A new NASA-funded study shows that the rate of growth of greenhouse gas emissions has slowed since its peak in 1980, due in part to international cooperation that led to reduced chlorofluorocarbon use, slower growth of methane, and a steady rate of carbon dioxide emissions.

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

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

Grow rates over time. Image: NASA

"The decrease is due in large part to cooperative international actions of the Montreal Protocol for the phase- out of ozone depleting gases," said Dr. James Hansen of NASA's Goddard Institute for Space Studies, 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. 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. Soot is 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.

The study also suggests that reduction of methane emissions and soot could yield a major near term success story in the battle against global warming, thus 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 CO2.

Though reducing these climate-forcing agents is important, scientists caution that limiting CO2 will still be needed to slow global warming over the next 50 years.

Hansen emphasizes that CO2 emissions are the single largest climate forcing, and warns that they need to be slowed soon and eventually curtailed more strongly to stabilize atmospheric conditions and stop global warming. Over the next few decades, Hansen said, it is important to limit emissions of forcing agents other than CO2, to buy time until CO2 emissions can be better managed.

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). 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 large 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 forcings and models. The warming in the GISS model is similar to the lowest of the IPCC results, despite the fact that the GISS model has a relatively high sensitivity to forcings.