[Editors] MIT: Methane gas levels increase again

[Jen Hirsch]

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Methane gas levels begin to increase again
--New surge ends a decade of stability
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For Immediate Release
WEDNESDAY, OCT. 29, 2008

Contact: Jen Hirsch, MIT News Office
E: jfhirsch at mit.edu, T: 617-253-1682


CAMBRIDGE, Mass. — The amount of methane in Earth’s atmosphere shot  
up in 2007, bringing to an end a period of about a decade in which  
atmospheric levels of the potent greenhouse gas were essentially  
stable, according to a team led by MIT researchers.

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 the Earth’s surface was approximately balanced by the  
rate of its destruction in the atmosphere.

However, since early 2007 the balance has been upset, according to a  
paper on the new findings being published this week in Geophysical  
Review Letters. The paper’s lead authors, postdoctoral researcher  
Matthew Rigby and Ronald Prinn, the TEPCO Professor of Atmospheric  
Chemistry, in MIT’s Department of Earth, Atmospheric and Planetary  
Science, 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, and  
is destroyed by reaction with the hydroxyl free radical (OH), often  
referred to as the atmosphere’s “cleanser.”

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  
from the Northern Hemisphere to the Southern Hemisphere. Hence,  
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.

A rise in Northern Hemispheric emissions may be due to the very warm  
conditions that were observed 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 OH.  
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 because of the inherent uncertainty in the  
current method for estimating global OH levels.

To help pin down the cause of the methane increase, Prinn said, “the  
next step will be to study this using a very high-resolution  
atmospheric circulation model and additional measurements from other  
networks.” But doing that could take another year, he said, and  
because the detection of increased methane has important consequences  
for global warming the team wanted to get these initial results out  
as quickly as possible.

“The key thing is to better determine the relative roles of increased  
methane emission versus an increase in the rate of removal,” Prinn  
said. “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, pound for  
pound, methane is 25 times more powerful as a greenhouse gas than  
carbon dioxide, the situation will require careful monitoring in the  
near future.

In addition to Rigby and Prinn, the study was carried out by  
researchers at Commonwealth Scientific and Industrial Research  
Organization (CSIRO), Georgia Institute of Technology, University of  
Bristol and Scripps Institution of Oceanography. These methane  
measurements come from the Advanced Global Atmospheric Gases  
Experiment (AGAGE) that is supported by the National Aeronautics and  
Space Administration (NASA), and the Australian CSIRO network.

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By David Chandler, MIT News Office