The atmosphere is composed of nitrogen (about 78 percent by volume), oxygen (21 percent), and various trace gases (totaling about 1 percent). If all trace gases were removed, these percentages for nitrogen and oxygen would remain fairly constant up to an altitude of about 80 kilometers. At the planet's surface, there is an approximate balance between the destruction (output) and production (input) of these gases.
The two most plentiful components of the atmosphere, nitrogen and oxygen, are of significance to life on Earth. Humans and animals cannot live without oxygen. By contrast, the trace noble gases, such as argon, neon, and helium, are not very active chemically. Water vapor is distributed inconsistently in the lower atmosphere; its concentration varies greatly from place to place and from time to time, and it can constitute from 0 to 4 percent of local air. This variable concentration is one reason that water vapor is so important in influencing Earth's weather and climate.
Water vapor provides the main physical substance of storms and precipitation, and its condensation into liquid water generates the large amount of energy (latent heat) necessary to initiate powerful and violent storms. Water vapor is also a greenhouse gas (GHG): It strongly absorbs longwave radiation and reemits this radiation back to the Earth, causing global warming. Clouds, which are generated from water vapor, also play an extremely important role in climate and climate change.
Another very important GHG is carbon dioxide (CO2). Observations indicate that the concentration of CO2 in the atmosphere has been rising steadily for more than a century. The increase of CO2 concentration indicates that CO2 is entering the atmosphere at a greater rate than its rate of removal. This rise is largely attributable to the burning of fossil fuels, such as coal and oil. Deforestation also contributes to the increase in atmospheric CO2 concentration. Estimates project that by sometime in the second half of the twenty-first century, CO2 levels will be twice as high as they were early in the twentieth century. Other GHGs include methane, nitrous oxide, and chlorofluorocarbons.
Ozone (O3) is another important gas for Earth's weather and climate. At Earth's surface, O3 is a major air pollutant, and it is closely monitored for its effects on air quality. However, at upper levels (about 25 kilometers high), O3 forms a shield for Earth's inhabitants from harmful ultraviolet solar radiation. For this reason, the loss of O3 high in the atmosphere as a consequence of human activity has become a serious global-scale issue. One of the examples of O3 depletion is the O3 hole found over Antarctica. Finally, aerosols, including particulate matter, are also important constituents of the atmosphere, affecting weather formation, air quality, and climate change.
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