CASE HISTORY The Fevered Planet

Organisms under stress are vulnerable to infection by microorganisms that normally would be present only at tolerable numbers. A planetary physician would view the greenhouse effect as just such a pathology afflicting Gaia — one where the stress of interglacial warming had allowed rapid multiplication of humans and their civilizations, with aggravating effect. Temperature records show clearly that the Earth’s temperature is high, though not as yet higher than in past episodes of interglacial fever from which the planet has recovered. More worrying are the signs of damage to the forest ecosystems that would normally provide the planet’s natural cooling mechanisms, and the biochemical disorder signalled by the rise of the greenhouse gases, including the specific human toxins, the CFCs.

The chart below details the human sources and the rising abundance of the principal greenhouse gases, together with their effectiveness as infrared absorbers, and their percentage contribution to any warming effect. Even with all this information, however, it is not easy for our planetary physician to predict the course of the fever. Take the greenhouse gases themselves. As you can see, carbon dioxide is by far the most abundant; released as a byproduct of all combustion processes, CO2 pollution is the price human civilization pays fur its need for energy. But by far the most effective of the greenhouse gases, molecule for molecule, are the CFCs. These gases are not only a danger to the atmosphere through their potential to destroy stratospheric ozone. They are also very potent infrared absorbers, about 10,000 times more so than carbon dioxide. Moreover, they absorb infrared at wavelengths between 8 and 13 thousandths of a millimetre, a part of the spectrum where the atmosphere otherwise is transparent. So, despite their low abundance, the CFCs are a serious and growing element in global warming.

Another potent greenhouse gas, whose action is complex and difficult to predict, is methane. It may be that in the next century methane will be the most significant of the greenhouse gases. At first this must seem surprising, because methane is not all that powerful an infrared absorber and is only present in the air at just under 2 parts per million. What is special about methane is that it can pass into the stratosphere unchecked. There it slowly oxidizes to give two molecules of water for each molecule of methane, and because it is so cold, ice crystals condense from the stratospheric air and form clouds. Unlike ordinary clouds, these are almost transparent to sunlight coming in, but quite opaque to the infrared radiation going out, so they act just like a greenhouse gas. The stratospheric ice clouds are already forming, and grow thicker year by year as increasing amounts of methane are produced by human livestock, rice paddies, and agriculture generally. Their presence was brought to our attention by that peculiar phenomenon the ozone hole over Antarctica. The decline of stratospheric ozone specifically over the South Pole is as much a result of the presence of these ice clouds as it is of the chlorine from the chlorofluorocarbons.