A new NASA study shows that the rate of growth of greenhouse gas emissions has slowed since its peak in 1980, due to reduced chlorofluorocarbon (CFC) use, slower growth of methane, and a steady rate of carbon dioxide emissions. Overall, the growth of emissions has slowed over the past 20 years, with the lower CFC the most important factor.

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

Although methane is much less abundant than carbon dioxide in the Earth?s atmosphere, the total warming effect of methane is substantial. 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. Cows are a major producer of methane gas in the atmosphere.

Another warming agent soot, which 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, where a larger proportion of the vehicles are diesel.

Reduction of methane emissions and soot could produce major lowering of the gases that cause, giving scientists more time to figure out ways to reduce carbon dioxide emissions. We already have technologies to reduce global air pollutants like methane, that are cheaper and faster than reducing CO2.

Reducing CO2 is still important if we hope to slow climate change over the next 50 years.Hansen says CO2 emissions are the single largest climate forcing, and warns that they need to be slowed soon and eventually curtailed in order to stop global warming.

Hansen says that 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

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