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Crreenhouse Gases, Climate Change, and Energy <br />d5 <br />(~. m__ ~Qr.. <br />-_ i~~ <br />Home> Greenhouse Gases Programs>Greenhouse Gases, Climate Change, and Energy <br />What Are Greenhouse Gases? <br />Many chemical compounds found in the Earth's atmosphere act as "greenhouse <br />gases." These gases allow sunlight to enter the atmosphere freely. When sunlight <br />strikes the Earth's surface, some of it is reflected back towards space as infrared <br />radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in <br />the atmosphere. Over time, the amount of energy sent from the sun to the Earth's <br />surface should be about the same as the amount of energy radiated back into space, <br />leaving the temperature of the Earth's surface roughly constant. <br />Many gases exhibit these "greenhouse" properties. Some of them occur in nature <br />(water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively <br />human-made (like gases used for aerosols). <br />Why Are Atmospheric bevels Increasing? <br />Levels of several important greenhouse gases have increased by about 25 percent since large-scale <br />industrialization began around 15Q years ago (Figure 1). During the past 20 years, about three-quarters of <br />human-made carbon dioxide emissions were from burning fossil fuels. <br />Figure 1. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide <br />,-- --, <br />7 <br />000 <br />380 <br />A <br />th <br />i <br />+ <br />A <br />h . <br />~ <br />p' <br /> eric <br />n <br />rupocen <br />tmosp <br />c / v <br />360 Concentrations Emissio-ns / 6,000 bj <br /> <br /> <br />c <br /> <br />y..._... ~ ~ N <br />C <br />000 <br />Fo <br />3AD 5, <br /> ~ <br /> <br /> <br />~~ 320 <br />~ <br />! . <br />. <br />4.000 <br /> ~ c <br /> <br /> <br />~ 300 <br />i <br />~' <br />i-~'~J - 5 <br />7} <br />3,000 r <br />`< I <br />V <br />, <br />o <br />I r, l_ <br />~_Y~-- c <br /> <br />260 <br />o <br />__.~.--.._..+ t <br />2,000 Q k <br />~ <br /> i <br />260 1.000 <br /> Q <br /> i <br />c <br /> 1 <br />0 <br />0 1750 1800 1850 1900 1950 2000 <br />t <br />Source: Oak Ridg? National Laboratory. Carbon Dioxide Informaficn Analysis C_nter, <br />~' http:7cdiec esdomlgav.+. <br />l u_____.. _-_.. <br />Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes <br />collectively known as the "carbon cycle" (Figure 2). The movement ("flux') of carbon between the <br />atmosphere and the land and oceans is dominated by natural processes, such as plant photosynthesis. <br />http://www.eia.doe.gov/oiaf/1605/ggecebro/chapterl,html 8/25/2005 <br />