Cows, rice paddies, and termites may change the climate
Climatologists are becoming increasingly concerned about methane, the gas that sometimes burns in a marsh. It pours into the atmosphere from jungles, swamps, and rice paddies. It escapes by the millions of tons from the digestive systems of termites and cattle. Like carbon dioxide, it traps some of the heat Earth radiates toward space, thus warming the air near the ground.Skip to next paragraph
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The methane effect has not been at the top of the climatologists' worry list. It has seemed minor compared with possible warming by carbon dioxide produced by burning fossil fuels. But as W.L. Chameides of the Georgia Institute of Technology pointed out recently in Nature, methane appears to have raised average global temperature about 0.23 degrees C. over the past three centuries. That's roughly 38 percent of the warming attributed to the rise in atmospheric carbon dioxide.
This is hardly a negligible effect. What is equally startling, if confirmed, is the finding by some researchers that the methane level has risen at a yearly rate of 1.7 percent since at least 1965.
Furthermore, Harmon Craig and his colleague, Chun-Chao Chou, at the Scripps Institution of Oceanography find that the atmospheric methane level remained stable from 27,000 BC to around AD 1580. Then it began to rise. ''Fossil'' air trapped in glacial ice indicates the level doubled from 0.7 ppm (parts per million by volume) to the present level of 1.65 ppm.
Chameides notes that natural processes which remove methane from the atmosphere may have become less effective, but considers this unlikely. He explains that various kinds of human activity are a more likely cause.
The prime source of the methane is fermentation due to bacteria in such wet environments as swamps or rice paddies. The bacteria aid the digestion of termites and cattle. Indeed, P.R. Zimmerman and J.P. Greenberg of the US National Center for Atmospheric Research, S.O. Wandiga of the University of Nairobi, and P.J. Crutzen of West Germany's Max Planck Institute for Atmospheric Chemistry have shown that termites may account for a substantial part of the 200 million tons of methane cycled through the atmosphere each year.
Chameides notes that the spread of rice paddies and of cattle raising would be expected to raise methane levels. Even the termite contribution may reflect human activity, for termite colonies multiply where forests have been cleared in the tropics.
Methane is active chemically in the air. It changes concentrations of other chemicals such as ozone or carbon monoxide, which also are heat-trappers. Thus a rise in methane level has complex implications.
As Chameides notes, if human activity is behind the rise, methane levels are likely to go on increasing. Thus meteorologists have a new problem - how to take account of the activities of cows, termites, and rice growers in trying to find out where the climate is headed. Return of an endangered species