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| == The evolution of Terra's atmosphere == | == The evolution of Terra's atmosphere == | ||
| The modern atmosphere is sometimes referred to as its third atmosphere; in order to distinguish the current ] ] from two notably different compositions. The original atmosphere was primarily ] and ], ] (from the still molten crust, and the sun) dissipated this atmopshere |
The modern atmosphere is sometimes referred to as its "third atmosphere"; in order to distinguish the current ] ] from two notably different compositions. The original atmosphere was primarily ] and ], ] (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 ], still heavily populated with ]es releasing ], ], and ]. This led to the "second atmosphere"; which was, primarily, carbon dioxide and ], with some ] and, virtually, no ]. This second atmosphere had ~100 times as much ] as the current atmosphere. It is generally believed that the ], caused by high levels of carbon dioxide, kept the Earth from ]. | |||
| During the next couple billion years, water vapour ] to form ] and ]s, which started to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. ] plants would ] and convert carbon dioxide into oxygen. Over time, excess carbon became locked in ], ]s (notably ]), and ]s. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, ] would also convert ammonia into nitrogen. | |||
| As more photosynthesizing plants appeared the levels of oxygen increased dramatically - leading to mass extinction of species adapted to the reducing atmosphere, and encouraging the evolution of more complex organisms. The appearance of the ] further favoured these new plants by protecting them from harmful ] radiation. At this time the carbon dioxide levels became very low - as it remains today. | |||
| As more plants appeared, the levels of oxygen increased significantly (while carbon dioxide levels dropped). At first combined with various ]s (such as ]s and further evolution. The appearance of an ] (a ] of oxygen ]s) ]forms were better protected from ] ]. The oxygen-nitrogen atmosphere is the "third atmosphere". | |||
| In modern times, the burning of fossil fuels has caused an increase in carbon dioxide in the atmosphere. This increases the amount of infrared radiation emitted from the atmosphere towards the earth's surface, thereby contributing to ]. The ] 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". | In modern times, the burning of fossil fuels has caused an increase in carbon dioxide in the atmosphere. This increases the amount of infrared radiation emitted from the atmosphere towards the earth's surface, thereby contributing to ]. The ] 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". | ||
Revision as of 02:25, 27 June 2003
On Earth, the 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 vapour and other gases. The atmosphere protects life on the planet by absorbing ultraviolet radiation of the sun, and reducing the temperature difference between day and night.
The temperature of the Earth's atmosphere varies with height above sea level:
- 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.
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 Terra'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. The oxygen-nitrogen atmosphere is the "third atmosphere".
In modern times, the burning of fossil fuels has caused an increase in carbon dioxide in the atmosphere. This increases the amount of infrared radiation emitted from the atmosphere towards the earth's surface, 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".