Organisms and the Atmosphere

THE ATMOSPHERE AND THE EXOGENIC HEAT ENGINE — As discussed in preceding classes, the atmosphere can be thought of as the exogenic heat engine’s power filter: it determines how much and what wavelengths and of the incoming sunlight are trapped, how long the entrapped energy remains trapped before being radiated back to space, and, through this, what the Earth’s overall average surface temperature will be. Atmospheric radiation trapping is accomplished by the so-called greenhouse gases. In order of importance in the modern atmosphere, these are water (both as vapor and as cloud droplets), carbon dioxide [CO2], and ozone [O3]. Minor greenhouse gases include methane [CH4] and nitrogen oxides [“NOx”]. The different compounds absorb at different, characteristic wavelengths; and the strength of absorption by each one depends on its abundance. Equilibration among O, O2, and O3 in the upper atmosphere absorbs most of the biologically damaging ultraviolet [uv] component of sunlight. Screening by water, another strong uvabsorber, is also important.