Pre-industrial rise in methane gas had natural and anthropogenic causes

Nov 22, 2013
Air bubbles trapped in an ice core from the West Antarctic Ice Sheet Divide ice core. Air samples were used to reconstruct the methane interpolar difference (IPD) over the late Holocene to constrain early anthropogenic emissions of methane. Credit: Logan Mitchell

(Phys.org) —For years scientists have intensely argued over whether increases of potent methane gas concentrations in the atmosphere – from about 5,000 years ago to the start of the industrial revolution – were triggered by natural causes or human activities.

A new study, which will be published Friday in the journal Science, suggests the increase in likely was caused by both.

Lead author Logan Mitchell, who coordinated the research as a doctoral student at Oregon State University, said the "early anthropogenic hypothesis," which spawned hundreds of scientific papers as well as books, cannot fully explain on its own the rising levels of during the past 5,000 years, a time period known as the mid- to late-Holocene. That theory suggests that human activities such as rice agriculture were responsible for the increasing methane concentrations.

Opponents of that theory argue that human activities during that time did not produce significant amounts of methane and thus natural emissions were the dominant cause for the rise in atmospheric CH4.

"We think that both played a role," said Mitchell, who is now a post-doctoral researcher at the University of Utah. "The increase in during the late Holocene came primarily from the tropics, with some contribution from the extratropical Northern Hemisphere.

"Neither modeled natural emissions alone, nor hypothesized anthropogenic emissions alone, are able to account for the full increase in methane concentrations," Mitchell added. "Combined, however, they could account for the full increase."

Scientists determine methane levels by examining ice cores from polar regions. Gas bubbles containing ancient air trapped within the ice can be analyzed and correlated with chronological data to determine methane levels on a multidecadal scale. Mitchell and his colleagues examined ice cores from the West Antarctic Ice Sheet Divide and the Greenland Ice Sheet Project and found differences between the two.

Ice cores from Greenland had higher methane levels than those from Antarctica because there are greater methane emissions in the Northern Hemisphere. The difference in methane levels between the hemispheres, called the Inter-Polar Difference, did not change appreciably over time.

"If the methane increase was solely natural or solely anthropogenic, it likely would have tilted the Inter-Polar Difference out of its pattern of relative stability over time," Mitchell said.

Since coming out of the ice age some 10,000 years ago summer solar insolation in the Northern Hemisphere has been decreasing as a result of the Earth's changing orbit, according to Edward Brook, a paleoclimatologist in Oregon State's College of Earth, Ocean, and Atmospheric Sciences and Mitchell's major professor. This decrease affects the strength of Asian summer monsoons, which produce vast wetlands and emit methane into the atmosphere.

Yet some 5,000 years ago, atmospheric methane began rising and had increased about 17 percent by the time the industrial revolution began around 1750.

"Theoretically, methane levels should have decreased with the loss of solar insolation in the Northern Hemisphere, or at least remained stable instead of increasing," said Brook, a co-author on the Science article. "They had been roughly on a parallel track for some 800,000 years."

Mitchell used previous models that hypothesized reasons for the methane increase – both natural and anthropogenic – and compared them to the newly garnered ice core data. None of them alone proved sufficient for explaining the greenhouse gas increase. When he developed his own model combining characteristics of both the natural and anthropogenic hypotheses, it agreed closely with the ice core data.

Other researchers have outlined some of the processes that may have contributed to changes in methane emissions. More than 90 percent of the population lived in the Northern Hemisphere, especially in the lower latitudes, and the development of rice agriculture and cattle domestication likely had an influence on methane emissions. On the natural side, changes in the Earth's orbit could have been responsible for increasing methane emissions from tropical wetlands.

"All of these things likely have played a role," Mitchell said, "but none was sufficient to do it alone."

Explore further: Sediment trapped behind dams makes them 'hot spots' for greenhouse gas emissions

More information: "Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget," by L. Mitchell et al. Science, 2013.

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User comments : 11

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ForFreeMinds
1.6 / 5 (25) Nov 22, 2013
Estimates of global population are 4 million people 10,000 years ago, and 5 million 5,000 years ago. http://en.wikiped...stimates

Given how big the atmosphere and oceans are, it's hard to see how so few people could have any affect on it. To me, this looks like a study designed to arrive at the preordained conclusion that humans affect the climate, in very small numbers, during the Bronze Age.

Where's the evidence that growing rice changed methane production? Another computer model based on a chemical chemical reaction that only looks at methane production, and not other variables or stabilizing feedback loops they keep missing with AGW?

By limiting one's variables, and not including all the chemical cycles and their effects on one another, it's not surprising one can construct models to generate outcomes. But they can't even get a computer model to explain the lack of warming we've seen in the last 20+ years as predicted.
goracle
2.7 / 5 (19) Nov 22, 2013
Estimates of global population are 4 million people 10,000 years ago, and 5 million 5,000 years ago. http://en.wikiped...stimates

Given how big the atmosphere and oceans are, it's hard to see how so few people could have any affect on it. To me, this looks like a study designed to arrive at the preordained conclusion that humans affect the climate, in very small numbers, during the Bronze Age.

...

