Some fine points on radiative forcing

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 wider ¹ 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 dike ³ and CO₂ cools rather than warms the planet.