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A methane mystery: Scientists probe unanswered questions about methane and climate change
Posted on 11 February 2014 by Guest Author
This is a re-post from Carbon Brief by Roz Pidcock
Climate scientists have a puzzle to solve. Methane is a powerful greenhouse gas and humanity produces lots of it - mainly from burning wood, mining fossil fuels and decaying waste in landfill.
But the amount we're emitting doesn't match up with how much scientists can detect in the atmosphere. More perplexingly, scientists don't know why atmospheric methane is on the rise again, after a decade of the amount in the atmosphere staying the same.
Methane gas is more than 50 times better at trapping heat than carbon dioxide - making it a much stronger greenhouse gas over several decades, molecule for molecule. About one fifth of the global warming linked to human activity comes from methane, scientists estimate.
Understanding where methane is coming from is important for working out how global temperatures might change in the future. But as a new perspective piece in the journal Science explains, there remain many unanswered questions.
A brief history
From the 1980s until about 1992, the amount of methane in the atmosphere rose sharply. Over the next ten years, the rapid growth slowed to just a quarter of what it had been.
In recent years the upward trend has come back but for a while in the early 2000's, the concentration stopped rising altogether, even falling slightly in some years.
You can see this in the graph below, where the red line shows the trend with the yearly ups and downs (in blue) taken out.
Source: Nisbet et al. (2014)
These figures come from directly measuring the amount of methane present in the atmosphere. But there's another way to estimate total methane emissions - measuring how much is coming from different sources across the globe.
Methane is emitted naturally from wetlands, but almost twice the amount produced naturally is emitted as a result of human activities, such as wood burning, agriculture, drilling and landfill.
Interestingly, during the early 2000s when measurements in the atmosphere recorded almost stable concentrations, measurements of methane sources on the ground told a different story.
When scientists combined all known sources of methane across the globe, the numbers they got suggested that far from staying still, methane emissions were rising quite dramatically.
"Top down" vs "Bottom up"
Lead author of the paper, Euan Nisbet from Royal Holloway University in London, says the mismatch could be because estimates of how much methane is being produced by different sources - so called 'bottom up" estimates - might not be all that accurate.
As Nisbet explained to Climate Central's Michael Lemonick:
"The measurements we make in the air are direct. Estimates of where methane is coming from, by contrast, are 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."
On the rise again
The mismatch in "bottom up" estimates of total methane emissions and direct measurements of the concentration of methane in air - known as "top down" estimates - is only part of the puzzle.
Scientists are also not sure why the amount of methane in the atmosphere started rising again in 2007 - and has continued to rise steadily since then. The authors say in the paper:
"Just when scientists thought the methane concentration had stabilised, it rose again … The renewed rise in the methane burden prompts urgent questions about the causes"
While scientists may not have all the answers just yet, Nisbet and colleagues explore some possibilities for what might be happening.
An Arctic solution?
In the poles, the main sources of methane are natural gas wells and pipelines. Scientists have also reported methane bubbling out of the shallow East Siberian Arctic Shelf as water temperatures are rising and beginning to thaw carbon-rich hydrate in the once-frozen sea bed.
But direct measurements of the overlying atmosphere don't appear to be registering more methane entering the atmosphere from Arctic waters. In fact, atmospheric concentrations of methane over the Arctic have closely tracked the global average since 2007, the new paper explains.
Some scientists have raised the prospect that thawing hydrates and permafrost - carbon-rich soils on land rather than in the sea bed - could release huge amounts of methane very quickly once they start to collapse, causing a dramatic jump in global temperature.
But the authors of the new paper discount that idea - suggesting that if methane is being released, it's not happening very quickly. They say in the paper:
"Long-term release of methane from hydrate is probable, but catastrophic hydrate emission scenarios are unlikely."
If the answer to why atmospheric methane has been increasing since 2007 doesn't lie in thawing Arctic permafrost, what else could be happening?
Leaky pipelines and landfill
In midlatitudes, where the UK and North America sit, methane emissions come mostly from the coal and gas industries, agriculture, burning wood, and landfill. The authors suggest these could have something to do with the rise in atmospheric methane since 2007. They say in the paper:
"In the United States, which has overtaken Russia as the largest gas producer, hydraulic fracturing is increasingly important. In Utah, fracking may locally leak six to 12 per cent of gas production to the air."
The authors also point out coal production has expanded, most notably in China. Methane is often found with coal and the suggestion is that some could be escaping as the coal is mined.
