In the continuing quest to find excellent descriptions of the greenhouse effect, Eli has come across J.S. Sawyers Nature article from 1972, Man Made Carbon Dioxide and the Greenhouse Effect, which was commented on by Skeptical Science and warehoused (bunnies can read it there) by the Weasel. Ray Pierrehumbert and David Archer included it in their climate change reader. Nevelle Nicholls wrote an appreciation of Sawyer's ground breaking work in 2007 under the title of A Warning We Ignored 35 Years Ago, unfortunately just as valid today 45 years later.
In four pages, Sawyer summarised what was known about the role of carbon dioxide in enhancing the natural greenhouse effect leading to warming at the earth's surface, and made a remarkable 28-year prediction of the warming expected to the end of the 20th century. His prediction can now be compared with what has been observed.Nicholls continues
While the IPCC (2007, ER) assessment devotes a volume to this subject, Sawyer could only conclude, after conceding that climate variations of only a fraction of a degree can have "considerable economic importance" that "although there may be no immediate cause for alarm about the consequences of carbon dioxide in the atmosphere, there is certainly need for further study".Sawyer's description of the greenhouse effect, as well as the rest of the paper, is masterful
Perusal of the IPCC volume devoted to the impacts of climate change on natural and human systems leaves one feeling far less sanguine than Sawyer was 35 years ago.
As carbon dioxide is one of the principal gases taking part in radiation exchange in the atmosphere and in the radiation of the Earth's heat content, a change in the content of carbon dioxide within the atmosphere is likely to influence the process. The chief effect of increasing carbon dioxide is that the gas which is radiating heat to space is found at a higher level in the atmosphere than before - the radiation from lower down in the atmosphere is absorbed by the extra carbon dioxide above and then reradiated to space. In the troposphere, at least, temperature decreases with height so the effective radiating temperature of the carbon dioxide becomes lower if the amount of the gas is increased and therefore less heat is radiated to space. Thus the additional carbon dioxide tends to act as a blanket which keeps the Earth warmer - the Earth has to get rid of the incoming radiation from the Sun, and the same amount can only be removed if the temperature of the atmosphere rises a little.He continues
An atmosphere at a higher temperature can hold more water vapour and the additional water vapour produces a similar blanketing effect to that produced by carbon dioxide. Manabe and Wetherald calculate that an increase of 100% in the content of carbon dioxide would increase the world temperature by 1.3 C if the water content of the atmosphere remained constant, but by 2.4 C if the water vapour increased to retain the same relative humidity. The increase of 25% of CO2, expected by the end of the century therefore corresponds to an increase of 0.6 C in world temperature, an amount somewhat greater than the climatic variations of recent centuriesHe did pretty well