About one-half of the solar radiation reaching the upper edge of the Earth's atmosphere is ultimately absorbed by the Earth's surface (see Fig. 2.3). The longwave infrared radiation emitted by the Earth's surface has an important role in the climate system because it provides the largest quantitative energy input to the atmosphere. Atmospheric gases that are transparent to shortwave solar radiation are largely opaque to longwave terrestrial radiation that occurs at wavelengths greater than 4.0 mm. The longwave thermal radiation is absorbed by atmospheric gases and reradiated back to the Earth's surface. The opacity of the atmosphere to longwave radiation and the atmospheric emission of thermal radiation back to the Earth's surface are the fundamental basis for the energy exchanges related to the phenomenon known as the greenhouse effect (Fig. 2.5). The radiant energy flux from the atmosphere to the Earth's surface reveals that the surface receives the greatest energy input from the atmosphere just as the atmosphere receives the greatest energy input from the Earth's surface.
The greater absorption and emission of thermal infrared photons by atmospheric gases is associated with the low photon energies of infrared radiation emitted by the Earth's surface and the corresponding low vibrational and rotational energy transitions of many polyatomic atmospheric gases. Diatomic molecules N2 and O2 have no dipole moment, and they lack vibration-rotation transitions at the small photon energies corresponding to terrestrial radiation (Peixoto and Oort, 1992). Therefore, these gases do not interact with electromagnetic radiation in the longwave spectrum.
Water vapor and carbon dioxide are trace gases present in significant amounts in the atmosphere. These polyatomic atmospheric gases display combinations of vibrational and rotational transitions that allow the molecules to absorb and emit photons at a large number of closely spaced frequencies. The water vapor molecule has a permanent dipole moment that supports pure rotation transitions beginning at about 25 mm and extending to longer wavelengths with greater and greater absorption. In addition, the bent triatomic water vapor molecule has a
F1 F4 F5
Fig. 2.5. A simple model of the greenhouse effect showing principal solar and thermal radiation fluxes. F1 is incoming solar radiation less albedo, F2 is solar radiation absorbed by the Earth's surface, F3 is terrestrial longwave radiation emitted by the Earth's surface, F4 is terrestrial longwave radiation passing through the atmosphere, F5 is atmospheric longwave radiation emitted to space, and F6 is atmospheric longwave radiation emitted back to the Earth's surface.
vibration-rotation band for bending at 6.3 mm that strongly absorbs terrestrial radiation at wavelengths greater than 12 mm. Carbon dioxide is a symmetric, linear molecule that can develop temporary rotational transitions to accompany vibrational transitions. It has a strong bending mode that supports a strong absorption region at photon wavelengths near 15 mm. This absorption band has a considerable effect even at low carbon dioxide concentrations. Still, carbon dioxide is second to water vapor in terms ofimportance in atmospheric radiative transfers. In this way, minor trace gas concentrations of these and other polyatomic molecules determine the infrared transmissivity of the atmosphere (Hartmann, 1994).
The practical climatic significance of the selective absorption and emission of radiant energy by atmospheric trace gases is that the atmosphere is warmed from the bottom. The absorption of radiant energy at the bottom of the atmosphere and the output of longwave radiation from the top of the atmosphere establishes an unstable environment that promotes constant turbulence within the troposphere or the lower layer of the atmosphere. The emerging pattern of global climate is an expression of the dynamic nature of the exchanges of energy and moisture sustained by the physical processes ultimately driven by the energy fluxes. Elucidating the underlying processes is aided by conceptualizing the patterns of several components dynamically linked within the climate system.
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