Methane Acceleration

A recent paper by Nisbet et al. notes that methane (CH4) in the atmosphere isn’t just increasing, it’s accelerating; the increase has gotten faster recently.

There was a time — from about 1999 until 2007 — when atmospheric methane wasn’t increasing at all. It had been, before 2000, but it remained steady for that 8-year period. But in 2007 it started rising again, as is plain to see in the following graph. What is not plain to see is that around 2014 it started rising even faster; the years 2014 through 2018 saw very rapid increase in CH4 levels.


Nisbet et al. identify the increased rate of rise in those four years, and consider the implication for the Paris climate agreement. The stated goal is to keep global temperature rise “well below 2°C.” So far, all our plans, our computer models, our strategies that have a decent chance of accomplishing that goal have relied on no increase in CH4, some even rely on decreasing CH4 in the air. The fact that it’s going the wrong way, at increasing speed, is a genuine threat to our chances of success.

I took the CH4 data from Cape Grim (shown above) and first removed the annual cycle, yielding de-seasonalized data. Then I fit two models. One is a lowess smooth, shown here in red (deseasonalized data in black):

The other is a piecewise-linear model in blue (deseasonalized data in black, the thin red lines show when the slope changes occur):

The slope change times were chosen to give the best fit. According to this model, the most recent slope change happened right around 2014. Especially useful is the fact that both models enable me to estimate how fast CH4 is changing. In fact the piecewise-linear model estimates the average rate of increase during each linear “piece.”

What do these models say about the rate of increase? This:

Red shows the estimated growth rate of CH4 from the lowess-smooth model, blue the estimate from the piecewise-linear model. Both make clear the increase in the growth rate that happened about 2014.

What’s the source of the increase? One clue is the 13C/12C isotopic ratio, which has been decreasing lately. Fossil-fuel CH4 is usually “heavier” (i.e. higher in 13C than other sources) while microbial CH4 (from biological sources) is lighter. But we don’t have enough geographical coverage of 13C/12C isotopic ratio to nail down the source of that change. Nonetheless, Nisbet et al. speculate on several possible causes of the acceleration of atmospheric methane.

One possibility is an increase in microbial CH4, from wetlands and cattle, which would account for the atmosphere’s decreasing 13C/12C ratio. Another possibility is a change of the fossil-fuel mix toward natural gas, which has a lighter 13C/12C ratio than coal. Yet another possibility is a decrease in the atmosphere’s ability to break down CH4, which would likewise account for both the increased amount, and the decreasing 13C/12C ratio. The most worrisome possibility — although not yet a likelihood thank goodness — is a dramatic increase in microbial CH4 from permafrost melt or other feedback sources.

And of course we cannot rule out the possibility of “all of the above.”

It’s imperative that we cease the increase of atmospheric methane, if at all possible.


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40 responses to “Methane Acceleration

  1. How implausible is RCP 8.5? Not implausible enough, here comes methane. Crummy news for hopes of slowing the sixth extinction. Thanks for the thorough analysis and review of the methane issue.

  2. Very nice.

    How was the slope off the lowess-smooth obtained? No criticism … I want to be able to do it myself. Is it a package which let’s you specify the deriv like some splining “predict” functions do? Or did you just calculate a numerical derivative from the values of the lowess?

  3. Given that methane is not decreasing does it make sense to use its instantaneous GWP value of ~120x instead of the 100 year 12x value? I’ve never found a good answer but the implications are huge.

    The not having clarity on where new methane is coming from needs to be resolved in a hurry. I hope methaneSAT can help to this end thought it appears to be targeted towards industry emissions. At least we might be able to infer the other half. But I imagine emissions from clathrate gasification will be isotopically indistinguishable from those due to fossil fuel production.

    The rise of methane is additionally problematic as its absorption bands are different than those of CO2. In other words its never going to be in the “shadow” of CO2 and since it is measured in ppb instead of ppm it would probably take getting to an uninhabitable Earth state before there is any meaningful diminishment in the warming potential of added methane due to absorption band saturation.

    Lastly, it’s very interesting how methane emissions appear to switch to new rates (plateau or increase) in a very distinct phase-transition sort of way.

