GEOSC 21 SYL - GEOSC 021 Earth and Life Spring 2009 Earth's...

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GEOSC 021 Earth and Life Spring 2009 Earth’s Atmosphere (36 pts) Invididual Assignment Name: Atmospheric Evolution The composition of the Earth’s atmosphere has changed greatly since the formation of the earth. For example, very early in Earth’s history free oxygen (O 2 ), was virtually absent because oxygenic photosynthesis, the major source of free oxygen in today’s atmosphere, had not evolved yet. During the Proterozoic Eon a historic biological event occurred - an organism evolved that was capable of producing O 2 by splitting water during photosynthesis: CO 2 + H 2 O CH 2 O + O 2 (The formula "CH 2 O" is shorthand for sugars, an important building block for biological tissues.) The organisms were cyanobacteria, the first life forms able to produce oxygen as a by- product of photosynthesis. The first solid evidence for cyanobacteria in the fossil record comes from 2.7 billion year old sedimentary rocks from western Australia. Several hundred million years later, at about 2.3 billion years ago, we see the first geologic evidence for an appreciable rise in the concentration of O 2 in Earth's atmosphere. Earth’s Current Atmosphere Constant components (remain the same over recent time and location) Gas Composition (Percentage by volume) Nitrogen (N 2 ) 77% Oxygen (O 2 ) 21% Argon (Ar) 0.93% Neon, Helium, Krypton 0.001% Variable components (vary depending on time and location) Gas Composition (parts per million) Carbon Dioxide (CO 2 ) 370 (in 2002) Water Vapor (H 2 O) 0.1 (South Pole) - 40,000 (tropics) Methane (CH 4 ) 1.7 Nitrogen oxide (N 2 O) 0.3 Ozone (O 3 ) 0.01 (at the surface) Earth’s Energy Balance and the Greenhouse Effect
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Although both nitrogen and oxygen are essential to human life, they have little effect on climate. The variable components, which make up far less than 1% of the atmosphere, have a much greater influence on both short-term weather and long-term climate. For example, variations in water vapor in the atmosphere are familiar to us as relative humidity. The most abundant naturally occurring greenhouse gas is water vapor, followed by carbon dioxide, methane, and nitrous oxide. Simply put, the surface of the Earth is warmer than it would be in the absence of an atmosphere.
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This note was uploaded on 09/09/2009 for the course GEOSC 021 taught by Professor Zellpaul during the Spring '08 term at Pennsylvania State University, University Park.

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GEOSC 21 SYL - GEOSC 021 Earth and Life Spring 2009 Earth's...

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