练习:

　　The Greenhouse effect and Global warming

　　The heart-trapping capacity of the atmosphere is popularly known as the “greenhouse effect”. Despite public controversy surrounding global warming, the natural greenhouse effect has been long established as fact in the scientific community.

　　Indeed, carbon dioxide, and other gases, the earth’s natural climate would be about 33℃ cooler than it is. Life would have evolved quite differently in such a climate.

　　Most scientists believe that the rapid expansion of agriculture and industrial activities over the last several hundred years have generated significant increases in carbon dioxide and methane in the atmosphere. However, experts disagree about whether such changers have caused the increase of approximately 0.5℃ in the earth’s surface temperature that has been documented over the past century. Many scientists cite the fact that 1997 was the warmest year on record, following a decade in which 9 of the 11 hottest years of this century were reported, as strong, but circumstantial, evidence that human activities have altered the earth’s climate. Other experts, however, believe this temperature trend is a natural variation.

　　Also disputed is whether projectedworld population growth to more than 10 billion people by the year 2100 will result in a doubling or tripling of atmospheric concentrations of carbon dioxide. If such a buildup occurs, as many scientists predict, global surface temperatures could increase by anywhere from about one Celsius degree to about 4 Celsius degrees during the next century. The higher half of range involves temperature changes outside of those experienced by human civilizations since the end of the last ice age some 10,000 years ago. During the last ice age, average global temperatures were only about 5 Celsius degrees cooler than the present period.

　　A temperature increase of several degrees Celsius could result in a sea level rise ranging from about 10 cm to about 1 m. A sea level rise of about 0.5 m would be noticeable primarily in the most vulnerable, low-lying islands and coastal areas. Larger sea level rises would result in extensive flooding of lowland beaches, wetlands, and coastal settlements. Moreover, a higher sea level base would increases the risk of catastrophic storm surges in coastal areas, since flooding would likely extend island well beyond historic levels.

　　Warmer temperature could also intensify droughts in some regions, destabilize ecosystems, and cause the decline or extinction of some species. Since carbon dioxide enhances photosynthesis, some scientists have calculated that higher concentrations in the atmostphere would increase the productivity of crops and forests. But others have point out that carbon dioxide increase and a warming climate could encourage the spread of destructive pests, including weeds and disease-carrying insects.