The leading greenhouse gases include carbon dioxide, methane, nitrous oxide, and halocarbons. Image to left - Inland Wetlands: Sources of methane include natural sources like wetlands, gas hydrates in the ocean floor, permafrost, termites, oceans, freshwater bodies, and non-wetland soils. So, the true effect of a single GHG emission on climate becomes very hard to single out. When gases are altered, their contribution to the greenhouse warming effect also shifts. Once GHGs like methane and the molecules that create ozone are released into the air, these gases mix and react together, which transforms their compositions. "For example, the amount of methane in the atmosphere is affected by pollutants that change methane's chemistry, and it doesn't reflect the effects of methane on other greenhouse gases," said Shindell, "so it's not directly related to emissions, which are what we set policies for." "The gas molecules undergo chemical changes and once they do, looking at them after they've mixed and changed in the atmosphere doesn't give an accurate picture of their effect," Shindell said. However, Drew Shindell, a climatologist at NASA's Goddard Institute for Space Studies, New York, NY, believes we need to look at the GHGs when they are emitted at Earth's surface, instead of looking at the GHGs themselves after they have been mixed into the atmosphere. When it comes to measuring the impacts of greenhouse gases on our climate, scientists typically look at how much of each gas exists in the atmosphere. Singling out how much each greenhouse gas (GHG) contributes overall to climate warming can be a tricky task. Click on image to enlarge Credit: Changsheng Li Previous studies have shown that new rice harvesting techniques can significantly reduce methane emissions and increase yields. Fossil fuels, cattle, landfills and rice paddies are the main human-related sources. Image to right - Rice Paddies in China: This image shows Chinese farmers transplanting rice in paddy fields in Yunnan Province, China, July 1999. Our Earth is much like that greenhouse, where a mixture of gases in our atmosphere acts together like a pane of glass, letting the sun's rays in, and without letting as much warmth escape out to space. The glass lets the sun's warming rays in, but doesn't let as much of that warmth escape outdoors. Even on a cold winter day, standing inside a greenhouse can be downright balmy if the sun is shining outside.
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