    • BB, This is something I’ve wondered for some time. I seem to recall that a Realclimate answer was given to this, a few years ago, and it indicated the situation wasn’t as simple as might be thought. However, like you, I think that an increasing level of methane (and even a non-decreasing level) would seem to suggest that, at least in the short term, a much higher C02 equivalent factor should be applied. Even with the lower figure, we’re over 490 ppm CO2e. A higher factor would likely have us quite close to the doubling of CO2e that would lead to over 2C of warming quite quickly.

      I’d love to hear other thoughts on this, though.

    • bostonblorp,
      I don’t see where you get your GWP values from.
      The usual GWP values for Methane are ~30x for 100 years and ~80x for 20 years. “Instantaleous” values are not usually considered but if you calculate them they are not greatly different to the 20-year value. [If you consider the last 5-years’ climate forcings – NOAA’s AGGI readily provide such numbers – and the last 5-years’ atmospheric burdens of CO2 & CH4 – NOAA ESRL MLO data – the increase in CO2 forcing is 10x the CH4 forcing from an increase of atmospheric burden of 270x by volume which is 745x by weight. So the instantaleous GWP for CH4 would be 745/10 = ~75.]
      I wonder if you have picked up on some source that has reasoned that, if the 100year value is so much lower than the 20year value, then the instantaneous value will be thus much greater again. This will not be true if CH4 levels do not reduce quickly over the first few years, as they do for CO2. [Note, there is a little confusion spread with CO2 draw-down times due to the concept of ‘Atmospheric Fraction’ which is the ratio of Emissions/Atmospheric-Increase. The Af is not some instantaneous effect but results from the draw-down of percentages of CO2 from years-gone-by as well as current emissions. Thus GWP should not be calculated using Af.]

      As for whether GWP is appropriate for such considerations within AGW, if we had no control over rising CH4 levels, it may be so. But the vast majority of today’s CH4 emissions are directly due to humanity, as are the CO2 emissions which causes climate forcing that are 10x bigger and far far more long-lasting than CH4.
      Only when CO2 emissions are reduced to the point where the peak atmospheric CO2 levels are in sight will GWP(20y) become an appropriate measure. Until that time, GWP(100y) or certainly something longer than 20 years will remain the appropriate measure.

    • “…since it is measured in ppb instead of ppm…”

      I don’t claim any great expertise on this, but I do recall physicist Ray Pierrehumbert explaining on RealClimate that the larger effect of methane emissions is due, not to an inherent greater strength of its greenhouse properties relative to the CO2 molecule, but precisely to the fact that it is measured in ppb, not ppm. Thus, a given quantity emitted has a greater relative effect on CH4 abundance than the same quantity would on CO2 abundance. And GH warming is a log function, defined on doublings of concentration.

      On a “molecule-for-molecule” basis (I understood him to say), the CO2 molecule is actually the more powerful of the two.

      Yes, this is a little-noted facet of the issue, and yes, people often speak as if the stronger GWP of methane is true on a “molecule-for-molecule” basis, without reference to extant atmospheric concentrations. But Dr. Pierrehumbert ought to know; he wrote, if not *the* book, at least a noted book on the topic, and has been called “one of the field’s broadest thinkers.”

      • @Doc Snow,

        Yeah, comparative forcings depend a lot on definitions of units: Do you want molar forcings (if you will)? Or forcings per unit mass? As noted elsewhere here, it also depends upon where in the spectrum the absorption happens … Which line. And you are correct. In “Short-lived climate pollution“, Professor Pierrehumbert remarks:

        Gases present in low concentrations tend to have higher radiative efficiency than gases like CO2 that start off with relatively high concentration (Supplemental Material). CH4 has a much higher radiative efficiency than CO2 simply because its concentration is low, and not because its molecular structure makes it an intrinsically better greenhouse gas (Pierrehumbert 2010, section 4.5.4). The use of linearized radiative efficiency always overestimates the radiative forcing when concentration changes are large enough. Among the well-mixed greenhouse gases, only CO2, CH4, and N2O exhibit significant nonlinearity over the range of anthropogenic concentration changes. (See Supplemental Material.)

        There’s a lot of analysis in the primary paper and in its Supplementary information. Professor Pierrehumbert expresses his well-versed opinion that “global warming potential” should be retired, because it is useless.

