On the basis of radiation transfer theory, Raymond Pierrehumbert (PHYSICS TODAY, January 2011, page 33) claims that as the ditch in emission spectra seen from space widens, increased atmospheric carbon dioxide renders Earth’s cooling less efficient. Across the ditch, emission by CO2 varies with the temperatures of the strata from which the emission occurs. As the concentration of CO2 increases, its emission to space takes place from progressively higher levels. Near the edge of the ditch, where the emission is from the troposphere, the levels become colder. However, across the 14.5- to 15.5-μm (650–690 cm−1) band they become warmer.
From 1970 to 1997, CO2 increased by about half of its glacial–interglacial range. The clear-sky ditch became wider1 but also shallower,2 and Earth’s cooling became slightly more efficient. When the effects of clouds are included, the brightness temperature across the atmospheric window is some 30 °C colder than seen in Pierrehumbert’s figure 3a. Clouds partly obscure emissions by CO2 from the troposphere, rendering the widening of the ditch less important than the figure would suggest. Above broadband emitters colder than around –55 °C, the ditch becomes a dike3 and CO2 cools rather than warms the planet.
