From Watts Up With That?
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.
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.
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.
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 CO2 concentration.
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.
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 CO2 rate 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 CH4 emissions.) 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.
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