        In the end, he says:

        We are not arguing that SLCP mitigation is irrelevant, only that it should take a backseat to CO2 mitigation at least until CO2 emission is on a secure path to trend to zero. The first order of business is to get the world on a pathway to a CO2 emissions scenario like RCP4.5 or lower, whereas at present we appear to be headed toward the nightmare of RCP8.5, if not worse. If we fail to avoid that peril, nothing we do about SLCP will matter much.

        I take away that burning CH4 is not a good idea because it produces CO2. But worrying about CH4 leaks might be a distraction, as long as outright use is being curtailed.

      • from Pierrehumbert: “The first order of business is to get the world on a pathway to a CO2 emissions scenario like RCP4.5 or lower, whereas at present we appear to be headed toward the nightmare of RCP8.5, if not worse.”

        Methane is on the rise, but it’s adding insult to the injury already underway from CO2. As it happens, I think that many of the solutions/responses to the CO2 will also reduce the methane load. It’s interesting to watch methane and see the science about its increase, but in terms of survival, the first order of business is to get the world on a pathway to a lower CO2 emission scenario. If we don’t do that, methane will kick us after CO2 knocks us down. I see no sign that we are making significant progress on CO2. That’s too bad. We have known the risks of CO2 emissions for quite a while. Just can’t kick the habit.

        Cheers,

        Mike

      • Thanks for elaborating, ecoquant!

      • Doc Snow,
        The Pierrehumbert book is more an educational book than a reference book. Even so, it crams in a fair amount into its 500 pages.
        The point that a GHG’s power is dependent on circumstances is set out in 4.5.4 Greenhouse effect of CO2 vs CH4.

        “The common statement that methane is, molecule for molecule, a better greenhouse gas than CO2 is true only for situations like the present where methane is present in far lower concentrations than CO2. In this situation, the greater power of a molecule of CH4 to reduce the OLR results simply from the fact that the greenhouse effect of both CH4 and CO2 are approximately logarithmic in concentration. … If methane were the most abundant long-lived greenhouse gas in our atmosphere, and CO2 were present only in very small concentrations, we would say instead that CO2 is, molecule for molecule, the better greenhouse gas.”

        And Fig 4.35 demonstrates that when properly compared, CO2 is the stronger GHG, a point the text also makes plain –

        “For example, with an absorbed solar radiation of 300W/m2 the surface temperature can be sustained with either 464ppmv of CO2 or 35,600ppmv of CH4. … Methane is, intrinsically speaking, a considerably worse greenhouse gas than CO2.”

        Although, just in case this is misinterpreted, the situation facing humanity is as set out by smallbluemike above. CO2 is the big bad guy, far far more more long-lived than methane (anybody care to calculate the GWP of methane for 10,000 years?) and, with today’s emissions, enough CO2 is being emitted to provide 10x the increase in climate forcing provided by the emissions of short-lived methane.

      • @Al Rodger,

        Another book I recently discovered (and bought!) is Professor Mark Z Jacobson‘s second edition of Fundamentals of Atmospheric Modeling, and he addresses these in his 9.4.1.2 and 9.4.1.3.

        The other book is the second edition of G. W. Petty, A First Course in Atmospheric Radiation, 2006. Petty devotes his Chapter 9 to the detailed physical chemistry of absorption by atmospheric gases.

      • Interesting. I’ve been under the misapprehension that methane was more potent molecule for for molecule. What I can’t quite seem to reconcile is the fact that CH4 is 1800ppb = 1.8ppm so a fraction of the concentration of CO2, but on forcing graphs like this:

        https://en.wikipedia.org/wiki/Radiative_forcing#/media/File:Radiative-forcings.svg

        methane accounts for about a quarter as much forcing as CO2. If CH4 is not more potent, what am I missing?

        [Response: CH4 is more potent “per molecule” only when CO2 concentration high (like now) but CH4 concentration is low (like now). So, in the present situation, CH4 really is more potent per-molecule. But if the concentrations of CO2 and CH4 were equal, it wouldn’t be.]

      • “I see no sign that we are making significant progress on CO2…”

        I suppose it depends on what you consider ‘significant.’ Certainly, the most essential question is, what is the atmospheric concentration doing? That’s the ultimate ‘top down’ question, and in that respect I’d have to agree with you that things don’t look good.

        The ‘bottom up’ perspective is a bit better, though: The structural reform of energy production is well underway, IMO, with coal on the ropes and natgas next. Transportation is starting to change, too, and agriculture will be following. I have to believe, logically, that those efforts will start to show in the concentration numbers.

