P> <P> Air pollution is the introduction into the atmosphere of chemicals, particulate matter or biological materials that cause harm or discomfort to organisms . Stratospheric ozone depletion is caused by air pollution, chiefly from chlorofluorocarbons and other ozone - depleting substances . </P> <P> The scientific consensus is that the anthropogenic greenhouse gases currently accumulating in the atmosphere are the main cause of global warming . </P>

Very high in the earth's atmosphere the temperature can be 700