Wetland culprits
But while the authors say there's likely to be some contribution from human activity, the link with rising atmospheric concentrations is yet to be made conclusively. The chemical "fingerprint" of the methane in the atmosphere suggests a natural source, rather than an anthropogenic one.
Decomposing vegetation in tropical wetlands is the single biggest natural source of methane. And as Nisbet explains to Climate Central, the amount wetlands are kicking out has increased:
"In the southern hemisphere especially, but also in the northern tropics, a series of really wet years has caused wetlands to expand"
The question of how much methane we're putting into the atmosphere is a lot less well understood than other greenhouse gases - better measurements are clearly needed. So while methane may not be the biggest source of warming - that distinction goes to carbon dioxide - this shouldn't diminish efforts to resolve the many unanswered questions, the scientists warn.
Two other notable anthropogenic sources of methane worth mentioning are our cattle herding and wet rice agriculture.
It is interesting to note that the global bottom-up methane emission estimates are more than the top-down measurements. Local and regional studies generally show the opposite tendency (see this recent SkS post by gws).
I am not quite sure what to make of that. I suppose that most of it can be attributed to a lack of adequate local and regional measurements and uncertainties in natural and human emissions rates used in the bottom-up calculations.
The other figure in the article shows a remarkable change in the distribution and amount of global emissions, starting in 2007. The authors point out that the recent rise in methane emissions is dominated by non-fossil sources, which perhaps suggests a link to global climate change, but exactly to what factors exactly is not clear.
The article did not discuss processes that remove methane from the atmosphere. I assume these would primarily be oxidation and absorption by the oceans. Is it clear these have a steady rate, or could these rates change?
Mike, see Eli's post Passing Gas on the rapid but rather complicated pathway of CH4 conversion to H2O + CO2 in the tmosphere. It is dependent on the presence of the OH radical.
#3
Mike,
Methane in the atmosphere is oxydised to CO2 and water. This process is dependent of the concentration of Methane in the atmosphere as well as the ceoncentration of a catalyst (which may be fairly constant).
This means, the more methane you have the faster it will be removed. If you have a stable emission of methane, the rate of oxidation will rise until the rate of emission and rate of removal will be equal. The concentration in the atmosphere will level out.
a new rise in methane concentration means, that emission is speeding up again to higher levels than before. Unfortunately the mechanisms of oxidation in the atmosphere are not fully understood. If this would be the case you could read the concentration of methane as a direct proxy of emission rates.
Note, that methane in the atmosphere has a far less remaining time than CO2. Methane will be eliminated by oxidation, CO2 can not be removed by fast working mechanisms, only by geological processes.
@3&5
Actually, the article did not discuss sinks much because oxidation of methane in the troposphere is very well understood. It constitutes the major sink of methane, minor sinks being soil (not ocean, which is a source) uptake by methanotrophs and oxidation in the stratosphere. The oxidation sink it maintained by OH radicals and there is no evidence that the abundance of that radical is changing. Hence, as we do know methane sinks better than its sources, the renewed tropospheric increase is indeed thought to be a proxy for changing source strength.
The graph from the paper, presented at the AGU Fall Meeting in Dec. 2013 and posted by Andy @2, is a spatio-temporal extrapolation of the tropospheric methane growth rate calculated from gas samples obtained from the global sampling network. The network is comparatively sparse, thus the fine detail of the map should not be overinterpreted. It is, however, detailed enough to conclude that the largest growth rates since 2007 have occurred in the tropics and northen midlatitudes (2009, 2012). Not much more can be said than what is in the article. However, from what is known about the sources, particularly the largest source, natural wetlands, wet and warm years in regions like the Amazon, have significant effects on tropospheric methane, and so do years with large biomass burning emissions (e.g. Simpson et al., JGR 2006).
If the tropics were to get not just warmer, but also wetter (that is the expectation), there is thus likely a positive feedback loop on warming via increasing wetland methane emissions. The same may be true for biomass burning if on average more burning will occur as a result of warming and drying in seasonally dry tropical and subtropical regions.
The discrepancy between bottom-up and top-down budget estimates is actually nothing new. The inventories are often outdated, and rarely carefully checked against atmospheric data. Not too surprising because maintaining inventories is cheap compared to measurements. That does not mean the inventory is always bad, but it cautions against trusting it, especially when it is old and/or based on limited input data. The global inventory is not necessarily biased the same way as the US inventory, so while recent US data create a perception that emissions are generally underestimated, that does not automatically mean the same should be true for global emissions.
