Atmospheric Concentrations Of Greenhouse Gases
Prior to the modern influence of human activities on atmospheric chemistry, the naturally occurring greenhouse gases had fairly stable atmospheric concentrations: carbon dioxide about 280 ppm (or parts per million by volume), methane 0.7 ppm, and nitrous oxide 0.285 ppm. (Human activities do not appear to affect the concentration of water vapor, which varies naturally over time.) Today, the atmospheric concentration of CO2 has increased to about 364 ppm, while that of CH4 is 1.7 ppm, and N2O is 0.304 ppm. The concentrations of chlorofluorocarbons (CFCs), and other completely man-made, or synthetic, greenhouse gases, have increased from essentially zero to about 0.7 ppb (parts per billion by volume).
Atmospheric concentrations of the greenhouse gases have increased particularly quickly since the middle of the twentieth century, coinciding with rapid human population growth and intensive global industrialization. The combined effects of fossil fuel use and deforestation have increased the atmospheric concentration of CO2. Fossil fuels, like oil, natural gas, and coal contain carbon in their chemical structure that, when liberated by combustion, combines with oxygen to create CO2. Trees, like all plants, take in CO2, incorporate carbon in their structure, and emit O2 back into the atmosphere; deforestation destroys carbon "sinks" that lower the atmospheric concentration of CO2. Fossil-fuel mining, decomposition of organic materials in human and livestock waste treatment facilities, and flooding in rice agriculture have led to increased emissions of CH4. Agricultural fertilizers, and combustion of fossil fuels and solid wastes account for increased N2O emissions. Industrial processes emit a variety of powerful synthetic greenhouse gases like CFCs, hydrofluorcarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6).
The greenhouse gases vary greatly in their ability to absorb infrared radiation. On a per-molecule basis, methane is about 25–40 times more absorptive than carbon dioxide, nitrous oxide is 200–270 times stronger, and CFCs are 3–15 thousand times more effective. CO2, however, has by far the largest atmospheric concentration, and has experienced the greatest increases; CO2 is responsible for about 60% of the human contribution to increased atmospheric heat retention.
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