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Global Warming

Our planet has warmed and cooled A22 .............. times during the 4.65 billion years of its history. At present Earth appears to be facing a rapid warming, which most scientists believe results, at least in part, from human activities. The chief A23 .............. of this warming is thought to be the burning of fossil fuels, such as coal, oil, and natural gas, which releases into the atmosphere carbon dioxide and other substances known as greenhouse gases. As the atmosphere becomes richer in these gases, it becomes a better insulator, retaining more of the heat provided to the planet by the Sun. The average surface temperature of Earth is about 15'C. Over the last century, this average has A24 .............. by about 0.6 Celsius degree. Scientists predict further warming of 1.4 to 5.8 Celsius degrees by the year 2100. This temperature rise is expected to A25 .............. polar ice caps and glaciers as well as warm the oceans, all of which will expand ocean volume and A26 .............. sea level by an estimated 9 to 100 cm, flooding some coastal regions and even entire islands. Some regions in warmer climates will receive more rainfall than before, but soils will dry out faster between storms. This soil drying may A27 .............. food crops, disrupting food supplies in some parts of the world. Plant and animal species will shift their ranges toward the poles or to higher elevations seeking cooler temperatures, and species that cannot do so may become A28 .............. .