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考点规范练5(必修1 Module 5) .阅读理解 The “greenhouse effect”is the warming that happens when certain gases in the Earth’s atmosphere trap heat.These gases let in light but keep heat from escaping,like the glass walls of a greenhouse. First,sunlight shines onto the Earth’s surface,where it is absorbed and then radiates(放射)back into the atmosphere as heat.In the atmosphere,greenhouse gases trap some of this heat,and the rest escapes into space.The more greenhouse gases are in the atmosphere,the more heat gets trapped. Scientists have known about the greenhouse effect since 1824,when Joseph Fourier calculated that the Earth would be much colder if it had no atmosphere.This greenhouse effect is what keeps the Earth’s climate livable.Without it,the Earth’s surface would be an average of about 60 degrees Fahrenheit(华氏温度)cooler.In 1895,the Swedish chemist Svante Arrhenius discovered that humans could increase the greenhouse effect by making carbon dioxide,a greenhouse gas. He kicked off 100 years of climate research that has given us a good understanding of global warming. Levels of greenhouse gases(GHGs) have gone up and down over the Earth’s history,but they have been fairly constant for the past few thousand years.Global average temperatures have stayed fairly constant over that time as well,until recently.Through the burning of fossil fuels and other GHG emissions(排放物),humans are increasing the greenhouse effect and warming the Earth. | 
