Rapid rise in methane gas may speed worldwide climatic changes

A WILL-O'-THE-WISP flitting elusively over a marsh may provide a symbol for scientists concerned about possible pollution-driven climate change. Such flickering marsh fires - fueled by methane bubbling up from decaying vegetation - have intrigued people for millennia. But now this planet-warming gas is accumulating in the atmosphere so rapidly it could soon rival carbon dioxide (CO2) in its potential climatic effect. Released by burning fossil fuels, CO2 is now accumulating at a rate that computer projections suggest could warm Earth by several degrees in less than a century - the so-called greenhouse effect.

The action of some of the smallest living beings - bacteria - in digesting plant material in the oxygen-free environment of swamps, rice paddies, and the rumens of sheep and cattle may seem an unlikely agent for planetwide climatic change. But this is the most likely source of most of the methane.

For example, studies by David Lowe and Roger Sparks at the New Zealand Institute of Nuclear Sciences indicate that about three-fourths of the atmosphere's methane is of recent biological origin, as opposed to methane escaping from natural gas and oil operations. The spread and intensification of agriculture as the human population has grown may well have boosted biological methane production, although this has not been scientifically demonstrated.

Donald R. Blake and F. Sherwood Rowland of the University of California at Irvine have published the most recent study of methane buildup. They reported in Science magazine earlier this month that the worldwide average methane concentration in the lower atmosphere has risen 11 percent in just under a decade - from 1.52 parts per million by volume (p.p.m.) in January 1978 to 1.684 p.p.m. in September 1987. It seems to be building up at a more or less steady annual average rate of 1 percent a year. Methane is some 200 times less abundant than is CO2. But it is 20 times as strong as CO2 in its heat-trapping action. So it is a significant greenhouse gas.

These direct measurements of methane in air samples taken widely over the planet confirm studies based on methane absorption of certain infrared wavelengths of sunlight. These latter studies indicate the air's methane concentration was been growing at 1 percent a year from 1951 to 1981. In contrast, ancient air samples (from 3,000 to 200 years ago) trapped in ice cores taken in Greenland and Antarctica show methane concentrations less than half those now measured.

It is now obvious that an environmentally important gas is accumulating at an extraordinary rate in the air. Its buildup seems due, at least indirectly, to human activity. But scientists do not know exactly what this link to human activity is. Here is yet another area where extensive international research is needed to better define a potentially important environmental effect.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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