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November
20, 2006
SEVEN-YEAR
STABILIZATION OF METHANE
MAY SLOW GLOBAL WARMING
Levels
of atmospheric methane, an influential greenhouse gas, have
stayed nearly flat for the past seven years, following a rise that
spanned at
least two decades, researchers say. This finding indicates that methane
may no longer be as large a global warming
threat as previously thought, and it provides evidence that methane
levels can
be controlled.
Scientists also found that pulses of increased methane were paralleled
by
increases of ethane, a gas emitted during fires. This is further
evidence, they
say, that methane is formed during biomass burning and that large-scale
fires
can be a big source of
atmospheric methane.
Professors Sherwood Rowland and Donald Blake of the University
of California,
Irvine,
and researchers Isobel Simpson and
Simone Meinardi, say that one reason for the slowdown in the growth of
methane
concentration may be leak-preventing repairs made to oil and gas
pipelines and
storage facilities, which can release methane into the atmosphere.
Other reasons may include slower growth or actual decrease
in methane emissions from coal mining, rice paddies, and natural gas
production, they say.
"If one really tightens emissions, the amount of methane in the
atmosphere
10 years from now could be less than it is today. We will gain some
ground on
global warming if methane is not as large a contributor in the future
as it has
been in the past century," said Rowland, a co-recipient of the 1995
Nobel Prize for discovering that
chlorofluorocarbons in such products as aerosol sprays and coolants
were
damaging the Earth's protective ozone layer. The research will be
published November
23 in Geophysical Research Letters.
Methane, the main ingredient of natural gas, warms the atmosphere
through the
greenhouse effect and helps form ozone, a component of smog. Since the
Industrial Revolution in the late 1700s, atmospheric methane has more
than
doubled. About two-thirds of methane emissions can be traced to human
activities, such as fossil-fuel extraction, rice paddies, landfills,
and cattle
farming.
Scientists in the Rowland-Blake laboratory use canisters to collect
sea-level
air in locations from northern Alaska
to
southern New Zealand.
They then measure the amount of methane in each canister and calculate
a global
average.
From 1978 to 1987, the amount of methane in the global troposphere
increased by
11 percent, a more than one percent increase each year. In the late
1980s, the
growth rate slowed to between 0.3 percent and 0.6 percent per year. It
continued to decline into the 1990s, but with a few sharp upward
fluctuations, which
scientists have linked to non-cyclical events such as the eruption of Mount Pinatubo in 1991 and
Indonesian and boreal wildfires in 1997 and 1998. Then, from December
1998 to December 2005, the
samples showed a near-zero growth of methane, ranging from an annual
0.2
percent decrease to a 0.3 percent gain.
Along with methane, the scientists also measured levels of other gases,
including ethane, a by-product of petroleum refining that also is
formed during
biomass burning, and perchloroethylene, a chlorinated solvent often
used in the
dry cleaning process. Ethane levels followed the peaks and valleys of
methane over time, but perchloroethylene
showed a different pattern. This finding provides evidence that biomass
burning
can on occasion, as in Indonesia
in 1997 and Russia
in 1998, be a large source of atmospheric methane, the researchers say.
They say there is no reason to assume that methane levels will remain
stable in
the future, but the fact that leveling off is occurring now indicates
that
society can do something about global warming. Methane has an
atmospheric
lifetime of about eight years. Carbon dioxide, the main greenhouse gas
that is
produced by burning fossil fuels for power generation and
transportation, can last
a century and has been accumulating steadily in the atmosphere.
"If carbon dioxide levels were the same today as they were in 2000, the
global warming discussion would leave the front page," Rowland said.
"But to stabilize this greenhouse gas, we would have to cut way back on
emissions. Methane is not as significant a
greenhouse gas as carbon dioxide, but its effects are important.
The world needs to work hard to reduce emissions of all greenhouse
gases."
NASA and the Gary Comer Abrupt Climate Change Fellowship supported this
research.
##
Contact:
Harvey
Leifert
American
Geophysical Union
202-777-7507
hleifert@agu.org
This
text derived from:
http://www.agu.org/
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