The radiative forcing due to clouds and water vapor

As the previous chapters have noted, the climate system is forced by a number of factors, e.g., solar impact, the greenhouse effect, etc. For the greenhouse effect, clouds, water vapor, and CO2 are of the utmost importance. The emergence of computers as a viable scientific tool in the 1960s in conjunction with the availability of spectroscopic data enabled us to treat the numerous complexities of infrared-radiative transfer in the atmosphere. While such calculations set the stage for estimating accurately (decades later in the 1990s) the radiative forcing due to greenhouse gases and clouds, they did not yield the necessary insights into the physics of the problem nor did they yield any explanation of the relevant phenomenon. Such insights needed physically based analytic approaches to the problem. It is in this arena that Dr. Robert Cess excelled and provided the community with important insights into numerous radiative processes in the atmosphere of Earth and other planets including Mars, Venus, Jupiter, and Saturn. A few examples that are relevant to the main theme of this chapter are given below. Within the lower atmosphere of many planets (first 10 km of Earth; 5 km for Mars; and 60 km for Venus) the greenhouse effect is dominated by pressure-broadened vibration–rotational lines (e.g., CO2 and CH4) or pure rotational lines (H2O) of polyatomic gases. Typically, the absorption and emission of radiation occurs in discrete bands with thousands of rotational lines within each band. Even with modern day supercomputers it is impossible to estimate the radiative transfer due to all of these lines and bands through the atmosphere for the entire planet. Thus a three-dimensional characterization of the radiative heating rates from equator to pole using the line-by-line approach is impractical. What is normally done is to use