Earth's atmosphere

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	Earth's atmosphere Earth's atmosphere consists of nitrogen (78.1%) and oxygen (20.9%), with small amounts of argon (0.9%), carbon dioxide (variable, but around 0.035%), water vapor, and other gases. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night. 75% of the atmosphere exists within 11km of the planetary surface.

Temperature and the Atmospheric Layers

The temperature of the Earth's atmosphere varies with altitude; the relationship between temperature and altitude varies between the different atmospheric layers:

troposphere - 0 - 7/17 km, temperature decreasing with height.
stratosphere - 7/17 - 50 km, temperature increasing with height.
mesosphere - 50 - 80/85 km, temperature decreasing with height.
thermosphere - 80/85 - 640+ km, temperature increasing with height.

The boundaries between these regions are named the tropopause, stratopause and mesopause.

Pressure

Gravity "pulls" the atmosphere towards the Earth's surface. The average atmospheric pressure, at sea level, is 1 atm. Much of this pressure is the result of air "pushing down" on air beneath it; such that, pressure decreases in an inverse relationship with height. The atmospheric pressure drops by ~50% for every 5.5km altitude increase.

Various Atmospheric Regions

Atmospheric regions are also named in other ways:

ionosphere - the region containing ions: approximately the mesosphere and thermosphere up to 550 km.
exosphere - above the ionosphere, where the atmosphere thins out into space.
ozone layer - or ozonosphere, approximately 10 - 50 km, where ozone is found.
magnetosphere - the region where the Earth's magnetic field interacts with the solar wind from the Sun. It extends for tens of thousands of kilometers, with a long tail away from the Sun.
Van Allen radiation belts - regions where particles from the Sun become concentrated.

The "Evolution" of the Earth's Atmosphere

The modern atmosphere is sometimes referred to as its "third atmosphere"; in order to distinguish the current chemical composition from two notably different compositions. The original atmosphere was primarily helium and hydrogen, heat (from the still molten crust, and the sun) dissipated this atmopshere.

About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes releasing steam, carbon dioxide, and ammonia. This led to the "second atmosphere"; which was, primarily, carbon dioxide and water vapor, with some nitrogen and, virtually, no oxygen. This second atmosphere had ~100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.

During the next couple billion years, water vapour condensed to form rain and oceans, which started to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. Photosynthesizing plants would evolve and convert carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.

As more plants appeared, the levels of oxygen increased significantly (while carbon dioxide levels dropped). At first combined with various elements (such as [[iron); but, eventually the atmosphere began to "fill" with oxygen - resulting in mass extinctions and further evolution. The appearance of an ozone layer (a compound of oxygen atoms) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".

Global Warming

In modern times, the burning of fossil fuels has caused an increase in atmospheric carbon dioxide; increasing the levels of infrared radiation "trapped" within the atmosphere; and, thereby, contributing to global warming. The IPCC concluded, in their Climate Change 2001 Report[?], that "most of the observed warming, over the last 50 years ,is likely to have been due to the increase in greenhouse gas concentrations".

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