The amount of methane in
Earth's atmosphere shot up in 2007, bringing to an end approximately a
decade in which atmospheric levels of the potent greenhouse gas were
essentially stable. The new study is based on data from a worldwide
NASA-funded measurement network.
Methane levels in the atmosphere have more than tripled since
pre-industrial times, accounting for around one-fifth of the human
contribution to greenhouse gas-driven global warming. Until recently, the
leveling off of methane levels had suggested that the rate of its emission
from Earth's surface was being approximately balanced by the rate of its
destruction in the atmosphere.
However, the balance has been upset since early 2007, according to
research published this week in the American Geophysical Union's
"Geophysical Review Letters." The paper's lead authors, Matthew Rigby and
Ronald Prinn of the Massachusetts Institute of Technology, say this
imbalance has resulted in several million metric tons of additional methane
in the atmosphere.
Methane is produced by wetlands, rice paddies, cattle, and the gas and
coal industries. It is destroyed in the atmosphere by reaction with the
hydroxyl free radical, often referred to as the atmosphere's "cleanser."
"This increase in methane is worrisome because the recent stability of
methane levels was helping to compensate for the unexpectedly fast growth
of carbon dioxide emissions," said climate modeler Drew Shindell at NASA's
Goddard Institute for Space Studies in New York.
"If methane continues to increase rapidly, we'll lose that offsetting
effect. We will use NASA's climate modeling capability to improve our
understanding of what is causing the increase and project future methane
levels."
One surprising feature of this recent growth is that it occurred almost
simultaneously at all measurement locations across the globe. However, the
majority of methane emissions are in the Northern Hemisphere, and it takes
more than one year for gases to be mixed between the hemispheres.
Theoretical analysis of the measurements shows that if an increase in
emissions is solely responsible, these emissions must have risen by a
similar amount in both hemispheres at the same time.
The scientists analyzed air samples collected by the NASA-funded
Advanced Global Atmospheric Gases Experiment ground network from 1997
through April 2008. The network was created in the 1970s in response to
international concerns about chemicals depleting the ozone layer. It is
supported by NASA as part of its congressional mandate to monitor
ozone-depleting trace gases, many of which also are greenhouse gases. Air
samples are collected and analyzed at several stations around the world.
According to the researchers, a rise in Northern Hemispheric emissions
may be a result of very warm conditions over Siberia throughout 2007,
potentially leading to increased bacterial emissions from wetland areas.
However, a potential cause for an increase in Southern Hemispheric
emissions is less clear.
An alternative explanation for the rise may lie, at least in part, with
a drop in the concentrations of the methane-destroying hydroxyl free
radical. Theoretical studies show that if this has happened, the required
global methane emissions rise would have been smaller and more strongly
biased to the Northern Hemisphere. At present, however, it is uncertain
whether such a drop in hydroxyl free radical concentrations did occur.
"The next step to pin down the cause of the methane increase will be to
study this using a very high-resolution atmospheric circulation model and
additional measurements from other networks," Prinn said. "The key is to
determine more precisely the relative roles of increased methane emission
versus a decrease in the rate of removal. Apparently we have a mix of the
two, but we want to know how much of each is responsible for the overall
increase."
It is too early to tell whether this increase represents a return to
sustained methane growth, or the beginning of a relatively short-lived
anomaly, according to Rigby and Prinn. Given that methane is about 25 times
stronger as a greenhouse gas per metric ton of emissions than carbon
dioxide, the situation will require careful monitoring in the near future
to better understand methane's impact on future climate change.
SOURCE NASA
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