The complicated world of methane
As governments struggle to find practical routes forward with positive outcomes for CO2 mitigation, attention is turning to dealing with other greenhouse gases, particularly methane. A number of methane emission initiatives are now underway or being planned, for example those within the Climate and Clean Air Coalition.
Methane seems like an obvious place to start. Anthropogenic emissions are about 250 million tonnes per annum. A tonne of methane emitted now has a short term (20 years) impact on atmospheric warming which is some 80 times greater than a tonne of CO2. This means that over the period of twenty years, the methane will add 80 times the amount of heat to the atmosphere as the carbon dioxide. But methane breaks down in the atmosphere quite quickly with a ‘half life’ of about seven years, so on a 100 year basis (with the methane effectively gone) the impact of a tonne of methane emitted now compared to a tonne of CO2 is much less. The factor falls to about 28, but even with a lower multiplier reducing methane still seems to be a worthwhile endeavour. While agricultural methane may require real lifestyle changes to bring down, e.g. less meat consumption, industrial methane emission management looks like something that can be done. Often mitigation may be a case of good housekeeping, such as monitoring and maintaining pipelines to minimize fugitive emissions.
While most articles about methane simply use the GWP (Global Warming Potential over 100 years) of 28 and present data and economics on that basis, a few dig deeper. Of note is the work of the Oxford-Martin School who present a number of policy papers on methane. In the more popular press, Burning Question author Duncan Clark has written about methane.
Both follow a similar line of reasoning. They note that methane and CO2, while both greenhouse gases, behave very differently with regards their impact on the actual goal of the UNFCCC, to limit eventual peak warming to 2°C or less. As noted, methane is a relatively short lived gas in the atmosphere, whereas CO2 is a long lived gas that accumulates in the atmosphere. This means there is another dimension to the issue, time. The point in time at which they are emitted relative to each other and the shape of any reduction pathway relative to the other is important. Duncan Clark describes this in the following way:
The difference between carbon dioxide and methane is a bit like the difference between burning coal and paper on a fire. Both generate plenty of heat but whereas the coal burns steadily for a long time and accumulates if you keep adding more, the paper gives an intense burst of warmth but one that quickly disappears once you stop adding it.
Their conclusions are similar. Peak warming is largely dictated by the cumulative amount of CO2 emitted over time. If a certain amount of methane is also emitted, the timing of that emission is what matters. Methane that is emitted today will immediately impact the rate of warming, but long before we reach peak warming (assuming CO2 emissions are eventually brought under control and warming actually peaks) the methane will have left the atmosphere and been converted to carbon dioxide, in which case it’s impact on peak warming is based only on the CO2 that remains from the methane. We may have accelerated warming in the short term but peak warming will remain largely unchanged. In this case, the warming potential of methane expressed in terms of its impact on peak temperature falls sharply and comes close to the stoichiometric conversion of methane to carbon dioxide, which is about 3, i.e. a tonne of methane when combusted or oxidised in the atmosphere gives rise to about three tonnes of carbon dioxide. Conversely, methane that is emitted much later, say when we are close to peak warming, will directly add to whatever level of temperature we happen to reach.
Does this mean that we shouldn’t bother about methane today? Unfortunately the answer is an ambiguous one. If we are confident that the world will quickly and decisively reduce CO2 emissions then of course we must also be reducing methane and other greenhouse gases as well. If we don’t, then we will still have a methane problem at the time peak CO2 induced warming occurs, in which case we will almost certainly overshoot our peak warming goal, i.e. 2°C, with the additional warming from the other greenhouse gases. But if we don’t address the CO2 issue, then addressing the methane issue now doesn’t offer a lot of benefit for later on. Instead, the benefit that we do get is less short term warming as we will have removed the intense burst of warming that the methane is providing.
Of course, since we don’t know how well or otherwise the task of CO2 mitigation will proceed (despite the fact that the track record is pretty poor), we feel obliged to act on methane now in case the CO2 mitigation picks up. At least we know that we will slow down the near term rate of warming by doing so.
Not surprisingly, it turns out that dealing with methane and atmospheric warming is just as complex as dealing with CO2. In the case of CO2, many are convinced that steps such as efficiency measures can curtail warming, when all they are probably doing is geographically or temporally shifting the same CO2 emissions such that the eventual accumulation in the atmosphere is unchanged. In the case of methane, treating it as if it were interchangeable with CO2 but with a convenient and high multiplier may make us feel that modest effort is delivering great benefit, when it may be the case that little benefit is being delivered at all.
In both cases it is the science that we have to look at to decide on the appropriate strategy, not expediency and certainly not sentiment.