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A Delicate Balance
Describing the atmosphere as “the protective bubble we live in”, the U.S. National Oceanic and Atmospheric Administration (NOAA) recognizes its’ importance to humans. The composition of the Earth’s atmosphere is thus vital to us as well. There were times in our planet’s approximately 4.5 billion year history when the composition was vastly different than it is today. It is a delicate balance of gasses which allows for the diversity of life now experienced.
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What Do the Percentages Mean?
All concentrations discussed are given in percentages, and are based on the lower atmosphere, considered to be from about 25 kilometers up to the Earth’s surface. These percentages are actually comparative measures of volume, or the amount of space taken up by the gas relative to the other gases.
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Nitrogen makes up about 78% of our atmosphere. Plants rely on certain forms of nitrogen in order to live. The Nitrogen Cycle involves the passing of the gas from the atmosphere, through living things, and back to the atmosphere. Without this cycle the ability of plants, and thus animals, to produce proteins would not exist. It is thought that Nitrogen concentrations have not varied significantly in our planet’s history.
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Oxygen represents about 21% of the earth’s atmosphere. Its’ primary importance to us is clear; animals need it to breathe. As pointed out in Out of Thin Air…, approximately 5 million years ago oxygen levels were much higher than today; 100 million years ago they were much lower. (Ward 2006)
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Argon (.93%) is part of the group of gases called noble gases. These are relatively inert, meaning they exhibit low chemical reactivity. The discovery of argon is accredited to Sir William Ramsey and Lord Rayleigh in 1894. Argon is often isolated and used for industrial purposes.
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The gas form of water is considered the third most abundant gas. The amount of water vapor in our atmosphere at one time varies from close to 0% up to about 4%. This is because water can also exist as a liquid or solid; the amount of water vapor in the air depends on how many water molecules happen to be in the gaseous stage of the hydrologic cycle. It is the most abundant “greenhouse” gas.
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Other Variable Gases
Carbon dioxide (.036%), methane (.00017%), nitrous oxide (.00003%) and ozone (.000004%) are all variable gases that can change concentrations in relatively short periods of time. Another commonality of these gases is their link to climate change. All are “greenhouse” gases except ozone. According to NOOA, the forming of ozone holes in the upper atmosphere is related to global warming, though the gas is still “distinct”. (NOAA 2008)
The Vostok ice core data provides a good example of the variability of carbon dioxide. It shows that about 350,000 years ago concentrations were close to half of what they are today. The other variable “greenhouse” gases exhibit similar extreme variations.
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Trace gases, which are not considered variable are, neon (.0018%), helium (.0005%), and hydrogen (.00005%). Neon and helium are both noble gases, similar to argon. Hydrogen can be used in fuel cells, though its’ lack of abundance means processes, such as electrolysis, must be done to separate the gas from hydrocarbon chains. Unlike the noble gases, it is also highly reactive in certain conditions.
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Why the Variations?
Long term and relatively short term variations occur in the chemical make up of the air around us. The variable nature of the composition of the Earth’s atmosphere is due to a combination of “natural” and anthropocentric phenomena.
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NOAA. A Pleo Perspective On Global Warming. August 20, 2008. http://www.ncdc.noaa.gov/paleo/globalwarming/what.html (accessed January 5, 2010).
Ward, Peter D. Out of Thin Air: Dinosaurs, Birds and Earth's Ancient Atmoshpere. Joseph Henry Press, 2006.