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The Greenhouse Effect
The overall temperature of earth is increased not only by direct solar radiation, but also by the greenhouse effect. The greenhouse effect is a natural process by which heat energy from the surface of earth is absorbed and emitted in all directions by atmospheric greenhouse gases such as carbon dioxide (C02), methane (CH4), nitrous oxide (N20), fluorinated gases, tropospheric ozone (03), and water vapors. About 90% of this heat energy is re-absorbed and re-emitted by greenhouse gases, and the cycle continues to repeat itself. The greenhouse effect traps heat energy in the atmosphere, increasing the temperature of the surface of earth and the lower atmosphere. Without the greenhouse effect, the average global temperature of earth would be about 58.6° F colder than it is today. The amount of heat energy trapped in the atmosphere due to the greenhouse effect depends on the amount of greenhouse gases in the atmosphere. Some greenhouse gases, such as carbon dioxide, methane, nitrous oxide, troposhperic ozone and water vapors, occur naturally and are emitted via natural processes, and from human activities. Other greenhouse gases, such as fluorinated gases, are created and emitted through human activities alone. Greenhouse gases that contribute the most to the greenhouse effect include carbon dioxide, followed by methane, then fluorinated gases, then nitrous oxide and troposhperic ozone.

Carbon Dioxide

Carbon dioxide (C02) is emitted naturally through the carbon cycle, forest fires and volcanoes, as well as human activities such as deforestation (which may be due to farming, ranching, overpopulation, real estate development, roads, logging, growing only one or a few plants in large areas, etc.), overgrazing, the burning of fossil fuels (i.e., oil, coal, natural gas and petroleum), burning and decomposition of biomass (i.e., solid waste and plant matter such as plants, trees and wood), and as a result of chemical reactions such as those produced by manufacturing of cement. Human activities have already increased the amount of C02 in the atmosphere by at least 35% since the Industrial Revolution. C02 emissions contribute acid rain and the acidification of oceans, causing a further decrease in PH levels as C02 is dissolved.

Methane

Natural methane (CH4) sources include wetlands, oceans, freshwater, soil, permafrost, termites, organic decay, wildfires, gas hydrates, natural gas (which is typically composed of about 70-90% methane, up to 20% ethane, propane and butane, 0-8% carbon dioxide, 0-5% nitrogen, 0-5% hydrogen sulfide, 0-0.2% oxygen, and trace amounts of rare gases) and geological sources such as thawing methane clathrates. Global warming causes the melting of clathrates, which releases methane gas into the atmosphere. Primary methane sources from human activities include rice cultivation, the raising of livestock (i.e., cattle, pigs, sheep, goats and poultry), landfills (due to the anaerobic decomposition of organic waste), coal mining, waste management, the burning of biomass, and fossil fuel extraction, processing, storage and distribution. The amount of methane in the atmosphere has increased by more than 150% since 1750, and human activities account for at least 50% of all methane emissions worldwide.

Fluorinated Gases

Synthetic fluorinated gases such as hydrofluorocarbons (HFCs), perfluorocarbons and sulfur hexafluoride (SF6) are now the most common substitutes used for ozone depleting substances such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and halons. Although fluorinated gases are usually emitted in smaller quantities and do not deplete the ozone, they are still very potent greenhouse gases. As such, they are often referred to as 'High Global Warming Potential Gases', or 'High GWP Gases'. GWP Gases are emitted from a number of industrial processes such as aluminum production, semiconductor manufacturing, electric power transmission, magnesium production and processing, and the production of HCFC-22. F-Gases (i.e., CFCs, HFCs and SF6) are typically thousands of times more potent greenhouse gases than carbon dioxide, yet they are still being used as refrigerants, aerosol spray propellants, cleaning solvents, foam blowing agents, electronics manufacturing, etc.

Nitrous Oxide

Nitrous oxide (N20) is produced naturally from biological sources in soil and water, such as microbial action in tropical forests. Nitrous oxide is also produced from human related sources and activities, such as agricultural soils (due to synthetic or excessive nitrogen in soil fertilizers and manure being converted to nitrous oxide by soil bacteria), burning of fossil fuels, industrial activities such as the production of adipic acid, nitric acid and nitrogen oxides, and the burning of solid waste such as animal manure and raw sewage. Chemical fertilizers containing synthetic or excessive nitrogen, phosphorus and/or potassium are often used in the growing of plants, which soaks into the ground and runs off into water sources, causing unnatural plant growth and eutrophication (low levels or lack of oxygen in water), as bacteria try to reduce the excess dying organic matter. Water that has been polluted from this agricultural runoff eventually reaches oceans, where it causes low oxygen areas called 'ocean dead zones'. Carbon dioxide, sulfur dioxide and nitrogen oxides also react with water molecules in the atmosphere to produce acids, resulting in acid rain, ocean acidification and acidic soil. Human activities account for about 6% of the nitrous oxide produced and released into the atmosphere, which has been increasing at a rate of 0.2 to 0.3% each year.

Troposhperic Ozone

Troposhperic ozone (03) is created naturally by the action of sunlight on oxygen, and artificially from smog, which is created as nitrogen oxides, sulfur oxides, hydrocarbons and other combustable gases from factories and vehicle exhausts react to sunlight. Smog and haze reduce the amount of sunlight received by plants to carry out photosynthesis, causing the production of tropospheric ozone, which damages plants. Increased surface UV can also increase tropospheric ozone at ground level, which is toxic due to its strong oxidant properties.
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