The importance of reducing methane emissions

Since I’ve written quite a lot about methane, I thought I would promote Realclimate’s definitive CO2/CH4 comparison post. It highlights that the large relative rise in atmospheric methane concentrations means that methane has made a significant contribution to modern warming. It also points out that when you also include the non-direct effects of increases in methane, it is responsible for about 0.60C of the warming since 1750, compared to about 1oC for CO2.

The Realclimate post also discusses the difference between CO2, which is long-lived and hence a stock pollutant, and methane, which is short-lived and hence a flow pollutant. This means that if we stop emitting CO2, the CO2-driven warming will stabilise, while if we stop emitting methane, the methane-driven warming will mostly reverse. One should be careful, though, of assuming that if we stopped emitting methane we’d cool by ~0.6oC, because the cooling impact of other short-lived forces is also comparable to the warming impact of increases in methane. Hence, our committed warming is roughly comparable to the warming due to CO2.

I have for a long time shared the concerns expressed in this Realclimate post; we should be careful of thinking that reducing methane emissions can buy us time, because if we do so at the expense of reductions in CO2 emission we’d be committing future generations to more warming than if we’d prioritised CO2 emission reductions.

However, I have come to realise that it is also important to also aim for reductions in methane emissions. One simple reason is that if we do continue to increase methane emissions, then it could contribute significantly to future warming. Given that methane is relatively short-lived in the atmosphere, we could aim to stabilise methane emissions, which would lead to little further methane-driven warming.

However, this would then mean that we would essentially be sustaining methane’s contribution to the warming since 1750 (~0.6oC), making it extremely difficult to achieve some of our stated targets, for example limiting warming to < 2oC. Also, since reductions in CO2 emissions would probably also result in reductions in the emissions of the short-lived species that have a cooling effect, we could actually end up with a period of quite rapid warming.

So, if we do start reducing methane emissions, we would reverse some of the methane-driven warming, counter-acting the impact of reductions in the short-lived pollutants that have a cooling influence. It would also probably give us a better change of limiting warming without overshoots. As the Realclimate post highlights, there are also other benefits (air pollution, crop yields, public health). However, since long-term warming depends mostly on how much CO2 we emit, it is key to do this in conjuncion with reductions in CO2 emissions, rather than instead of reductions in CO2 emissions.

Links:

..and Then There’s Physics – methane, post I’ve written about methane.
The definitive CO2/CH4 comparison post – recent Realclimate post comparing CO2 and methane.
Losing time, not buying time – 2010 Realclimate post warning against thinking that methane emissions could buy us time.

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13 Responses to The importance of reducing methane emissions

  1. Thank you. Gavin at RC said: “Whatever way you slice this it implies that CH4 reductions have an outsize effect on climate, as well as an undeniably positive impact on air pollution, crop yields and public health (mainly through ozone reductions). It is therefore not a complicated decision to pursue methane reductions, taking care not to assume that doing so gets you off the hook for reducing CO2, whatever the EPA says. ”
    Exactly right.
    Cheers
    Mike

  2. Ben McMillan says:

    Looks like a pretty comprehensive discussion on methane, both here and over at realclimate.

  3. Mark Brinkley says:

    Help me with my chemistry here….what happens to methane after it’s been in the atmosphere for some time. Doesn’t it degrade into CO2? If so, how is this accounted for in the calcs.

  4. Mark,
    Yes, methane decays to CO2 and water. If the methane was biogenic (for example, from agriculture) then the carbon was probably already part of the fast carbon cycle (oceans, artmosphere, biosphere) so isn’t really a new carbon and hence this doesn’t need to really be accounted for [Edit: I should have made clearer that I mean that the CO2 that the methane decays into doesn’t need to be accounted for since it was already part of the carbon cycle]. If the methane is fossil methane, then this is the addition of a carbon that has been sequestered for a long time and does essentially add a “new” carbon and will contribute to long-term warm.

    However, the amount is relatively low. Fossil methane emissions are something like 0.1 GtC per year (happy to be corrected if this is wrong) while anthropogenic CO2 emissions are just over 10 GtC per year (about 100 times more carbon is directly emitted as CO2 than in fossil methane). Hence, once the methane has decayed to CO2 and water, the additional amount of CO2 coming from the decay of methane is relatively low when compared to the amount we’re directly emitting as CO2.

  5. I should probably add, though, that even though the amount of carbon emitted through fossil methane is small compared to how much we emit directly as CO2, that fossil methane is adding new carbon to the atmosphere is certainly another reason for reducing these emissions.

  6. Russell says:

    Since Direct Air Capture of CO2 is already available on a retail basis from Climeworks new ORCA plant in Iceland:
    https://vvattsupwiththat.blogspot.com/2021/09/great-moments-in-carbon-capture.html

    up-market carbon offset entrepreneurs should next tout capturing CH4 directly from the air, to allow the escalation of virtue signaling by those whose footprint reduction portfolios already contain carbon sequestered at painful expense , like Air Vodka synthesized rom flue gas in Brooklyn, or the Certified Vegan DAC synthetic diamonds advertised in Vogue by Aether Inc.:
    https://vvattsupwiththat.blogspot.com/2021/06/virtue-is-35000000000-ton-carbon-tax-on.html

    The astromomical energy cost of capturing methane directly from the air should not deter true environmentalists from underwriting CH4 capture plants, as DAC carbon synthetic diamonds made by chemical vapor deposition already sell for ~ $7,000 a carat, or $35,000,000,000 a Tonne.

