阅读理解

　　Is the world warming up? According to many reports the answer is yes．Eleven of the hottest years since 1850 appeared between 1995 and 2006．Last year, the United Nation’s Intergovernmental Panel on Climate Change(IPCC)reported the earth was about 0.75℃warmer than it was in 1850．While this doesn’t sound like a lot, a small difference in average(平均)temperature can make a big difference in climate(气候)．During the last ice age, the planet was only about five degrees colder than now．

　　It’s all about the speed at which temperatures are changing．In the past, temperatures moved up or down step by step．But in the 20^th century, especially since 1976, temperatures have likely risen more quickly than during any century in the last 1,000 years．Warming may bring improved harvests(收获)to northern countries such as Canada and Russia．However, many species(物种)won’t like these conditions．They begin moving towards cooler places and cause lots of trouble to human world．

　　The IPCC reported that human activity is very likely responsible(负责任的), by increasing the greenhouse effect(效应)．Some gases cause the air to take in heat energy．Without the greenhouse effect, the earth’s average temperature would be -18℃ rather than the present comfortable 14.6 degrees．The main greenhouse gases are carbon dioxide(CO₂), water vapor(蒸汽)and some other gases．Burning oil, coal and waste adds to the Carbon dioxide, and it has increased 35％since 1850．

　　Many researchers have found that natural forces alone don’t explain the temperature increases over the last 30 to 40 years．Their study shows that as carbon dioxide goes up, temperatures will rise too．But some scientists don’t agree．They think the effect of human produced greenhouse gases is very small compared to the normal changes that the climate causes．

　　When asked whether humans are causing the world warming, Carl Wunsch, a professor at the Massachusetts Institute of Technology says,“It seems very likely from what I’ve seen．Is it 100％sure?No．There’s something to do with natural changes．”Does that mean we should have a“wait and see”attitude(态度)?Not according to Wunsch．He points out we buy house insurance(保险)not because we are sure our homes will be on fire, but because it’s a danger we’d rather avoid．

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