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How does the way we define methane emissions impact the perception of its effects on global warming?

Posted on 4 May 2020 by Justine_Wickman

11-15% of anthropogenic greenhouse gas emissions (GHG) are a result of raising livestock (Yusuf et al. 2012; Gerber 2013). The discussion surrounding the value of low-meat diets, such as vegan and vegetarian diets, in reducing global warming through reduced GHG emissions is ongoing. Livestock produce methane and COemissions, which are currently defined by their 100-year or 20-year Global Warming Potential (GWP) where all gases are treated as equivalent to COand the way it acts in the atmosphere.

This approach has limitations due to the variation between long-lived gases like COand short-lived gases like methane, in the way that they are broken down in the atmosphere and their resulting effect on global warming (Shackley & Wynne 1997). New methods to define the global warming effect of greenhouse gases that take into account these differences are changing the way that we look at livestock emissions.

One way to visualise the differences in short-lived and long-lived GHGs is to imagine pouring water into a cup with a hole in the bottom at the same rate as the water was falling out the hole.  In this scenario, the amount of liquid in the cup would remain constant. If the cup did not have the hole, the amount of water in the cup would increase. This is the same for GHGs. 

Methane released into the atmosphere is like pouring water into the cup with the hole: the ongoing process of methane conversion means that it does not inevitably accumulate over time in the same way as CO2. However, CO2 is long-lived in the atmosphere; therefore, as more COis emitted it accumulates, like pouring water into an intact cup. 

This allows stark differences to be seen with the effect of these gases.  So, how can a single metric of measurement give a true picture of their effects on global warming? Conventionally, GWP100 (100-year Global Warming Potential) measurements are standardised to the equivalent of a single burst of COemissions (Epa 2017). This does not take into account the difference in the heating effect of short-lived gases like methane compared to long-lived gases such as CO2. A new metric that can encompass the warming effect of both short-lived gases and long-lived gases has been developed, GWP* (Allen et al. 2018)

There are two parts to the understanding of the effect that this has on defining methane emissions, one more complex than the other. The simple observations that can be made relating to methane emissions is as follows. Methane has a more potent warming effect than CO2; however, due to its short-lived nature, with constant emissions there is no increase in temperature as the amount in the atmosphere reaches an equilibrium where it is broken down at the same rate that it is added. The effect is an elevated but constant temperature (Figure 1b). In contrast, COconcentration has a practically linear effect on warming meaning that temperature increases with constant COemissions (Figure 1a).  

 Figure 1: a) The effect of constant COemissions on warming showing that warming increases. b) The effect of constant emissions of methane on warming showing that warming is elevated but stable.

However, as previously mentioned this does not fully explain the extent to which methane is broken down in the atmosphere because methane degrades into a small amount of CO2, which is long-lived. Therefore, short-lived methane emissions have an influence on the COlevels so that even when the warming effect of the original methane emissions have gone, the effect of the resulting COis still prevalent (Figure).

Why is this important?

Livestock have been linked to emissions of both COand methane.  They directly produce methane as a by-product of their digestion and in their manures. A single molecule of methane has a stronger warming effect compared to a single molecule of CO2. Therefore, it has been widely claimed that reducing intake of meat would have a huge positive effect on the climate warming issue (Yusuf et al. 2012). If everyone became vegan overnight so that livestock associated methane emissions stopped, the temperature would decrease, and the warming caused by livestock methane would be undone in a relatively short time (Figure 2), discounting the warming caused by the resulting CO2.  

 

Figure 2: Effect of reducing livestock numbers so that they are producing no methane emissions and the associated change in warming when discounting the resulting CO2produced when methane is degraded.

However, this new definition of methane emissions that takes into account that it is short-lived gives a different perspective of the potential effects of stopping meat intake and therefore its associated methane emissions. After this initial reduction, the temperature would level out again at a lower level.  Also, while emissions are not increasing, no further addition in temperature as a result of methane would occur. In contrast, all COemissions cause an increase in temperature, regardless if they are comparatively less than before because they will still cause an overall increase in the amount of COin the atmosphere.

Therefore, with the aim to reduce global warming, reducing methane emissions (e.g. through reduction of meat intake) is helpful to provide an immediate reduction in temperature, but long-lived gases such as COstill need to be reduced in order to have a long-term effect on global temperature (Allen et al. 2018). 

References 

Allen, M. R. et al. (2018). A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation. npj Climate and Atmospheric Science, 1 (1), p.16.