    Even at its 1.8 ppmv ambient concentration, direct CH4 capture could undercut Vegan Diamond sequestration by orders of magnitude. Its profitability could be doubled, and climate justice promoted, by siting plants in neighborhoods near oil refineries, where they could simultaneously save enegy and mitigate air quality by sucking asthma-inducing CH4 from the hydrocarbon-rich air.

  7. Mark Brinkley says:

    Ken,

    Thanks very much for such an erudite explanation.

    However, it brings forth a supplementary question. Why exactly is methane so high in GWP? What is the chemistry driving its effect on the atmosphere? (The same question might well be asked of NO2?)

    Regards,

    Mark

  8. Mark,
    The reason it’s so high is essentially because the radiative response to a change in an atmospheric greenhouse gas concentration isn’t linear and depends on the initial concentration of the species in the atmosphere (for CO2 the response is logarithmic, while for methane it depends on the difference between the square roots of the initial and final concentration). This means that the impact of an increase in concentration decreases as the baseline concentration goes up. Our emission of CO2 has increased atmospheric CO2 concentrations by about 50%, while methane emissions have increased methane concentrations by 150%. So, even though we’ve emitted much less methane than CO2, the initial atmospheric methane concentration was low enough that these emissions have increased the atmospheric methane concentration by a much larger relative amount than our emission of CO2 has increased atmospheric CO2 concentrations. This then results in the radiative response to the increase in methane concentration being quite large even though the amount of methane that has been emitted is quite low when compared to the amount of CO2 that’s been emitted.

  9. The radiative efficiency of any GHG depends first on the structure of the molecule which determines its radiative absorption lines as radiation interacts with molecular vibrations. It turns out that asymmetric vibrations are necessary, so diatomic molecules like O2 and N2 have no absorption lines in the IR spectrum. The second important factor is where the radiative lines fall and whether there are gases in the atmosphere with high absorption that would interfere with absorption at those radiative lines. This includes self-interference for gases with very high concentrations. CO2 has the highest concentration, so has saturated at the center of the highest absorbing lines: that’s why additional radiative forcing is logarithmic. CH4 and N2O also have high concentrations, so their radiative forcing behaves as the square root of concentration. However, low concentration gases (e.g., HFCs) have linear relationships between concentration and forcing.

  10. Russell says:

    Technology Review very obligingly published an essay on direct methane capture the day after this one appeared :

    https://www.technologyreview.com/2021/09/27/1036284/methane-removal-slow-global-warming/

  11. raypierre says:

    ATTP, you’ve got it all wrong. If we don’t get CO2 emissions to net zero, and pretty darn soon, nothing else matters — if we don’t get to net zero CO2 emissions, accumulating CO2 will always negate anything we do with methane, HFC’s or other short-lived stuff. What you say would make sense if we were making good progress to net zero CO2 emissions, but we aren’t. Until we are, methane is just a distracting sideshow. Sure it makes sense to build incentives for low or negative cost methane abatement (e.g. forcing capture of methane from abandoned Permian Basin wells), but methane and other SLCP only comes into its own in a world where we have gotten a handle on CO2 emissions. We are not in that world, not even close. Also, it is easy to say “do both,” but easier said than done. In some cases, it may be possible to regulate some methane sources out of existence, but money spent on methane abatement is also money that could be spent on CO2 abatement, and it is clear which should come first. Methane effects on climate are mostly reversible. CO2 effects, with present or near-future technology, is not.

  12. Ray,

    If we don’t get CO2 emissions to net zero, and pretty darn soon, nothing else matters — if we don’t get to net zero CO2 emissions, accumulating CO2 will always negate anything we do with methane, HFC’s or other short-lived stuff.

    Indeed, I agree. So, if I made it sound otherwise then I was wrong. I guess I can see how methane could be an important lever in terms in influencing the rate of near-term warming, but this shouldn’t be seen as some kind alternative to CO2 emission reductions, and we shouldn’t focus on this if it risks ignoring the importance of reducing CO2 emissions.

    In some cases, it may be possible to regulate some methane sources out of existence, but money spent on methane abatement is also money that could be spent on CO2 abatement, and it is clear which should come first. Methane effects on climate are mostly reversible. CO2 effects, with present or near-future technology, is not.

    Yes, but I assume that there must be some action that could influence methane emissions that wouldn’t affect attempts to reduce CO2 emissions. However, as I think I said in an earlier post, if roughly 1/3 of methane emissions come from fossil fuel extraction then prioritising reductions in fossil fuel use would presumably also have an impact on methane emissions.

  13. russellseitz says:

    Raypierre, when it comes to GHG vapor leakage around large hydrocarbon deposits, plate tectonics can be a problem along with capping and sequestration economics:

    https://vvattsupwiththat.blogspot.com/2021/10/after-we-remove-all-methane-air.html

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