The Misguided Crusade to Reduce Anthropogenic Methane Emissions

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Clyde Spencer

2023

ABSTRACT

The role of anthropogenic methane (CH₄) in global warming is exaggerated. The atmospheric concentration of carbon dioxide (CO₂), as measured at Mauna Loa Observatory (MLO), is above 420 ±0.7 ppmv (parts per million-volume) and is increasing about 2.8 ppmv annually. CH₄ has a concentration of about 1.9 ppmv and is increasing about 0.014 ppmv annually. The anthropogenic contribution to the annual CH₄ increase is a fraction of the total, probably about one-third, albeit the official estimate was increased in recent years. CH₄ has more potential for warming than CO₂, but it is effectively gone in about a decade, having a commonly cited long-term impact that is only about 32X that of an equal weight of CO₂. Accounting for the CO₂ warming equivalence on a mole-fraction basis reduces the equivalence factor to less than 12X. It is the long-term impact we need to be concerned about because of an arbitrary temperature threshold claimed to be threatening our survival after 2050. With CO₂ being more than 200X as abundant, even with the greater potential impact of CH₄, the Global Methane Pledge will, at most, achieve a 0.58% annual decrease in CO₂-equivalent CH₄.

INTRODUCTION

More than 100 member nations, represented at the 26th Conference of the Parties (COP26) to the UN’s Framework Convention on Climate Change, committed to the Global Methane Pledge (GMP). The GMP was created using information provided by Krane (2022) and others; it is a commitment to reduce the estimated 33% of total annual methane (CH₄) emissions derived from fossil fuels, by at least 30%, compared to 2020. The intent is to reduce methane emissions because of the claimed greater warming potential of CH₄ than CO₂.

From the International Energy Agency (IEA):

“…, the most common [metric] is the global warming potential (GWP). This can be used to express a tonne of a greenhouse-gas emitted in CO₂ equivalent terms, in order to provide a single measure of total greenhouse-gas emissions (in CO₂-eq).

The Intergovernmental Panel on Climate Change (IPCC) has indicated a GWP for methane between 84-87 when considering its impact over a 20-year timeframe (GWP₂₀) and between 28-36 when considering its impact over a 100-year timeframe (GWP₁₀₀). This means that one tonne of methane can considered to be equivalent to 28 to 36 tonnes of CO₂ if looking at its impact over 100 years.”

Paraphrased from Chapter 8 (Physical Science Basis) of the IPCC Fifth Assessment Report, the GWP of a greenhouse gas is defined as the integrated radiative forcing, over some time interval, resulting from a pulse emission of that greenhouse gas, as compared to an equal mass of CO₂. After some caveats about the definitions and utility of the metric, it says, “Thus, the name ‘Global Warming Potential’ may be somewhat misleading, and ‘relative cumulative forcing index’ would be more appropriate.” They continue with, “The GWP has become the default metric for transferring emissions of different gases to a common scale; often called ‘CO₂ equivalent emissions’ … The GWP for a time horizon of 100 years was later adopted as a metric to implement the multi-gas approach embedded in the United Nations Framework Convention on Climate Change (UNFCCC) and made operational in the 1997 Kyoto protocol.” This provides precedence for using a 32X multiplier rather than the commonly quoted 85X.

From Figure 1, below, the average annual CH₄ emissions-increase over the period January 2018 – January 2022 was 0.014 ppmv. Therefore, the pledged annual reduction in fugitive CH₄ would be less than 10% (30% of 33%) of 0.014 parts per million-volume, or 0.0014 ppmv annually. That is, the annual increase in CH₄ emissions would decline from 0.014 to 0.013 ppmv. One should probably consider that an upper-bound, considering the history of compliance with IPCC goals.

