Wetland input: A waterfall on Siberia's Kutamarakan River. One explanation is that unusually warm conditions over Siberia throughout 2007, led to an increase in methane from bacteria in thawed wetlands.
Credit: iStockphoto/Zastavkin
SYDNEY: The amount of methane in Earth's atmosphere shot up in 2007. This brings to an end a decade in which levels of the potent greenhouse gas were relatively stable, says a report in the journal Geophysical Review Letters.
Methane levels in the atmosphere have more than tripled since pre-industrial times. Though much less methane is generated than carbon dioxide, it is more effective at trapping heat from the Sun, meaning that it accounts for around 20 per cent of the human contribution to greenhouse gas-driven global warming.
Atmospheric cleanser
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'.
Because of this, until recently, experts thought that the methane produced by human activity was being roughly balanced by the rate of its destruction in the atmosphere.
However, the new study reports that something has upset this balance since early 2007. The paper's lead authors, Matthew Rigby and Ronald Prinn both of the Massachusetts Institute of Technology, in Boston, USA, say this imbalance has already resulted in several million metric tons of additional methane in the atmosphere.
Compensating effect
"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 modeller Drew Shindell at NASA's Goddard Institute for Space Studies in New York City.
"If methane continues to increase rapidly, we'll lose that offsetting effect. We will use NASA's climate modelling capability to improve our understanding of what is causing the increase and project future methane levels," said Shindell who is involved with a NASA network that collected the data for the study.
The scientists analysed air samples collected by this 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.
One surprising feature of this recent growth in methane, said the researchers, 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 become mixed between the hemispheres.
Theoretical analysis of the measurements puzzlingly 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.
Warmer wetlands
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 methane-destroying hydroxyl free radicals in the atmosphere.
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. More data is needed, though, to test if there has been a drop in the level of hydroxyl free radicals.
"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.
Trend or anomaly?
"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," he added.
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.
With NASA and MIT.
