More methane a mystery
2008-11-02 18:36
Washington - Levels of climate-warming methane - a greenhouse gas 25 times as potent as carbon dioxide - rose abruptly in the earth's atmosphere last year, and scientists who reported the change don't know why it occurred.
Methane, the primary component of natural gas, has more
than doubled in the atmosphere since pre-industrial times, but
stayed largely stable over the last decade or so before rising
in 2007, researchers said on Wednesday.
This stability led scientists to believe that the emissions
of methane, from natural sources like cows, sheep and wetlands,
as well as from human activities like coal and gas production,
were balanced by the destruction of methane in the atmosphere.
But that balance was upset starting early last year,
releasing millions of metric tonnes more methane into the air,
the scientists wrote in the Geophysical Research Letters.
"The thing that's really surprising is that it's coming
after this period of very level emissions," said Matthew Rigby
of the Massachusetts Institute of Technology (MIT).
"The worry is
that we just don't understand the methane cycle very well."
Another surprise was that the rise in methane levels
happened simultaneously at all the places scientists measured
around the globe, instead of being centred near known sources
of methane emissions in the Northern Hemisphere, said Rigby,
one of the study's lead authors along with Ronald Prinn, also
of MIT.
A rise in methane in the Northern Hemisphere might be due
to a year-long warm spell in Siberia, where wetlands harbour
methane-producing bacteria, the scientists said, but had no
immediate answer on why emissions also rose in the Southern
Hemisphere at the same time.
Not enough 'cleanser'?
There is considerably less methane than carbon dioxide in
the atmosphere.
Pre-industrial concentrations of methane were
about 700 parts per billion - that is, for every billion
molecules of air, there were only 700 of methane - but that
level rose gradually to 1 773 parts per billion by the late 20th century, Rigby said in a telephone interview.
The rise in 2007 was about 10 parts per billion over the
course of a year, a real jump for such a short period of time.
By contrast, there are about 385 parts per million of
carbon dioxide in the atmosphere. However, methane is much
better at locking in the solar radiation that heats up the
planet.
Methane is destroyed by reaction with an atmospheric
"cleanser" called the hydroxyl free radical, or OH. The researchers theorised that the rise in methane might be due in part to a decline in OH.
The researchers said it was too soon to tell whether the
one-year rise in the amount of atmospheric methane is the start
of an upward trend or a short-lived anomaly.