By limiting one's variables, and not including all the chemical cycles and their effects on one another, it's not surprising one can construct models to generate outcomes. But they can't even get a computer model to explain the lack of warming we've seen in the last 20+ years as predicted.

So, it's 20 years of no warming now? Others from the denialsphere use 17 years, or 10. You can't even coordinate the misinformation right!
Maggnus
3.5 / 5 (13) Nov 22, 2013
Free form rambling.

Its hard to see how the so few denialists remaining can continue to have an affect on bored internet surfers. To me, it looks like a statement from a person who has already decided what he will believe, any evidence to the contrary be damned. That someone cannot understand the scientist's stated purpose that :
"Neither modeled natural emissions alone, nor hypothesized anthropogenic emissions alone, are able to account for the full increase in methane concentrations,""
suggests they either cannot comprehend what they read, or have a predetermined mindset that any study that even remotely suggests humans can affect the Earth's atmosphere are, by default, wrong.
There is growing evidence that contrarianism is a form of mental defect. By limiting one's ability to critically analyze the statements of non-scientific bloggers, it is not surprising that such a poor understanding of climate is manifested in such poorly informed opinions.
baudrunner
1.3 / 5 (12) Nov 22, 2013
It makes logical sense to have slowdowns and speedups in the climate change cycle, with those cycles riding a logarithmically increasing slope. As cold water from ice melt in the north moves toward tropical zones, ambient temperatures decrease around those currents. Ice loss contributes to warmer ambient temperatures near the pole and greater ice loss occurs during peak melt seasons. It is a relatively slow wavelength cycle in terms of the human experience but one with enough recurrences to display a trend in our lifetimes. We will experience serious climate change, requiring us to go so far as to move our coastal cities farther inland. We should learn from lessons of the past.
24volts
1.3 / 5 (12) Nov 22, 2013
Shouldn't those numbers include animals along with people in those programs? ... There were very large herds of buffalo and probably other creatures that lived in large herds that fit into that time frame. There are so many biological sources of methane I don't see how they can really apply all the possible variables.
NeutronicallyRepulsive
1.5 / 5 (15) Nov 23, 2013
Yeah, same crap as AGW. Just blame humans for everything because you can't really hold anyone/anything else accountable. The humans probably hunted down the dinosaurs to extinction too according to these schmucks. Nevermind there were none. It is all offshoot of the Gaia crap theories, idea of delicate balance in nature, etc. There are plenty of other sources, we're undeniably minor one, but it is like blaming one smoker, smoking a daily cigarette, for causing the smog in a city full of burning coal. Why him? Because you love the heat from burning coal, and you can't really blame a coal. The guy is also the only one who has money, and you can guilt him to pay for smoking. Or scare him with visions of massive demise.
24volts
1.7 / 5 (12) Nov 23, 2013
well, I can see voleure and open has made the ratings rounds again and clicked everyone as a '1'
Now all we need is toot, blotto, nom, dalariada, thescicilian and father brrenk to show up........ all people that never post anything but constantly go through the messages and mess up the rating tags...
VENDItardE
1.3 / 5 (12) Nov 23, 2013
well, I can see voleure and open has made the ratings rounds again and clicked everyone as a '1'
Now all we need is toot, blotto, nom, dalariada, thescicilian and father brrenk to show up........ all people that never post anything but constantly go through the messages and mess up the rating tags...


they are all one and the same as vendicarE (or Scott to those who know and despise him).
Dug
1 / 5 (10) Nov 23, 2013
"None of them alone proved sufficient for explaining the greenhouse gas increase." Unless your base input assumptions are also imprecise. The precision we assume in our ability to measure and model past climate trends on a global level are highly questionable (as our climate prediction model failures have so clearly and repeatedly demonstrates) - but few do. Perhaps resolving the claimed source of the processes we assume to be so dangerous, would be a more productive and rewarding focus.
goracle
1.4 / 5 (9) Nov 23, 2013
well, I can see voleure and open has made the ratings rounds again and clicked everyone as a '1'
Now all we need is toot, blotto, nom, dalariada, thescicilian and father brrenk to show up........ all people that never post anything but constantly go through the messages and mess up the rating tags...


they are all one and the same as vendicarE (or Scott to those who know and despise him).

They all rate the opposite of VendicarE, etc., at least they rate comments counter to Vendicar's stance on climate change and the like.
NeutronicallyRepulsive
1.8 / 5 (10) Nov 24, 2013
Comment rating on this site became ridiculous due to AGW voting bots. It is obvious who is the crazy side with voting bots and it's not people who are skeptical to AGW. If your only argument is voting bot, then you already lost. And I have no reason not believe that supposed bot VendicarH is not bot of VendicarE. I reacted on him, and suddenly it happened. Correlation is strong enough for me. This discreditation is a sign of fanaticism, and also in line with Vendicar call everyone names ("tard" etc.) it is a juvenile, and ridiculous version of argumentation. Have your fanatic, unsupported "truth".. that have been caught many times to be statistical manipulation, blatant lies. I'm still skeptical about AGW, much more so about proclaimed adverse effects in the future, even MORE about ability of humans to do something about it, and convinced absolutely that Vendicar's methods are low.

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