Greenhouse Effect Ⅰ. Introduction Greenhouse Effect, the capacity of certain gases in the atmosphere to trap heat emitted from the Earth's surface, thereby insulating and warming the Earth. Without the thermal blanketing of the natural greenhouse effect, the Earth's climate would be about 33 Celsius degrees cooler—too cold for most living organisms to survive. The greenhouse effect has warmed the Earth for over 4 billion years. Now scientists are growing increasingly concerned that human activities may be modifying this natural process, with potentially dangerous consequences. Since the advent of the Industrial Revolution in the 1700s, humans have devised many inventions that burn fossil fuels such as coal, oil, and natural gas. Burning these fossil fuels, as well as other activities such as clearing land for agriculture or urban settlements, releases some of the same gases that trap heat in the atmosphere, including carbon dioxide, methane, and nitrous oxide. These atmospheric gases have risen to levels higher than at any time in the last 420,000 years. As these gases build up in the atmosphere, they trap more heat near the Earth's surface, causing Earth's climate to become warmer than it would naturally. Scientists call this unnatural heating effect global warming and blame it for an increase in the Earth's surface temperature of about 0.6 Celsius degrees over the last nearly 100 years. Without remedial measures, many scientists fear that global temperatures will rise 1.4 to 5.8 Celsius degrees by 2100. These warmer temperatures could melt parts of polar ice caps and most mountain glaciers, causing a rise in sea level of up to lm within a century or two, which would flood coastal regions. Global warming could also affect weather patterns causing, among other problems, prolonged drought or increased flooding in some of the world's leading agricultural regions. Ⅱ. How the Greenhouse Effect Works The greenhouse effect results from the interaction between sunlight and the layer of greenhouse gases in the Earth's atmosphere that extends up to 100 km above Earth's surface. Sunlight is composed of a range of radiant energies known as the solar spectrum, which includes visible light, infrared light (红外线), X-rays, and ultraviolet light. When the Sun's radiation reaches the Earth's atmosphere, some 25 percent of the energy is reflected back into space by clouds and other atmospheric particles. About 20 percent is absorbed in the atmosphere. For instance, gas molecules in the uppermost layers of the atmosphere absorb the Sun's X-rays. The Sun's ultraviolet (紫外线的) radiation is absorbed by the ozone layer, located 19 to 48 km above the Earth's surface. About 50 percent of the Sun's energy, largely in the form. of visible light, passes through the atmosphere to reach the Earth's surface. Soils, plants, and oceans on the Earth's surface absorb about 85 percent of this heat energy, while the rest is reflected back into the atmosphere—most effectively by reflective surfaces such as snow, ice, and sandy deserts. In addition, some of the Sun's radiation that is absorbed by the Earth's surface becomes heat energy in the form. of long-wave infrared radiation, and this energy is released back into the atmosphere. Certain gases in the atmosphere, including water vapor, carbon dioxide, methane, and nitrous oxide, absorb this infrared radiant heat, temporarily preventing it from dispersing into space. As these atmospheric gases warm, they in mm emit infrared radiation in all directions. Some of this heat returns back to Earth to further warm the surface in what is known as the greenhouse effect, and some of this heat is eventually released to space. This heat transfer creates equilibrium between the total mount of heat that reaches the Earth from the Sun and the amount of heat that the Earth radiates out into space. This equilibrium or energy balance—the