        A good step here in the US would be to get rid of the Trump maladministration, which, I’m convinced has as a primary mission the promotion of fossil fuel. (Yeah, a shocker, right?) Let’s work toward that, by whatever means make sense in our respective situations.

        And let’s support the youth climate strikes later this month: the week of action kicks off September 20. There’s an event near you, and even if you do no more than show up (and maybe live stream your experience!) you’re making a strong statement. Don’t let the media ignore us!

        https://strikewithus.org/

      • @Doc Snow, @smallbluemike,

        I need to keep reminding people, especially friend environmentalists, that the situation isn’t hopeless, that we can always do something, even if the price of doing something (per Stocker) is going up by the year. Yes, there are all kinds of positive things developing, Doc, and I’m enthusiastic about them. The hardest thing to change, however, is I think cultural, and that is a basic norm of consumption, something which is actually pretty recent: No older than the 1960s. This is the throw-away-rather-than-repair mentality, or the idea that an individual’s time is worth more than X amount of stuff. It’s also the unfortunate feature that, because of these values, there is overproduction built into the economy, and that means, like a housing and real estate industry which is booming in part because of people flipping houses on spec, these jobs and profits are based upon thin air.

        These are so fundamental it’s even hard to get environmental progressives to take these seriously. People protest pipelines but they won’t get behind a movement to discourage natural gas heating for new construction. People advocate plastic bag bans and use of plastic when paper is arguably worse in upstream GHG emissions, plastic is lighter than glass (requiring less fuels burnt for transport, etc), and plastic, once broken down, is consumed by microbes in oceans and apparently to some degree even in landfills. And I find it incredible that people want somehow to do away with corporations as a construct. Sure, some have abused with propaganda and misinformation. But others have championed doing good things: Unilever, Virgin, and, now, even a declaration that shareholder interests aren’t necessarily top-of-the-heap in corporation management.

        Still, we are making progress, even if it’s not fast enough. It’s all about the engineering job ahead, a big one, but still do-able.

      • wrt to significant progress being made on addressing ghg emissions and accumulations: I think it’s kind of silly to suggest that significant progress is being made on a post about the acceleration of methane accumulation in the atmosphere.

        CO2 is the ghg we should all be most concerned about. It is the big dog in the sky (and oceans) and will be the primary focus of concern about ghg for our lifetimes. How are we doing on CO2? https://tamino.wordpress.com/2018/01/20/is-co2-still-accelerating/

        Are we making progress on reducing ghg emissions and accumulation? Yes. There is a lot of great work being done both to push conversion to cleaner energy and to reduce consumption. Is it possible to live with zero impact? I think there is a movie/documentary about that. Lots of people are engaged in activism to address the ghg problem. Is that stuff progress? Yes, it is. Can you call that progress significant when both CO2 and CH4 accumulation/emissions have not slowed and appear to be continuing to accelerate? I cannot. If you can, go with God. I am happy for you.

        I think significant progress can be claimed when the rate of increase begins to fall. I am not talking about neutral emissions, I am simply talking about seeing several years in a row where the CO2 rate of increase is flat and then begins to drop. So, given a background rate of 2.4 to 2.5 ppm for CO2 increase at this time, I need to see several years in a row where that rate remains steady, then a couple of years where we see that rate of annual increase begin to drop. The actual rate has to be teased out of annual data that has ENSO fluctuations etc. but it’s like porn, I think I will know it when I see it. I haven’t seen it yet. That’s my bar for significant progress.

        We can go over the climate cliff still trying to figure out how to avoid the crash. Any progress, however small, on that project is great. Activism is great. Do it, if you have the energy. I have done that for a long time and I am now done with that. I want to spend my retirement time helping my kids and grandkids with any projects that can use my skills. I don’t think my activist skills and time have produced much of value for them. I love Greta and I think it is time to pass the torch to a younger generation of activists. I wish them great luck. I hope they will turn out to be much more effective than I have been on that project.

        Warm regards

        Mike

      • I don’t have a lot of faith in the emissions estimates, when the atmospheric concentration seems to be going up just as quickly, if not more quickly, than previously.

