More Heat Over Greenhouse Gases

In the December issue of Physics Today (page 12), Robert Whitten takes Barbara Goss Levi to task concerning her article about greenhouse gases. However, I am not convinced that Levi’s response gets to the nub of the problem. Whitten’s question about “how the mean atmospheric temperature can remain essentially constant while warming the oceans,” touches on some very basic physics. The question can best be addressed in the context of a simplified, globally-averaged model of Earth’s atmosphere that one finds in texts on meteorology and climate. See, for example, chapter 1 of J. T. Houghton’s The Physics of Atmospheres (Cambridge U. Press, 1986), in which Earth is treated as a “gray” spherical body with uniform radiant properties, immersed in a beam of solar plane waves.

First, the mean planetary radiant temperature of about 255 K must remain approximately constant, independent of any altered greenhouse emissions, if one assumes that the planetary reflection coefficient, emissivity coefficient, and solar constant all remain unchanged. (These assumptions can be relaxed, but that would only cloud the issue). This model reflects the steady state achieved in the power balance between the constant incoming shortwave radiation from the Sun and the constant outgoing long-wave infrared terrestrial radiation. A temperature of 255 K corresponds to a height of some 5 km in the atmosphere; but Earth’s average surface temperature is around 288 K, the elevation of 33 K being due to absorption and reemission of terrestrial radiation by greenhouse gases in the atmosphere’s lowest layer. Feedbacks and other complications aside, the surface temperature rises in line with an increased effective absorption coefficient, as is shown in the wonderfully simple model calculations in Atmospheric Science by J. M. Wallace and P. V. Hobbs (Academic Press, 1977). However, the average temperature of the atmosphere, in the sense described above, cannot change.

Second, the oceans are in thermal contact with the adjacent atmospheric layers, and therefore their temperatures are intimately connected. While the actual transfer mechanisms—radiation, eddy transfer, and so on—may well be complicated and do affect the quantitative outcome, the warming tendency of the enhanced greenhouse effect cannot be disputed.

This short letter necessarily glosses over a lot of detail, but I hope that Whitten’s suggestion of “pseudoscience” can at least be laid to rest.