Limiting Methane, Soot Could Quickly Curb Global Warming

NEW YORK, New York, January 16, 2002 (ENS) - The rate of growth of greenhouse gas emissions has dropped since its peak in 1980 due to concerted efforts by governments around the world, according to a new report from The Goddard Institute for Space Studies, a division of the National Aeronautics and Space Administration (NASA).

Researchers have shown that global warming in recent decades has probably been caused by carbon dioxide (CO2) emitted by the combustion of coal, oil and gas, and by other greenhouse gases including chlorofluorocarbons (CFCs), methane, tropospheric ozone, and black carbon soot particles.

Overall, the growth rate of emissions has slowed over the past 20 years, with the CFC phase-out being the most important factor, according to the study.

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Dr. James Hansen (Photo courtesy Goddard Institute for Space Studies)
"The decrease is due in large part to cooperative international actions of the Montreal Protocol for the phaseout of ozone depleting gases," said Dr. James Hansen, director of NASA's Goddard Institute for Space Studies in New York. "But it is also due in part to slower growth of methane and carbon dioxide, for reasons that aren't well understood and need more study."

The findings appeared in the December 18 issue of the "Proceedings of the National Academy of Sciences." Hansen co-authored the paper with Makiko Sato of Columbia University, New York.

The warming effect of methane is about half as large as that of CO2, and when methane increases it also causes a rise in tropospheric ozone levels. Tropospheric ozone is a principal ingredient in smog, which is harmful to human health and reduces agricultural productivity.

Methane is a naturally occurring gas, a product of a variety of biological processes. But in terms of climate change, it is the unnatural concentration of the gas from human induced factors that has researchers concerned.

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Landfills like this one in Southampton, UK, release the greenhouse gas methane into the atmosphere. (Photo courtesy Government of Southampton)
In the case of garbage disposal, methane enters the atmosphere as a byproduct of decomposition. As anaerobic bacteria break down carbon based garbage like banana peels, methane is produced as a waste gas. Some landfills capture methane and burn it as fuel, but if it enters the atmosphere, methane reduces the Earth's ability to cool by absorbing more reflected heat from the planet than would otherwise occur.

Other sources of methane include rice cultivation, industrial production, and cattle herds.

The rate of methane growth has slowed during the past decade, and it may be possible to halt its growth entirely and eventually reduce atmospheric amounts, Hansen and Sato suggest.

Another warming agent deserving special attention, according to the authors, is soot, a product of incomplete combustion. Diesel powered trucks and buses are primary sources of airborne soot in the United States. Even larger amounts of soot occur in developing countries.

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Smoke and soot contribute to global warming (Photo courtesy James Hansen)
The study suggests that reduction of methane emissions and soot could yield a major near term success story in the battle against global warming, providing time to work on technologies to reduce future carbon dioxide emissions.

Currently, technologies are within reach to reduce other global air pollutants, like methane, in ways that are cheaper and faster than reducing carbon dioxide, the scientists say.

Hansen emphasizes that CO2 emissions are the single largest factor that has contributed to climate change since 1850. He warns that CO2 emissions must be slowed soon and eventually curtailed more strongly to stabilize atmospheric conditions and stop global warming.

If fossil fuel use continues at today's rates for the next 50 years, and if growth of methane and air pollution is halted, the warming in 50 years will be about 1.3 degrees Fahrenheit (0.7 Celsius), the Goddard study shows.

That amount of warming is significant, according to Hansen, but it is less than half the warming in the "business as usual scenarios that yield the specter of imminent disaster."

The climate warming projected in the Institute scenario is about half as great as in the typical scenario from the report of Intergovernmental Panel on Climate Change (IPCC). This is because the IPCC considers a large range of climate change agents and models. The warming in the Goddard model is similar to the lowest of the IPCC results, despite the fact that the Goddard model has a relatively high sensitivity to agents of climate change.