        However, when looking at the totality of human degradation of our environment, I don’t even see so-called renewable energy and transport developments as encouraging. Collectively, we seem determined to continue with life as we know it. Just changing the energy systems (if it were possible to power our civilisation with so-called renewables) is not going to alter the continued harm we do to the planet in other ways. Most of the damage we’ve done so far has been done before climate change became a significant factor.

        The climate crisis has become, and will be, the most significant issue going forward but even if we manage to get to a net zero emissions situation (and I have almost no expectation of that), we will still continue to live unsustainably. Sadly, that is just what humans do (maybe all species do that but numbers are usually kept in check).

      • @Mike Roberts,

        Would your hopes be raised at all by realizing that zero GHG emissions logically implies (a) consumption as a driver of economies is stopped, and (b) population on the planet is stable?

        “(a)” is true because a substantial amount of the emissions OECD countries produce are because of the never ending cycle of buying more stuff, and building ever bigger suburbs and homes. Can’t get to zero GHG emissions unless these stop.

        “(b)” is true because a substantial amount of GHG emissions comes from agriculture, not fossil fuel power, but just agriculture, and not (principally) beef, but much of everything else. Accordingly, this will need to be zeroed out in some way, but that’s useless if the world is getting more hungry.

        I think zero emissions of greenhouse gases is a pretty darn good sustainability target, because reaching it implies a bunch of other things.

      • Tony McLoed,
        OK, so bear with me a little bit while I try to explain the relative effectiveness of CH4 and CO2

        First, a spectral absorption line isn’t really a “line” at all. It looks like a Cauchy distribution–a high peak, but with wide tails on the side. There is enough CO2 in the atmosphere that really all the infrared light within the central peak gets absorbed before it leaves the atmosphere. But increasing CO2 still further does absorb more IR in the tails of the CO2 spectral lines. That’s why CO2 only raises the temperature logarithmically as it increases in concentration.

        For CH4, you are still working with the center of the absorption line, so the effect is essentially linear in CH4 concentration. Make sense?

      • Mike R–

        …even if we manage to get to a net zero emissions situation (and I have almost no expectation of that), we will still continue to live unsustainably.

        Mike, this is a statement of ‘negative faith’–an assessment for which there is precisely zero evidence.

        Humanity lived for hundreds of millennia in (relatively) sustainable ways, so there’s clearly nothing inherent in our makeup that mandates unsustainability. And in fact, human life was relatively sustainable until quite recently in historical terms. Our current patterns are massively unsustainable, but also transient. That will only become more obvious as we continue to observe what the results of our choice are. Pain is motivating for us, and there’s lots of pain coming, so change we will. Your inability to imagine how–or mine, for that matter–is beside the point.

      • Doc,

        an assessment for which there is precisely zero evidence

        Not zero evidence, surely? Homo sapiens is a species and so has a characteristic behaviour for that species. Up to now, the species has lived unsustainably, though its previously small population might have resulted in a largely sustainable behaviour if it moved when depleting resources, allowing those resources to recover. With more than 7.5 billion people living largely in the same place with increasingly hard borders between regions, the unsustainable behaviour can’t be modified.

        Of course, it’s impossible to state explicitly that humans will not somehow alter their collective behaviour in a sustainable manner but, if you claim that it is possible, you will yourself have to examine the evidence closely, if there is any evidence to such a claim. (By the way, “relatively sustainable” is another way of saying “unsustainable”). In this, I’m referring to humans trying to continue living in societies which resemble the current ones. Yes, as ecosystems break down and the environment deteriorates, humans will increasingly be forced into significant changes which may tend towards a less destructive existence but they will not do this willingly.

        Individuals may appear to move towards sustainability but our societies show no signs of doing that. So you may well be able to do better, as I can (and as I intend to do) but, collectively, there is not much evidence of such a change.

      • @Mike Roberts, @Doc Snow,

        I’m not aware of any direct evidence, as you say, Mike. There might be some in the annals of archaeology or anthropology but I do not know.

        However, if it’s considered how people apparently operate, as summarized in the Prospect Theory of Kahneman and Twersky, being optimists in the face of losses, how they are behaving is exactly as would be expected. Unfortunately, as Kahneman himself has said, there’s only pessimism to be had with this realization.

      • ecoquant,

        True, one cannot be optimistic at the realisation that humans will always act like humans. But, to my mind, it is pointless pretending that humans could start acting like a different species, if only we wish hard enough that it were so.

