Greenhouse gases are atmospheric gases that absorb heat, and then re-radiate the heat. The process of continual absorbing and radiating creates a cycle that retains heat in the atmosphere; this cycle is called the greenhouse effect. Human activities have resulted in increasing levels of greenhouse gases in the atmosphere, resulting in an enhanced greenhouse effect. The enhanced greenhouse effect is causing a global warming trend that is disruptive to ecosystems worldwide. Greenhouse gases include carbon dioxide, water vapor, methane and nitrous oxide.
Human carbon dioxide emissions are the single most significant cause of global warming. Approximately two-thirds of human-caused carbon dioxide comes from burning fossil fuels, with an additional third resulting from deforestation. Carbon is stored in plant matter, such as trees and plants, within forests. Fossil fuels are mostly created by the anaerobic decomposition of buried plant matter, usually over the course of millions of years. When fossil fuels are burned, and forests destroyed, the stored carbon is released into the atmosphere as carbon dioxide. As of 2011, atmospheric carbon dioxide levels were approximately 35 percent above normal, and rising.
Water vapor is the most common greenhouse gas, and the one with the greatest overall effect on atmospheric heat retention. Due to the enhanced greenhouse effect, levels of water vapor in the atmosphere increase due to a positive feedback loop. Warmer conditions cause increased evaporation of water, with the warmer atmosphere able to hold larger amounts of water vapor. Therefore, when human greenhouse emissions cause warming, increased water vapor levels are a secondary effect. The higher water vapor levels then trap yet more heat, creating the feedback loop.
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Methane, the main component of natural gas, is a potent greenhouse gas that traps about 20 times as much heat as carbon dioxide. Atmospheric methane emissions occur during natural gas drilling, coal mining and other industrial processes. The digestive systems of livestock produce approximately 35 percent of human-caused methane emissions. Some scientists predict that warming trends will melt arctic permafrost, resulting in large releases of methane, and a positive feedback loop that will accelerate global warming.
Nitrous oxide exists in much smaller concentrations in the atmosphere, but is a very efficient greenhouse gas, trapping approximately 300 times as much heat as carbon dioxide. Human nitrous oxide emissions are produced mainly by the agricultural sector. When nitrogen-rich fertilizers make their way into underground aquifers and rivers, they breakdown to produce atmospheric nitrogen, with nitrous oxide as a byproduct. Human-caused nitrous oxide emissions account for between 6 and 10 percent of the enhanced greenhouse effect.