Assuming that the equivalent long-term global warming potential of CH₄ is about 32X that of CO₂, the reduction goal would be equivalent to 0.044 ppmv of CO₂ (32 x 0.0014). The net annual increase in atmospheric CO₂ is about 2.8 ppmv recently, having ranged from about 2.0 to 3.6 ppmv over the last 20 years. [See Fig. 4 in Spencer (2021)]. Thus, the first-order estimate of the reduced warming impact of reducing anthropogenic CH₄ would be less than 1.6% of the temperature increase attributed to the net annual CO₂ increase. This approximation will be refined further, below. We can use this initial estimate as a sanity check on the refined estimate.

Fig. 1. Globally averaged, monthly mean, marine surface, atmospheric methane concentration.
(https://gml.noaa.gov/ccgg/trends_ch4/)

(https://gml.noaa.gov/ccgg/trends_ch4/)

Unlike CO₂, atmospheric CH₄ does not always increase. As can be seen in Figure 1, above, CH₄ concentrations plateaued during the decade from 1999 through 2009. It isn’t certain why that happened. It does suggest that we need to learn more about the CH₄ cycle.

As recently as 2017, NOAA dismissed the claim that fugitive emissions resulting from fossil fuels were driving the increase in atmospheric CH₄. However, that is inconvenient for the political activity at COP26 and 27. From Figure 1, above, the seasonal range (≈0.023 ppmv), which probably represents the natural emissions, is about twice the annual net increase (0.014 ppmv), which probably represents mostly anthropogenic emissions.

PROBLEMS

A problem is that the atmospheric concentration of CO₂ is reported invariably as parts per million by volume (ppmv). Most commonly, one sees graphs of the increase in CH₄ in units of parts per billion by volume (ppbv), as shown in Figure 1. Many people find it difficult to make direct comparisons because the scale used for CH₄ inflates the amount subjectively by three orders of magnitude. For clarity and objectivity, CH₄ should be reported in the same units as CO₂ when comparing the relative warming potential.

Krane (2022) cites a source that claims “Over 20 years, methane causes 85 times more warming than the same amount of carbon dioxide.” There is a problem with this also. Chen and Zou (2022) state that “Based on the assessment in IPCC AR6, the methane perturbation lifetime is about 12 years. Others, such as NASA, claim that the methane is gone in 10 years. While opinions vary on the details, experts agree that CH₄ only has a lifetime of about 10-12 years in the atmosphere. CH₄ is converted to CO₂ and is then counted in the monthly CO₂ measurements as part of the CO₂ flux. To simplify things, we could assume that most of the impact occurs in the first decade after release; however, our concern should be with the long-term impact. As noted above, the UNFCCC settled on the 100-year time-frame GWP for the 1997 Kyoto protocol.

There is yet another more important problem: Infrared radiation is absorbed by individual molecules of CO₂ and CH₄. Therefore, the proper measure of the potential warming is the relative number of molecules, or the mole fraction, not the bulk mass (molecular weight).

The claim for the mid-range, long-term warming potential of CH₄ is 32 times that of CO₂. However, that equivalence is for equal weights of the two gases! The multiplier is a poor choice for comparison because CO₂ is more than two orders of magnitude more abundant than CH₄ in the atmosphere. As pointed out previously, CH₄ and CO₂ atmospheric concentrations are reported typically as a volume fraction, not a weight fraction. Because they have different molecular weights, equal molecular concentrations of the two gases do not weigh the same. CH₄ has a molecular weight (16.0 g/mole) about 36.4% of CO₂ (44.0 g/mole). That is to say, when the equivalent warming potential of CH₄ is calculated for the same volumetric concentration of CO₂, or molecules in a given volume, the warming potential is significantly less than 32X. Specifically, 1.0 ppmv of CH₄ has less than 12 times (11.6X) the long-term warming potential of 1.0 ppmv of CO₂, not 32X. Rarely does the news media, or even the climatology community, make this distinction, quoting instead, the equal-mass (weight) equivalences. That makes the problem seem more threatening.

Not surprisingly then, there are some contradictions in a SciTechDaily article about methane ‘super-emitters.’ It quotes NASA Administrator Bill Nelson egregiously saying, “Reining in CH₄ emissions is key to limiting global warming.” The concentration of CO₂ in the atmosphere is currently above 420 ±0.7 ppmv. CH₄ concentration is currently about 1.9 ppmv, which is equivalent to 22 ppmv of CO₂, or about 5.2% of the CO₂concentration.

