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  3. Greenhouse Effect Ⅰ. Introduction Greenhouse Effect, the capacity of certain gases in the atmosphere to trap heat emitted fr

Greenhouse Effect Ⅰ. Introduction Greenhouse Effect, the capacity of certain gases in the atmosphere to trap heat emitted fr

admin2010-04-30  36
问题                                            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 exchange of energy between the Earth’s surface, atmosphere, and space—is important to maintain a climate that can support a wide variety of life.
    The heat-trapping gases in the atmosphere behave like the glass of a greenhouse. They let much of the Sun’s rays in, but keep most of that heat from directly escaping. Because of this, they are called greenhouse gases. Without these gases, heat energy absorbed and reflected from the Earth’s surface would easily radiate back out to space, leaving the planet with an inhospitable temperature close to-19℃, instead of the present average surface temperature of 15 ℃.
    To appreciate the importance of the greenhouse gases in creating a climate that helps sustain most forms of life, compare Earth to Mars and Venus (金星). Mars has a thin atmosphere that contains low concentrations of heat-trapping gases. As a result, Mars has a weak greenhouse effect resulting in a largely frozen surface that shows no evidence of life. In contrast, Venus has an atmosphere containing high concentrations of carbon dioxide. This heat-trapping gas prevents heat radiated from the planet’s surface from escaping into space, resulting in surface temperatures that average 462℃—too hot to support life.
Ⅲ. Understanding the Greenhouse Effect
    Although concern over the effect of increasing greenhouse gases is a relatively recent development, scientists have been investigating the greenhouse effect since the early 1800s. French mathematician and physicist Jean Baptist Joseph Fourier, while exploring how heat is conducted through different materials, was the first to compare the atmosphere to a glass vessel in 1827. Fourier recognized that the air around the planet lets in sunlight, much like a glass roof.
    In the 1850s British physicist John Tyndall investigated the transmission of radiant heat through gases and vapors. Tyndall found that nitrogen and oxygen, the two most common gases in the atmosphere, had no heat-absorbing properties. He then went on to measure the absorption of infrared radiation by carbon dioxide and water vapor, publishing his findings in 1863 in a paper titled "On Radiation Through the Earth’s Atmosphere."
    Swedish chemist Svante August Arrhenius, best known for his Nobel Prize-winning work in electrochemistry, also advanced understanding of the greenhouse effect. In 1896 he calculated that doubling the natural concentrations of carbon dioxide in the atmosphere would increase global temperatures by 4 to 6 Celsius degrees, a calculation that is not too far from today’s estimates using more sophisticated methods. Arrhenius correctly predicted that when Earth’s temperature warms, water vapor evaporation from the oceans increases. The higher concentration of water vapor in the atmosphere would then contribute to the greenhouse effect and global warming.
    Today scientists around the world monitor atmospheric greenhouse gas concentrations and create forecasts about their effects on global temperatures. Air samples from sites spread across the globe are analyzed in laboratories to determine levels of individual greenhouse gases. Sources of greenhouse gases, such as automobiles, factories, and power plants, are monitored directly to determine their emissions. Scientists gather information about climate systems and use this information to create and test computer models that simulate how climate could change in response to changing conditions on the Earth and in the atmosphere. These models act as high-tech crystal balls to project what may happen in the future as greenhouse gas levels rise. Models can only provide approximations, and some of the predictions based on these models often spark controversy within the science community. Nevertheless, the basic concept of global warming is widely accepted by most climate scientists.
Ⅳ. Efforts to Control Greenhouse Gases
    Due to overwhelming scientific evidence and growing political interest, global warming is currently recognized as an important national and international issue. Since 1992 representatives from over 160 countries have met regularly to discuss how to reduce worldwide greenhouse gas emissions. In 1997 representatives met in Kyoto, Japan, and produced an agreement, known as the Ky0to Protocol, which requires industrialized countries to reduce their emissions by 2012 to an average of 5 percent below 1990 levels. To help countries meet this agreement cost-effectively, negotiators are trying to develop a system in which nations that have no obligations or that have successfully met their reduced emissions obligations could profit by selling or trading their extra emissions quotas to other countries that are struggling to reduce their emissions. Negotiating such detailed emissions trading rules has been a contentious task for the world community since the signing of the Kyoto Protocol. A ratified agreement is still not yet in force, and ratification received a setback in 2001 when newly elected U.S. president George W. Bush renounced the treaty on the grounds that the required carbon-dioxide reductions in the United States would be too costly. He also objected that developing nations would not be bound by similar carbon-dioxide reducing obligations. However, many experts expect that as the scientific evidence about the dangers of global warming continues to mount, nations will be motivated to cooperate more effectively to reduce the risks of climate change.
Mars has a thin atmosphere that contains low concentrations of ______.
选项
答案heat-trapping gases
解析 参见第2个小标题HOW THE GREENHOUSE EFFECT WORKS第5段第2句：Mars has a thin atmosphere that contains low concentrations of heat-trapping gases．
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