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The "greenhouse effect" is the warming that happens when certain
gases in the earth's atmosphere trap heat. These gases let in
light but prevent heat from escaping, like the glass walls of a
greenhouse. First, sunlight ___1___
onto the earth's surface, where it is absorbed, and then
radiates back into the atmosphere as heat. In the atmosphere,
greenhouse gases trap some of ___2___
heat, while the rest escapes into space. The more greenhouse
gases are in the atmosphere, the more heat gets trapped.
Scientists ___3___ 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 ___4___.
Without it, the earth's surface would be an average of about 60
degrees Fahrenheit cooler. In 1895, a Swedish chemist, Svante
Arrhenius discovered that humans could enhance the greenhouse
effect by making carbon dioxide a greenhouse gas. This finding
kicked ___5___
100 years of climate research that has given us a better
understanding of global warming.
Levels of greenhouse gases (GHGs) have gone up and down over
the earth's history, but they ___6___
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 ___7___, humans are
enhancing the greenhouse effect and warming the earth. |