The recent annual increases in CH₄, resulting from all emissions (0.014 ppmv/yr) is equivalent to about 0.16 ppmv/yr of CO₂, which is about 5.8% of the annual CO₂ increase, and only about a quarter of the average monthly uncertainty in the Mauna Loa Observatory CO₂ measurements. Thus, assuming fossil fuels account for about 33% of the total annual CH₄ emissions, fossil fuel emissions only account for about 0.054 ppmv CO₂-equivalence, or about 1.9% of the annual CO₂ increase. Reducing that 1.9% by 30% (1.9% x 0.30% = 0.58%) is not going to limit global warming! It is naive to think so. The expected CO₂-equivalence reduction resulting from the Global Methane Pledge is less than the uncertainty of the measured monthly CO₂ concentrations.

SUMMARY

It is disingenuous for climate-change alarmists to focus on the large Absolute Global Warming Potential (AGWP) of CH₄ on a weight-equivalence, because CO₂ is more than two orders of magnitude more abundant than CH₄ on a molecular basis, which is how the concentration and annual flux are reported usually. Furthermore, CO₂ and CH₄ have not been equally abundant since at least the early-Proterozoic Eon, meaning the last two billion years. Using a molecular basis (parts per million-volume mole-fraction) to account for the lighter CH₄ reveals that the annual contribution to warming is a fraction of that claimed for CO₂ and, therefore, attempts to reduce warming will have insignificant effects.

The global rate of increase of CH₄ was about 0.014 ppmv/yr in 2020. The estimated fossil fuel contribution to that (33%) is 0.0046 ppmv and a 30% reduction of that is (0.30 x 0.0046) 0.0014 ppmv/yr. Converting that fractional flux to its long-term CO₂-equivalence (11.6 x 0.0014) gives an annual reduction of 0.016 ppmv/yr. That is, if all nations on Earth were to achieve the pledge made by the 100+ COP26 nation-members, there might be about 0.016 ppmv less CO₂-equivalence of CH₄ emitted into the atmosphere annually. That is 0.58% of recent annual CO₂ increases.

There was a 10% (by weight) average annual decline (monthly maximum decline > 18%) in anthropogenic CO₂ emissions during the COVID-19 pandemic shutdowns of 2020, with no observable decline in the atmospheric CO₂rate of seasonal increase, seasonal maximum, or net annual increase. [See Spencer (2022)] We have no empirical evidence that a 0.58% decrease in CO₂-equivalent CH₄ will have a measurable impact. I consider the claim that focusing on reducing CH₄ is the “cheapest, quickest way to reduce climate change without roiling the economy,” to be wishful thinking unsupported by the facts.

Approximately one-half to two-thirds of the annual emissions of CH₄ are natural and not amenable to reduction by humans. (In fact, the recent movement to re-introduce beaver in locations where they have become extinct may increase CH₄emissions.) Historically, Conference of the Parties (COP) nation-members do not have a good record of achieving their pledges. We would be doing extremely well to decrease all anthropogenic CH₄ emissions by half. Perhaps that is why the pledge goal is only 30% of the 33% (≈10%) attributed to fossil fuels. Thus, an expensive, concerted effort to reduce annual anthropogenic CH₄ emissions might reduce the CO₂-equivalence from 1.9% to about 1.3%. I would not consider that “key to limiting global warming.” The impact is truly lost in the noise.

REFERENCES

Lan, X., K.W. Thoning, and E.J. Dlugokencky: Trends in globally-averaged CH₄, N₂O, and SF₆ determined from NOAA Global Monitoring Laboratory measurements. Version 2023-02, https://doi.org/10.15138/P8XG-AA10

Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

https://theconversation.com/why-fixing-methane-leaks-from-the-oil-and-gas-industry-can-be-a-climate-game-changer-one-that-pays-for-itself-194346

https://www.iea.org/reports/methane-tracker-2021/methane-and-climate-change