What about the extensive horizontal drilling and fracking to release natural gas from tight shale formations. It seems that a lot of methane may be released before it can be captured.
tblakeslee @7: The important words here are "can" or "may" as you used. Have a look at this and this post. Like with any other industrial process, there is a chance things go wrong, and therefore risk management, proper planning, and execution is needed. If BP and the other companies involved had adhered to that, the Deepwater Horizon may have never been in the news. Suffice it to say, reality is different.
tblakelee, that would fall under the "mining fossil fuels" catagery in the opening sentence.
Perhaps the methane increase is due to dying trees. Interesting that the jump began in around 2008, which is the year I first noticed that they are all dying. From this post: http://witsendnj.blogspot.com/2012/10/spill-scarlet-rain.html
New information also comes from the Yale School of Forestry, where researchers have discovered high levels of methane emissions from trees that are dying from fungal infection, which they claim is a worldwide phenomena, and yet they cannot (in fact, don't even try to) explain WHY trees that are only at the beginning of maturity (80 to 100 years old) are dying prematurely all over the earth. I suspect they do not appreciate the implications of their own discovery beyond the acceleration of climate change. From SummitCountyVoice:
Sixty trees sampled at Yale Myers Forest in northeastern Connecticut contained concentrations of methane that were as high as 80,000 times ambient levels. Normal air concentrations are less than 2 parts per million, but the Yale researchers found average levels of 15,000 parts per million inside trees.
“These are flammable concentrations,” said Kristofer Covey, the study’s lead author and a Ph.D. candidate at Yale. “Because the conditions thought to be driving this process are common throughout the world’s forests, we believe we have found a globally significant new source of this potent greenhouse gas.”
“If we extrapolate these findings to forests globally, the methane produced in trees represents 10 percent of global emissions,” said Xuhui Lee, a co-author of the study and Sara Shallenberger Brown Professor of Meteorology at Yale. “We didn’t know this pathway existed.”
more at the link above.
Since methane is an energy source for a range of bacteria and bacteria can multiply at prodigious rates if the conditions are right, perhaps we are looking for a sink involving methane using bacteria to explain the missing methane. Also the comment (5) that the rate of a chemical reaction is very much dependent on the concentration of the reactants is very much to the point.
Incidentally, methane is a lot more powerful than 50times Carbon dioxide. A recent figure given in the NSIDC was X86. Reverse engineering the figures suggests as much as X140
http://mtkass.blogspot.co.nz/2013/03/the-real-strength-of-methane.html
thanks, witsend @10 for pointing out that paper. However, the study was about live, standing trees, not dead/decaying trees. Its sample size was very small, the variability (range) near 100% in most cases, and its extrapolation (from the mean) rather bold. If real, there is no reason to believe the source (~20 Tg/yr, thus relatively small) would not have existed before 2007/08, thus cannot explain the current rise.
In the figure by Andy@2, the biggest surge of emmisions happened at N mid-lattitude around 2002-2003. That was immediately followed by the biggest drop in the following year at the same location (is it really the same 'locality' in the raw data or just happens to be in the same band but somewhere else, e.g. NAmer vs. Europe/Asia?). These are the two biggest anomalies on this figure. Has anyone looked at explaining this? Do we have a 'smoking gun' here (but do not understand the processes involved yet) or is it just random noise from naturaql variabilities?
biomass burning provides at least a partial answer
What the summary above misses but the linked article states is:
"since 2007, atmospheric methane has become more depleted in 13C (14), an indication that growth is dominated by 12C-richer wetland and ruminant emissions"
That kind of lets FF and methane hydrates off the hook to some extent.
The first well to start the hydrualic fracturing boom in the Bakken was in 2006 if I remember correctly. I believe that is basically when it started everywhere and that is about when the methane concentrations shown in post #2 start to increase. But then again, it may have nothing to do with fracing if most of the methane is biogenic rather than thermogenic in origin.
There is a new study out that looks at methane leaks in the US. Here is the press release.
Quick summary: Emissions measured from continent-wide top-down sampling are bigger than the bottom-up measurements. This is mostly due to a few one-in-a-thousand very leaky components that may get undersampled either randomly or by selection bias. Fracking activity itself gets a comparitive free pass. Some of the recent top-down studies that have shown large release rates cannot be representative of the ccountry as a whole if the methane budget is to be balanced, many factors, such as natural seepage rates and emissions from abandoned wells are poorly known.
