Carbon dioxide (CO₂) is by far the most abundant greenhouse gas in the atmosphere; however, other gases are more prodigious trappers of heat. It is these other gases that are responsible for rapid global warming in recent decades, says a leading expert, James Hansen.

Hansen, a National Aeronautics and Space Administration scientist, says the quickest way to slow global warming is to cut these other heat-trapping greenhouse gases first.

According to Hansen's new study in the Proceedings of the National Academy of Sciences:

• Global warming in recent decades has been driven mainly by non-CO₂ greenhouse gases, such as chlorofluorocarbons, methane (CH₄) and nitrous oxide (N₂O), rather than the products of fossil fuel burning, CO₂ and aerosols.

• Although burning fossil fuels has raised atmospheric levels of CO₂, it also produces a haze of particles (soot) that reflects as much of the sun's energy back into space as the release of CO₂ has trapped in the air.

• In contrast to the difficulties of controlling CO₂, technologies already exist to capture or eliminate many of the other kinds of emissions, and many are already declining.

• If sources of CH₄ and ozone (O₃) precursors were reduced in the future, their effect on the climate could be reduced to near zero over the next 50 years.

• Because many of these gases also cause serious, costly health problems or can harm agriculture, there are "strong economic reasons for wanting to eliminate them," says Hansen.

Meanwhile, emissions of carbon dioxide from human activities may be decreasing; Hansen said they shrank slightly in 1998 and 1999, even as the global economy grew.

Thus, the world may find it easier and less costly to slow climate change by focusing on air pollution, concludes Hansen -- easier and less costly than scientists had previously thought.

Source: Andrew C. Revkin, "Study Proposes New Strategy to Stem Global Warming," New York Times, August 19, 2000; James Hansen, et al., "Global warming in the twenty-first century: An alternative scenario," Proceedings of the National Academy of Science, August 15, 2000.

For NYT text http://www.nytimes.com/library/national/science
/081900sci-environ-climate.html

For NAS abstract http://www.pnas.org/cgi/content/abstract/170278997v1