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)

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.

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