Washington, Oct 30 : Data from a worldwide NASA-funded measurement network has shown that the greenhouse gas methane is on the rise again, after a decade long period of stability.
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, according to the research, the balance has been upset since early 2007.
According to the research paper's lead authors, Matthew Rigby and Ronald Prinn of the Massachusetts Institute of Technology (MIT), 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," he added.
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.
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.
"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," he added.