From Molecules to the Global Atmosphere

However, simultaneously there is emission of infrared radiation from the Boltzmann distribution of molecules in excited states, which leads to a negative energy component. This emission process depends not only on the concentration of the gas but very sensitively on the temperature since this determines the population of the excited states that emit (Eq. (A)). As we shall see in some specific cases below, it is the balance between these two at any given altitude that determines the changes in fluxes and the ultimate impact of a change in a greenhouse gas concentration. 

The difference of 155 W m 2 between the 390 W m-2 emitted by the earth s surface (Fig. 14.2c) and the 235 W uT2 escaping from the atmosphere represents the amount of trapped radiation, the greenhouse effect.  

In summary, in a hypothetical world unperturbed by anthropogenic emissions, the presence of HzO, C02, and, to a lesser extent, 03, CH4, and NzO in the atmosphere leads to a natural greenhouse effect that results in an average surface temperature of about 288 K, rather than 254 K, which is expected in the absence of these gases. 

It is important to emphasize that because the greenhouse effect originates in radiative transfer processes in the earth-atmosphere system, the net effect of a 

FIGURE 14.7 Model-calculated atmospheric temperature changes as a function of altitude due to an increase in C02 from 3f5 ppm in 1960 to 370 ppm projected for 2000 (no feedbacks taken into account) (adapted from Rind and Lacis, 1993). 

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