      • Mike Roberts–

        Homo sapiens is a species and so has a characteristic behaviour for that species…

        Which is to adapt intelligently to an incredibly wide variety of circumstances, environments and challenges.*

        (Had a slightly lengthier response, which didn’t come through for some reason. But I’ll leave it at that, as this sub-thread is already quite unwieldy.)

        *I know, it’s often a lot tougher than that summary statement makes it sound. But that’s generally been the ‘bottom line’ reality, which is why we’re found from the Amazon to the Canadian archipelago.

      • Doc,

        Which is to adapt intelligently to an incredibly wide variety of circumstances, environments and challenges.

        As you mentioned, that is a simplification but I agree that a species’ behaviour will change depending on its environment (in the broadest sense) but the tendency will be not to change unless forced by the conditions. I see little evidence that humans are taking the deteriorating ecosystem and climate conditions into consideration, though some are seeing the “green” drive as a money spinner and getting the benefit. The lobbyists for so-called green energy systems nearly always highlight the economic benefits of the move because that is what people what to hear. If renewable infrastructure allows a continuation of business as usual for some period of time, humans will continue to damage our natural life support systems. Of course, that’s an opinion but one based on observation.

  4. You have to understand that RPC stands for representative concentration pathway, and the 8.5 refers to the watts/m^2 forcing. The RCP’s define the changes in the different greenhouse gases as well as aerosols through time, so that different models will have the same forcings.The differences among models are then from model dynamics, not the way that 8.5 W/m2 was achieved.

    The largest uncertainty in climate change is how people will respond to the threat.

  5. so, Mitch. On the RCP 8.5 thing: I am not sure I understand the point you are making. Doesn’t unanticipated methane emissions just push up the watts per sq meter forcing and move us closer to the RCP 8.5 pathway?

    As to the uncertainty about how people will respond, past behavior is generally a good predictor of future behavior. That being the case, I think it is pretty certain that people will respond ineffectively to the threat and that ghg emissions will not be suddenly curtailed. You can see some of this with the US policy regarding methane release from fossil fuel industry: https://arstechnica.com/tech-policy/2019/08/trump-admin-announces-plan-to-kill-2016-methane-emissions-limits/

    I don’t want be a buzzkill, but I am not confident that people will respond at scale to the threat, they may only react to the threat. Reactions might look like border walls or other means to reduce movement of human being who have become climate refugees. Reactions might also look like political turns to authoritarian/nativist leaders and a related impulse to react with military force based on nation state interest and bias.

    The US has a leader who is walking us away from nuclear weapon treaties and who has apparently wondered about using nuclear bombs against hurricanes. This is not a complete outlier, Brazil’s leader, appears to be insufficiently concerned about the global climate function of the Amazon forest.

    Maybe I am wrong about all that.

    Mike

    • This. Exactly this:

      I don’t want be a buzzkill, but I am not confident that people will respond at scale to the threat, they may only react to the threat. Reactions might look like border walls or other means to reduce movement of human being who have become climate refugees. Reactions might also look like political turns to authoritarian/nativist leaders and a related impulse to react with military force based on nation state interest and bias.

    • These aren’t really unanticipated methane emissions though. Methane concentration growth around now in the headline CMIP5 RCP8.5 scenario is just under 20ppb/year compared to observed just under 10ppb/year. In the newer baseline SSP scenarios all feature similar or greater methane growth compared to observed between 2010 and 2020. What we’re seeing with methane is what we expected to see with methane.

      A number of recent articles have talked about the mystery of methane growth, but to me that’s looking at things the wrong way. Most bottom-up emissions estimates suggest methane emissions have been rising steadily over the past few decades. That the atmospheric methane concentration “resumed growth” after several years of none is simply a return to the expected trend. The real mystery is not the growth, but the period without growth despite apparently increasing emissions.

  6. Are there, additional to the C-13/C-12 ratio, also measurements of C-14/C-12 ratio? Would be very helpful to identify the source(s).

    • @Uli,

      Saunois, et al, “Variability and quasi-decadal changes in the methane budget over the period 2000–2012”, Atmos. Chem. Phys. Dis-
      cuss., https://doi.org/10.5194/acp-2017-296, in review, 2017 is quoted as a source on this by Hansen, Sato, et al, “Young people’s burden: requirement of negative CO2 emissions“, Earth Syst. Dynam., 8, 577–616, 2017 [https://doi.org/10.5194/esd-8-577-2017].

