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reflectivity, Mars would be only 15°C cooler than Earth. 



However, these planets have atmospheres with very different 

 capabilites of absorbing infrared. The atmosphere of Mars has 

 almost no water vapor and a low level of CO2. According to 

 simple models of the greenhouse effect, its temperature should 

 rise only slightly. In contrast, according to the same models, 

 Venus' s atmosphere of 97 percent CO2 should raise its temperature 

 to a scorching 432°C (775°F), and Earth's atmosphere of 0.03 

 percent CO2 and considerable water vapor should warm it by 

 30°C to an average of 17°C (63°F). Space probes to Mars and 

 Venus and measurements on Eartn confirm that their actual 

 temperatures rise as predicted. 



This and other evidence fias led scientists to believe 

 that as atmospheric levels of CO2 and other greenhouse gases 

 increase, Earth's temperature will rise. Using more comprehensive 

 models of the greenhouse effect, it has been relatively simple 

 to estimate that if no other changes take place in atmospheric 

 composition (i.e., concentration of other greenhouse gases) or 

 the albedo (reflectivity) of the planet, a doubling of atmospheric 

 CO2 concentrations will warm the earth's surface temperature 

 1.2°C.j/ 



Projecting the extent of the future warming is complicated 

 by the fact that the initial warming will change reflectivity 

 and atmospheric composition in a way that will almost certainly 

 amplify the direct warming. For example, the 1.2°C temperature 



