Scientists concerned about climatic warming may find clues to the atmospheric buildup of heat-trapping gases in old studies of the sun. While carbon dioxide has been at the top of the climatologists' worry list, experts now recognize that other gases, such as methane, are equally important.
These gases not only trap heat radiating from Earth's surface, they also absorb certain characteristic wavelengths of sunshine as it filters down through the atmosphere. As a result, by studying solar spectra taken years ago, scientists can tell how much methane, for example, there was in the atmosphere in the past.
That's how Curtis P. Rinsland, Joel S. Levine, and Thomas Miles of the NASA Langley Research Center found that atmospheric methane may have increased as much as 40 percent in the past 35 years.
Their finding meshes with other studies -- such as analyses of air bubbles in Antarctic ice -- that indicate methane now is accumulating in the atmosphere faster than carbon dioxide.
Methane is a biological byproduct. Bacteria release it when they process organic material in the absence of air. The gas pours into the atmosphere from swamps and rice paddies. It escapes by the millions of tons per year from the digestive systems of sheep and cattle. The biggest single source may be the vast northern peatlands that ring the hemisphere between latitudes 45 and 65 degrees, according to Robert C. Harriss, also at the Langley Research Center, and several colleagues.
In recent years, climatologists have begun to take methane and other heat-trapping gases as seriously as they do carbon dioxide when they consider possibly harmful climatic warming in the next century.
One of the most extensive studies of this made so far has been done by Ralph J. Cicerone, director of the National Center for Atmospheric Research, together with Veerabhadran Ramanathan and other NCAR scientists. They reported last spring that these gases might amplify the warming effect of carbon dioxide 1.5 to 3 times over the next 50 years.
Besides methane, the NCAR study identified ozone, nitrous oxide, and chlorofluorocarbons 11 and 12 as major heat-trapping gases that are on the rise. The chlorofluorocarbons are used as spray-can propellants and refrigerants.
The scientists noted that their 50-year projection is subject to many uncertainties. Obtaining good data on how these gases have built up in the past is one of them. And that is where the solar spectra can help out.
Reporting their work recently in Nature, Drs. Rinsland, Levine, and Miles said that old solar spectra offer an important data base for assessing past concentrations of gases such as methane.
They used spectra taken in 1951 at the Jungfraujoch International Scientific Station more than two miles high in the Swiss Alps. This enabled them to compare atmospheric levels of methane and carbon monoxide in 1951 with modern data.