Greenhouse Emissions Growth Slowed Over Past Decade

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.

"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.

These agents can be categorized into three areas: greenhouse gases, other man-made (anthropogenic) forcings, and natural forcings. The greenhouse gases consist of carbon dioxide (CO2), methane (CH4), nitrous oxide (N20) and chlorofluorocarbons (CFCs). The other anthropogenic forcings consist of black carbon (soot, formed by incomplete combustion), reflective aerosols (tiny airborne particles that reflect sunlight back to space), soil or dust, land cover changes, and forced cloud changes. Natural forcings include changes of the sun's energy and changes of aerosols from volcanic eruptions.

The total "forcing" of climate since 1850 includes a "positive" effect from all the greenhouse gases, which would have a warming effect. Of the other anthropogenic forcings, black carbon has also had a "positive" effect, whereas the other factors including: aerosols, soil and dust, cloud changes, and land cover alterations have had "negative" or cooling effects. Of the natural forcings, an increase of the Sun's brightness has caused a positive forcing, while variations of volcanic aerosols have caused both positive and negative forcings.

Although the sum of all forcings coincidentally is similar to that for carbon dioxide alone, knowledge of each of the large forcings such as methane and black carbon (soot) is needed for development of effective policies.

Climate forcings, or factors that promote warming, such as emissions of carbon dioxide (CO2), methane (CH4), nitrous oxide (N20) and chlorofluorocarbons (CFCs) have increased in the world's atmosphere since the technology of the industrial revolution began pumping these into the atmosphere beginning in the 1800s.

In the graph, climate forcings due to CO2 increases are depicted in light blue, CH4 in dark blue, N20 in yellow, and CFCs in red. In a new study, James Hansen of NASA's Goddard Institute for Space Studies and Makiko Sato of Columbia University found that the growth rate of climate forcings have slowed substantially from almost 5 Watts/meter2 (W/m2) per century to about 3 W/m2 since their peak in 1980.

A watt is a unit of energy and a "watt per meter squared" is the amount of energy the forcing agents have over an area of one square meter. Typically, a forcing of 1Watt after 50 years of would yield a warming of 1.35ƒF (3/4ƒC) by 2050 in changing climate model simulations. The peak in the mid-1980s and drop in the late 1980s of CFCs is evident in the reduction of the red colored area on the graph toward the end of the 1980s.

"The decrease is due in large part to cooperative international actions of the Montreal Protocol for the phase-out of ozone depleting gases," Hansen said. "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."

Hansen and Sato report that emission trends need to be further reduced to approximately 2 W/m2 per century for the next 50 years to achieve a "moderate climate change scenario."