      Quoting Hansen, et al, from their appendix A8:

      Carbon isotopes provide a valuable constraint (Saunois et al., 2016) that aids analysis of which CH4 sources16 contribute to the CH4 growth resurgence in the past decade (Fig. 7). Schaefer et al. (2016) conclude that the growth was primarily biogenic, thus not fossil fuel, and located outside the tropics, most likely ruminants and rice agriculture. Such an increasing biogenic source is consistent with effects of increasing population and dietary changes (Tilman and Clark, 2014). Nisbet et al. (2016) concur with Schaefer et al. (2016) that the CH4 growth is from biogenic sources, but from the latitudinal distribution of growth they conclude that tropical wetlands17 have been an important contributor to the CH4 increase. Their conclusion that increasing tropical precipitation and temperature may be major factors driving CH4 growth suggests the possibility that the slow climate-methane amplifying feedback might already be significant. There is also concern that global warming will lead to a massive increase in CH4 emissions from methane hydrates and permafrost (O’Connor et al., 2010), but as yet there is little evidence for a substantial increase in emissions from hydrates or permafrost either now or over the last 1 000 000 years (Berchetet al., 2016; Warwick et al., 2016; Quiquet et al., 2015).

      Schwietzke et al. (2016) use isotopic constraints to show that the fossil fuel contribution to atmospheric CH4 is larger than previously believed, but total fossil fuel CH4 emissions are not increasing. This conclusion is consistent with the above studies, and it does not contradict evidence of increased fossil fuel CH4 emissions at specific locations (Turner et al., 2016). A recent inverse model study, however, contradicts the satellite studies and finds no evidence for increased US emissions (Bruhwiler et al., 2017). The recent consortium study of global CH4 emissions finds with top-down studies that the recent increase is likely due to biogenic (natural and human sources) sources in the tropics, but it is difficult to attribute the magnitude of the rise to tropical wetlands alone (Saunois et al., 2017).

      I just happened to have re-read this recently.

  7. @ecoquant: Thank!. But although these paper are interesting for the methane budget, they do not mention the radioactive C-14 isotope at all. Only C-13.

    Are there atmospheric methane C-14 measurements at all?

    I know it must be difficult due to the very small amount, but one can date very small samples by radiocarbon method now, so it should be possible.

  8. Found methane C-14 paper, with further references to others:
    https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018EF001064

  9. “a substantial amount of GHG emissions comes from agriculture, not fossil fuel power, but just agriculture”

    Nope. Not even close.

    Agriculture sector in USA is 9% of total emissions. Globally, total agriculture is between 13% -19%. Fossil fuels are multiples higher than agriculture.

    2hxc4mr

    63zl12

    • @Gingerbaker, I believe the figure referenced is about the change in CH4 not the absolute amount. There were references given earlier, I think, probably by me, that’s looked at this.

      Fletcher, Schaefer, “Rising methane: A new climate challenge”, Science, 2019.

      Turner, Frankenberg, Kort, “Interpreting contemporary trends in
      atmospheric methane”, PNAS, 2019.

      Lan, et al, “Long‐term measurements show little evidence for large increases in total U.S. methane emissions over the past decade”, GRL, 2019.

  10. As I understand it, estimates of the contributions of ag vary widely. I think that there is still no generally-accepted solution to the definitional and accounting issues involved. For example, take a diesel truck transporting cows to, or from, a feedlot: are those emissions due to ag, or to transportation? You can make the call either way, but the problem is that analysts have made the call *both* ways. And similarly in a fair few other aspects of the problem.

    • I have references someplace but setting fossil fuel supports aside like your trucks the contributions of ag to GHGs are pretty substantial. And it’s not all about beef. Rice and soy do a lot. And the sleeper is N2O, which comes from fertilizer runoff, is a centennial GHG, and for which there is no direct capture remedy. There are ancillary problems, such as ag at higher temps and CO2 needing more N and some fast afforestation proposals involve fertilizing new woods.

      • As I have been given to understand the problem, the most fundamental issue is that whereas we could be building soil, we are very often essentially strip mining it–which means that we’re creating soil-to-atmosphere carbon fluxes instead of the reverse. Some folks claim the numbers involved, if we largely reversed that, could be very significant. But my reading on this has barely scratched the surface so far.