The lesson learned

This has been a rather arduous chapter - certainly for the author, and no doubt for the reader as well, but hopefully to a lesser extent. The basic lesson, however, can be summed up in a few pithy remarks. The greenhouse effect relies on infrared optical thickness of the atmosphere and temperature decline with height. Real greenhouse gases do not make the atmosphere optically thick uniformly throughout the infrared spectrum. Rather, the optical thickness is concentrated in preferred gas-dependent spectral bands, and the main way the greenhouse effect gets stronger as the gas concentration increases is through the spread of the optically thick regions to every greater portions of the spectrum. There are two basic ways to get the temperature decline which is necessary to translate optical thickness into OLR reduction: radiative equilibrium in an atmosphere that is optically thick through at least part of the spectrum, or convection in an atmosphere where the radiative equilibrium is statically unstable either at the surface or internally. The tropopause height determines the blend of the two mechanisms in force. Both mechanisms can yield a surface temperature very much in excess of the no-atmosphere blackbody temperature, but a radiatively-dominated atmosphere is a very different place from a convectively-dominated atmosphere, since the latter has vigorous vertical mixing that can give rise to a stew of small scale turbulent phenomena. A mostly radiative-equilibrium atmosphere is a more quiescent place, in which mixing is dominated by the more ponderous large scale fluid motions. A central remaining question is how the tropopause height behaves as an atmosphere is made more optically thick. When do we approach an all-troposphere atmosphere, and when do we approach an all-stratosphere atmosphere? These issues are explored in Problems ?? and ??. We will return to the problem of tropopause height in Section 5.10.

Was this article helpful?

0 0

Post a comment