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Sources of Methane Emissions Still Uncertain: Study

Climate scientists still haven’t figured out the reason for the so-called “pause” in global warming — more accurately, a slowdown in the rate of warming over the past decade or so — even as emissions of heat-trapping carbon dioxide continue to increase. But there’s another sort of pause, involving a different greenhouse gas, that baffles them as well. From 1999 through 2006, methane concentrations in the atmosphere stopped increasing after more than a decade of strong growth. Then they took off again. “It is,” said Euan Nisbet, of the University of London in an interview, “a real puzzle.”

Credit: Aftab Uzzaman/flickr

It’s an important puzzle to solve, because while methane remains in the atmosphere for a shorter time than carbon dioxide, it’s much more efficient at trapping heat, which contributes to global warming. Pound for pound, says the Environmental Protection Agency, the impact of methane is more than 20 times greater than that of CO2 over a 100-year period.

Nisbet and two colleagues have now tried to tackle the issue in a new paper in the January 31 issue Science. And while they offer no definitive answer, they offer many possibilities. “In the Northern hemisphere there has been a very sharp increase in Chinese coal production,” Nisbet said. Methane-rich natural gas is common in coal deposits, and, Nisbet said, “the deeper coal mines now being exploited are quite gassy.”

Methane can leak from natural gas wells and pipelines, too, and while the conventional gas-drilling industry has been increasingly careful about stopping leaks, the scientists argue that the newer technique of hydraulic fracturing, or fracking, is probably much leakier.

But the scientists suggest that the greater contribution to skyrocketing methane levels has more to do with biological sources of the gas. Methane molecules are made of carbon and hydrogen atoms, and the carbon in biological methane tends to be slightly lighter than the carbon in methane associated with fossil fuels. And over the past decade or so, the proportion of lighter methane in the atmosphere compared to heavier methane has been rising. “I think this perspective is basically right,” said Martin Helmann, of the Max Planck Institute for Biogeochemistry, in Jena, Germany, in an email. Helmann was not involved in the research.

The authors of the Science paper have some ideas about why biological sources of methane may be increasing. “In the southern hemisphere especially,” Nisbet said, “but also in the northern tropics, a series of really wet years has caused wetlands to expand”—and vegetation decomposing in swamps and shallow lakes is a well known source of natural methane emissions. Another is cows, which generate methane as they digest their food, then belch it out into the air.

These explanations, however, aren’t at all definitive — another key point Nisbet and his co-authors make in the Science paper. “The measurements we make in the air are direct,” he said. “Estimates of where methane is coming from, by contrast, is much less reliable. You estimate the contributions from gas leaks, count up the cows, estimate the emissions from wetlands. There’s obviously going to be a lot of error.”

And in fact, there is: the estimates of how much methane should be going into the atmosphere are greater than what actually ends up there. Tracking methane emissions more accurately is crucial, said the scientists, and not just as an academic exercise.

“If we want to control greenhouse-gas emissions,” Nisbet said, “it’s obviously important to know where the emissions are coming from.” 

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Comments

By john harkness
on January 30th, 2014

Thanks for this reminder about the important role of methane as a forcing in gw.

But, again: the actual, correct statistical analysis of the temperature graph shows NO SLOWDOWN in the rate of GW over the last decade or so: http://tamino.wordpress.com/2014/01/30/global-temperature-the-post-1998-surprise/#more-6942

Time to put that meme to sleep for good.

A main post here on this important info would be of value.

Reply to this comment

By Michael Berndtson (Berwyn, IL 60402)
on January 30th, 2014

Here’s my guess. And it is a guess.

Something is happening between methane and near surface ozone. If near surface ozone is increasing in cities (I believe it is), there could be an increase in methane mineralization. Therefore, slowing down methane buildup in the atmosphere above the northern hemisphere (more cities) compared to the southern. Here’s an interesting paper I found by googling:

“Impacts of climate change, ozone recovery, and increasing methane on surface ozone and the tropospheric oxidizing capacity” by Olaf Morgenstern et al

http://onlinelibrary.wiley.com/doi/10.1029/2012JD018382/abstract

Abstract copied from the webpage:

“Using a stratosphere-troposphere chemistry-climate model, we compare the impacts of climate change, stratospheric ozone recovery, and methane increases on surface ozone and the tropospheric oxidizing capacity by 2050. Methane increases lead to a decreasing OH, particularly in the northern subtropics during summer. Stratospheric ozone recovery causes small increases of surface OH driven by increased stratosphere-troposphere exchange, occurring during parts of the year in the southern extratropics. Tropospheric warming is also associated with increasing OH, maximizing in the Northern Hemisphere in northern summer. In combination, OH is anticipated to decrease by approximately 8% in the tropospheric average by 2050 in the scenario considered here. In conjunction with these changes to OH, we model substantial changes in surface ozone in both hemispheres. Methane increases alone will lead to increasing surface ozone by up to 2–3 ppbv in the zonal mean, maximizing around 30°N. This increase is exacerbated during austral winter when increased stratosphere-troposphere flux of ozone causes an increase in surface ozone in the southern extratropics. Both increases are partially offset by decreases in surface ozone of up to 2 ppbv in the zonal mean, with substantial zonal asymmetries, due to global warming. We model substantial changes in the methane lifetime caused by the three factors. In the Arctic during summer, disappearing sea ice, in an ice-albedo feedback, causes substantially reduced surface ozone. Of the three factors considered here, methane increases are found to exert the strongest influence on surface ozone.”

Reply to this comment

By Ted (Philadelphia, PA)
on January 31st, 2014

I can’t view the article since it’s behind a paywall, so I may be misinformed. Is there new information on methane measurements in the article? Is there any evidence that they have for the natural methane increases? I guess this news article doesn’t give enough information to justify why this journal article was published in Science.

Reply to this comment

By chris
on January 31st, 2014

If you address the hiatus, make clear it is atmospheric temperature - but then Cowtan & Way shows Arctic underestimated.

Reply to this comment

By J. Beamsley (Sycamore, IL, 60178)
on March 17th, 2014

And how much is the melting permafrost to blame? The Siberian Ice Shelf release of methane—- While there is controversy over what factors are to blame for the ice shelf permeation, we do know it is certainly permeated and leaking. And perhaps an abrupt & huge belch of methane from the ice shelf is in our near future, which would dramatically & very quickly increase GW with little chance of reversal.
  http://www.nsf.gov/news/news_summ.jsp?cntn_id=116532

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