Concern about the enhanced greenhouse effect concentrates mainly on the role played by carbon dioxide, or CO2. Humanity produces more and more of it as the years go by, and less and less is absorbed as the rainforests of the lower latitudes are axed.
Other substances, however, less abundant in the atmosphere but more efficient at inducing global warming, are almost equally important. Methane is a case in point.
Like CO2, methane traps long-wave infrared radiation from Earth that would otherwise escape to space. Molecule for molecule, however, methane manages to prevent the escape of 25 times as much heat as carbon dioxide does.
It is only its relative sparseness in the atmosphere which prevents the results from being catastrophic - but, ominously, concentrations are increasing much more rapidly than those of CO2.
Methane is produced when certain bacteria break down organic matter in the absence of oxygen. These bacteria hate oxygen; until about 2 billion years ago they were in their element on this planet, but once oxygen from new forms of life had saturated the oceans and begun to bubble out into the atmosphere, the methane-loving microbes took refuge wherever oxygen could not reach - in swamps, in coral reefs, and in the guts of animals. There they have remained.
This gives clues as to possible sources of methane in today's world. Great quantities of the substance are produced, for example, in the vast paddy fields of Asia. In certain seasons and at various times of the day the roots of the rice seem to capture methane which has been generated in the effervescent decay along the muddy bottoms of the paddy fields; the gas is transported through the plants' vascular system and expelled directly into the air.
Methane is also formed under municipal refuse tips, and because of its flammable nature, gas from this source has now and then been known to cause a nasty accident on reclaimed land. And there are vast reserves of methane trapped in the bogs and swamps under the permafrost of northern latitudes: if global warming were to cause a rise in temperature, vast quantities of Siberian methane would be released to accelerate the warming process.
Ruminant animals, also, are a major source. Scientists have calculated that the average cow expels some 200 grams of methane into the atmosphere every day. Multiply this by the estimated 1,300 million cattle in the world, and it becomes understandable why one eminent expert on these matters described the three main causes of global warming as "cars, cows and chainsaws".