Epa, U. S. (2017). Understanding global warming potentials. Recuperado el, 8. [Online]. Available at: https://www.epa.gov/ghgemissions/understanding-global-warming-potentials.

Gerber, P., Steinfield, H., Henderson, B., Mottet, A., Opio, C., Dijkman, J., Falcucci, A., Tempio, G. (2013). Tackling Climate change through livestock: a global assessment of emissions and mitigation opportunities.FAO, Rome. 

Yusuf, R. O. et al. (2012). Methane emission by sectors: A comprehensive review of emission sources and mitigation methods. Renewable and Sustainable Energy Reviews, 16 (7), pp.5059–5070.

Shackley, S., Wynne, B. (1997). Global Warming Potentials: ambiguity or precision as an aid to policy? Climate Research, 8, p89-106.  

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Comments

Comments 1 to 5:

  1. Good to see efforts to expand awareness and improve understanding regarding methane emissions from human food livestock operations.

    Every action to reduce negative human impacts is helpful. But ending the use of natural gas for fuel is still a far more helpful action than reducing livestock methane emissions.

    An additional consideration is that extracting and burning fossil methane 'Adds new carbon to the surface recycling environment, and primarily as new GHGs including fugitive emissions of methane from the extraction, transport and processing operations'.

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  2. As the next decades are decicive whether we can keep global temperatures below 2°C - or even 1.5°C - we need to take the global warming potential (GWP) of methane on a 20-year timescale into account.

    GWP Methane

    In the US, Oil and gas is sector top source of methane emissions, ahead of agriculture.

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  3. There is multiple evidence that natural gas is no better than coal for mitigating climate change.

    Fracking and Shale Drilling Caused Spike in Climate-Warming Methane Pollution


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  4. With a truly epic sudden release of methane from, say, the sub-sea deposits in the Arctic, methane is more like X140 as effective as Carbon dioxide as a GHG.  Another wrinkle in this story is that the methane is converted to Carbon dioxide by the OH radicals in the atmosphere.  A large sudded output of methane could depleat OH and leave the methane in the atmosphere much longer. https://mtkass.blogspot.com/2013/03/the-real-strength-of-methane.html

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    Moderator Response:

    [DB] The potential for possible methane contributions to the atmosphere from clathrates/hydrates is better discussed here, and not in this post.

    Self-promotional advertising snipped.

  5. There is a flaw not recognised by this essay as well. In order for any reduction in livestock production to have an effect on CH4 emissions, would depend on how that vegetative material that used to feed livestock decomposed and produced CH4.

    Burning the forage would still produce CH4. Composting the forage would still produce CH4. Letting the forage abiotically slow oxidize would still produce CH4. Replacing the livestock with wild herbivores would still produce CH4. Burying the forage in landfills would still produce CH4. 

    The primary factor in CH4 emissions is the amount and type of vegetative material being recycled, not necessarily whether a cow or a wild herbivore like a bison or elephant or termite does the recycling. Yes different routes vary slightly but not anywhere near what those figures suggest.

    As the %'s for all those are a little different, but not all that much as you might think. More importantly to all these factors is the methanotroph to methanogen ratio in any particular environment where decaying vegetative matter is present. And of course you already mentioned that fossil methane from natural gas increases CO2 levels like any fossil fuel even after it oxidizes, while recent produced methane from decaying/digesting vegetative material does not effect long term CO2 levels any more than exhaling CO2.

    For this reason I think your use of the 11-15% of anthropogenic greenhouse gas emissions figure from  (Yusuf et al. 2012) and (Gerber 2013) to be very misleading, even if technically correct.

    The reason I say that is immediately after you state, 

    "If everyone became vegan overnight so that livestock associated methane emissions stopped, the temperature would decrease, and the warming caused by livestock methane would be undone in a relatively short time"

    This conclusion is not a given at all. Certainly not the "temperature would decrease" part. There is no way that even if we use the misleading 11%-15% figure, and even if we somehow figured a miraculous way to recycle all that vegetative material without producing any methane at all 0%, that 15% reductions in emissions would actually lower temperatures. Even 100% reductions in all anthropogenic greenhouse gas emissions wouldn't actually lower temperatures for a very long time. 15% reductions would certainly be helpful if possible, but it would slow warming, not actually cool.

    So while I do appreciate the point you were trying to make, the flaws in the Vegan argument go much deeper than you are exposing.

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