Gas powered power stations are better for the climate than coal, at least over 100 year periods, but given the leak rates in the natural gas supply system, there are no full life-cycle emissions advantages for vehicles that use natural gas to replace diesel or gasoline.
Here's a link to a video where the lead author discusses the results.
In my opinion methane is a problem primarily because of CAFO's. It is not a problem in a properly managed grassland/savanna biome. After all those biomes supported many millions and millions of grazers who were extirpated. The methane levels before they were extirpated were actually lower than now! According to the following studies those biomes actually reduce atmospheric methane due to the action of Methanotrophic microorganisms that use methane as their only source of energy and carbon. Even more carbon being pumped into the soil! Nitrogen too, as they are also free living nitrogen fixers.
http://blogs.uoregon.edu/gregr/files/2013/07/grasslandscooling-nhslkh.pdf
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC239458/pdf/600609.pdf
LINK
http://www.ncbi.nlm.nih.gov/pubmed/11706799
http://journal.frontiersin.org/article/10.3389/fmicb.2013.00225/full
http://www.sciencedirect.com/science/article/pii/0038071794903131
The CH4 uptake rate was only 20% of that in the woodland in an adjacent area that had been uncultivated for the same period but kept as rough grassland by the annual removal of trees and shrubs and, since 1960, grazed during the summer by sheep. It is suggested that the continuous input of urea through animal excreta was mainly responsible for this difference. Another undisturbed woodland area with an acidic soil reaction (pH 4.1) did not oxidize any CH4.
[RH] Shortened link that was breaking page format.
RedBaron @18:
You have this backwards. The current megafaunal biomass is approximately 7 times that prior to human induced mass extinctions. The biomass of human livestock alone is 4.5 times that of all megafaunal biomass prior to human induced mass extinctions. See Barnosky 2008:
So, quite aside from the fact that not all grazers produce methane in the quantities of cattle, the amount of cattle now far exceeds the quantity of wild grazers they have displaced. Even the estimated 60 million American Bison pre-1800 have been replaced by 90 million cattle in the US alone, a further 12 million in Canada, (plus 500,000 Bison).
Tom,
Lets say you are right, and we have more total ruminents now. (I am not convinced of that but not worth arguing) That is a good thing if the animals are part of the grazer/grassland biome which is a net sink. The only reason it is a problem is due to removing those animals from the biome and putting them in CAFOs instead.
What we have now is a loss of ecosystems services over a vast area of land, because a cropfield most certainly does not function the same in regards to methane oxidation as a grazer/grassland biome. In fact at least one of the sources I quoted above found it functioned at only 20% the effectiveness of an adjacent grazed area. Think of all the cropland in the world producing grains and operating at only 20% efficiency in methane oxidation!
It is not unusual for agriculture to actually produce more food than a natural ecosystem. It is managed after all. But grasslands can be managed as well. There is no need to overproduce grains, far more than we could possibly eat, and then feed them to livestock, just so we can count that productivity twice. More important to AGW mitigation is to bypass all the "middle steps" and go straight from grassland to animal so we don't loose that ecosystem service.
RedBaron @20, I was merely pointing out that your claim about megafauna was false. I am not interested in rediscussing your other views, which have already been sufficiently refuted here.
Rebuttal of Rudiman is not exactly the same as actually taking a minority view of methanotrophs and claiming you refuted their function in controlling atmospheric gasses is it? Do you honestly believe anything you posted on that other thread refutes the work of Dr Ralf Conrad? Really? Extraordinary claims require extraordinary evidences. You provided none at all regarding methanotrophs. Your post of biomass that includes human biomass has little relevance regarding herbivores existing on a grassland/grazer biome either. Except as the prairie was plowed at about the same time as the industrial revolution, the curve could support either hypothesis.
RedBaron @22, what I clearly refuted was the claim that largescale return to intensive grazing will solve greenhouse gas problems into the future. Your claims about methane above are of the same nature. However, seeing you challenge the point:
1) You say "Extraordinary claims require extraordinary evidences", but provide not significant evidence. A list of unexplained links coupled to out of context quotes is not evidence. It is a smokescreeen. You need to explain what the papers (and blogs) show in detail and how they support your views.
2) You claim @18 that "According to the following studies those biomes actually reduce atmospheric methane" but Conrad et al (1996) studies only the trace gas emissions and sinks of soils. It is not a study of the entire biome, which of course includes the animals on them. Ergo, your extraordinary evidence consists of exageration. (This is not suprise as the papers are available to the IPCC, who take them into account. It follows that if you come to signficantly different conclusions to the IPCC from those papers, you are likely misreading or exagerating them.)
