- 5 - 



The larger the percentage of infrared radiation blocked by 

 the atmosphere, the warmer the earth's surface temperature. 

 This feature of CO2 and certain other gases is known as the 

 "greenhouse effect." 



The greenhouse effect is an important factor determining 

 a planet's temperature, as observations of the temperatures of 

 other planets confirm (see Table 2-1). ^/ 



TABLE 2- 1 

 THE GREENHOUSE EFFECT ON THE INNER PLANETS: PREDICTION AND OBSERVATIONS 



Sunlight 

 Received 

 (watts per 

 square meter) 



Sunlight 



Reflected 



(percentage) 



Temperature 

 Without An 

 Atmosphere 



"Opacity" 

 Of Atmosphere 

 To Infrared* 



Predicted 

 Temperature 

 With Simple 

 Models** 



Actual 

 Temperature 



VENUS 2613 



EARTH 1367 



MARS 589 



75 

 30 

 15 



-40°C 

 -18°C 

 -56°C 



- 100 

 ~ 1 

 ~ .1 



429°C 

 17°C 

 52°C 



427°C 

 15°C 

 -53°C 



' Higher number indicates greater ability to trap infrared radiation. 

 *Average of two simple models: radiative equilibrium and convective 

 equilibruim. 



Source: Modified from Hansen, J. Lacis, A. and Rind, D., "Climate Trends Due to 

 Increasing Greenhouse Gases", Coastal Zone 83. New York: ASCE, 1983. 



Contrary to popular belief, Venus is not hotter than Earth 

 because it is nearer to the sun, but because its atmosphere is 

 97 percent CO2 . Although Venus receives more sunlight 

 (2613 watts per sguare meter) than Earth (1367 watts per square 

 meter), it reflects 75 percent of this radiation (which explains 

 its brightness), compared with 30 percent for Earth. If the 

 atmospheres of the planets did not differ in their ability to 

 absorb infrared, Earth, with its lower reflectivity, would be 

 23°C degrees warmer than Venus. Similarly, with its very low 



