Thus the temperature of the eatth, surface is controlled to a significant extent by the carbon dioxide and water content of the atmosphere. The effect of atmospheric moisture (humidity) is apparent in the Midwest. In summer when the humidity is high, the heat of the sun is retained well into the nigh, giving very high nighttime temperatures. On the other hand, in winter the coldest temperatures always occur on clear nights, when the low humidity allows efficient radiation of energy back into space. The atmosphere's water content is controlled by the water cycle (evaporation and precipitation), and the average content remains constant over the years. However, as fossil fuels have come into more extensive use, the carbon dioxide concentration has increased significantly. This increase, which was 16% from 1880 to 1980, has continued in the past two decade. Some projects indicate that the carbon dioxide content of the atmosphere may be double in the twenty-first century what it was in 1880. As a result, the earth's average temperature could increase by as much as 3℃,cause dramatic changes in climate and greatly affecting the growth the food crops. Most atmospheric scientists believe the warming trend has already started. For example, in 2005 the global average temperature of 14.6℃(58.3℉) was the warmest since records have kept starting in the late 1800s. Global average temperatures have risen 0.6℃ in the past 30 years and 0.8℃ in the past century. " " 可以当你的好朋友吗？ --

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