3) In fact, with regard to upland soild Conrad et al cite Holmes et al (1999), who conclude that globally, acidic soils are a net sink of 20-60 million tonnes of CH4 annually. Even at 60 million tonnes per annum, that is substantially less than CH4 emissions from cattle. That, of itself, makes it unlikely that the complete biome (upland grasslands plus grazers) is a net CH4 sink, and it is, it is certainly a small one. More importantly it means that even without grazers, the loss of grassland cannot account for even a very small fraction of total emissions anthropogenic emissions (just short of 500 million tonnes per annum in 2012).
These arguments just echo in form the arguments with regard to CO2 that refuted your positions on that gas. There is not substantial difference between the cases, and no reason to consider your claims of any more interest with regard CH4 than there was with CO2 unless you actually unpack all the numbers, including peer reviewed estimates of loss upland grassland to horticulture, CH4 sink per km^2 for grassland relative to CH4 emissions per km^2 for grazing cattle at expected herd densities etc
Tom,
Lets try and discuss the relevant parts only please.
I happen to agree with this part of your statement, right up until your conclusion:
How is it that we can look at exactly the same evidence and draw the exact opposite conclusion? I agree that now the cropland that supports animal husbandry is not nearly large enough to offset emission. It functions quite differently with regards to methanotroph activity. So why would citing the sink is only 20-60 million tonnes of CH4 annually somehow refute what I am saying? That's actually my point. Increase that sink by 5x or more and you get 100-300 tonnes of CH4 annually. that actually puts a big dent in the just short of 500 million tonnes per annum total anthropogenic emissions from all sources. By the time you add in abiotic oxidation and improvements in rice production, it appears as if we might be able to remove the "mystery CH4" in the OP, as well as actually reducing atmospheric CH4.
RedBaron @24, yes, all we need to do is increase the upland soils (dark blue on the map below) so that they cover 5 times their current area:
(Source)
How could there be any problem with that at all? /sarc
And please note that in the US, nearly all cropland is in lowland (ie, net methanogenic) soils, while in the rest of the world for the most part (by area), crop intensity is very low. The exceptions are in countries with very high population densities where a loss of cropland would result in a severe risk of famine.
Tom,
No, you misunderstand. I am not talking about increasing the upland soils so that they cover 5 times their current area. I am talking about changing the agricultural practises to those that increase the effectiveness of the methane sink by 5X or more. That's why I provided those additional studies and quotes from them. Of all the CH4 sources and sinks, the biotic sink strength is the most responsive to variation in human management, and examples I showed prove that increases of 5X or more is at least possible. Although I am fully aware that one published study is not vetted enough to proclaim it is necessarily possible on all oxic cropland. That would take additional studies of course. However since a very high % of intensively managed cropland is used to produce food for livestock, and that change in management to a forage based system was the change in management showing the highest increase in methanotroph numbers and activity, it leands me to believe all that is needed in confirmation studies. The principle itself seems sound.
BTW we are also using two different definitions of "upland soils". The definition I am using (and that of the citation I provided) is land above the level where water flows (i.e., well-drained, oxic soils), to differentiate from swamp, delta or paddy soils. That means most agricultural land excepting rice production.
RedBaron @26:
From the final study to which you linked, and partly quoted by you above:
Doing the maths, the CH4 uptake in the woodland is 6 times that on the arable plot, and 5 times that in the grazed grassland. If follows that the CH4 uptake in the grazed grassland in 1.2 times that in the arable plot. In other words, switching from cropping to grazing does not result in the 400% increase in methane uptake you desire, but only a 20% increase.
Meanwhile your second last link indicates that increasing soil carbon content (which increases water retention) will increase methane production, as will increasing water content (as by irrigating improved pasture), or fertilizing. Ergo, increasing net methane oxidization in soil is at the expense of increasing CO2 emissions from LUC; while intensive pasturing which does increase soil organic content (just not enough to reverse the atmospheric effects of the industrial revolution) will increase methane generation, and therefore decrease net methane oxidation in soils.
Your linked studies do not support the idea that methane oxidation can be increased by a factor of 5 in agricultural land, let alone in all upland soils by any definition.
Tom BTW
I just found this over at Real Climate. Might be an interesting read for you The early anthropocene hypothesis an update