Methane vs carbon dioxide trade-off

 From And Then There's Physics

There’s a really nice recent paper by John Lynch, Michelle Cain, David Frame and Ray Pierrehumbert on Agriculture’s Contribution to Climate Change and Role in Mitigation Is Distinct From Predominantly Fossil CO2-Emitting Sectors. It’s largely discussing why there are important differences between carbon dioxide (CO2), which is a stock pollutant, and methane (CH4), which is predominantly a flow pollutant.

The basic point is that the emission of CO2 increases the stock, which leads to a long-term increase in atmospheric concentrations and, consequently, to warming that will persist for a very long time. Methane, on the other hand, has a short atmospheric lifetime, decaying within decades to CO2 and water. Given that – for agricultural emissions – the carbon comes from plants, this doesn’t add a new carbon to the system, and hence doesn’t increase the stock. This isn’t strictly true for methane from natural gas, since that does add a new carbon to the system, but this is relatively small when compared to direct CO2 emissions from fossil fuels.

Left: A single emissions pathway (left) reported as CO2-equivalents using the 100-year Global Warming Potential.
Right: How the resulting warming depends on the gas-specific composition (credit: Lynch et al. 2021).

The key figure in the paper is the one above. The left-hand panel shows an example of an emission pathway based on using CO2-equivalents using the 100-year Global Warming Potential (GWP100). The right-hand panel shows the actual warming we would experience for different gas-specific compositions. CO2 warming (dark blue line) peaks when emissions gets to zero, but then remains at this level well after emissions have ceased (it’s essentially irreversible without some kind of artificial negative emission technology).

Methane (yellow line) initially produces more warming than would be expected based on its CO2-equivalence. However, when emissions start to go down, there is cooling, which continues well after emissions have ceased (for completeness, the pink line is 50% methane, 50% CO2, while the green line is N2O which has a reasonably long atmospheric lifetime).

The key point is that if one is using GWP100 to estimate CO2-equivalence, you would predict warming profiles that would be quite different to what would happen in reality. You would under-predict the impact of methane emissions initially, but then over-predict its impact later on.

The reason this is important is because any emission reduction pathways are likely to involve trade-offs. Consequently, as the paper highights,

reducing methane emissions at the expense of CO2 is a short-sighted approach that trades a near-term climate benefit with warmer temperatures for every year thereafter

and

If strong efforts are made to reduce agricultural emissions but prove expensive—in terms of monetary costs, political capital, public goodwill, or individual effort—and detract from efforts to eliminate fossil CO2 emissions then we will be climatically worse-off.

Essentially, the emission of a stock pollutant (CO2) leads to warming that will persist for a very long time, which is different to the impact of a flow pollutant (agricultural methane). The latter clearly does produce warming and, in fact, leads to more warming in the near-term than simple CO2-equivalent estimates would suggest. However, this warming would stabilise if emissions were to stabilise (unlike CO2) and can be reversed if these emissions are reduced (also, unlike CO2).

So, it would seem important to be aware of these differences when thinking of how best to decarbonise. Any strategy that prioritises short-lived pollutants over long-lived pollutants runs the risk of committing us to future warming that is essentially irreversible and that we could have avoided if we’d prioritised differently.

This isn’t to suggest that we should be ignoring the short-lived pollutants. They can have a large near-term impact which may be important if we wish to avoid crossing certain warming thresholds. There may also be other reasons for reducing these emissions (land use change, for example). I just happen to think that if we’re trying to assess the impact of different greenhouse gas emissions, it’s important to use a metric that properly represents this.

Links: (these are additional resources that might be useful)
Agriculture’s Contribution to Climate Change and Role in Mitigation Is Distinct From Predominantly Fossil CO2-Emitting Sectors, new paper by Lynch et al. (2021)
Losing time, not buying time, Realclimate post by Ray Pierrehumbert making the same basic point (from 2010).
Methane, a post I wrote in 2019 about the impact of methane.

This point is contentious, with many accusing those who make it of supporting fossil fuels.  In my opinion, though green methane (made using electricity from wind or solar) is to be preferred to natural gas for 'firming' renewables, natural gas is still preferable to coal, despite leaks ('fugitive emissions') because methane is comparatively so short-lived in the atmosphere.  Especially since coal mining may produce as much methane as extracting natural gas.   So for example, a grid with 90% renewables and 10% gas is in the short-term much worse for global warming than a 100% coal grid, but in the longer term is 95% less warming.  

The real problem would be a huge and sustained expansion in methane leaks, because that would mean continually rising methane levels in the atmosphere.  Stable methane emissions would lead to stabilised methane levels in the atmosphere within 10 years, with no additional warming (from the methane) after that.   The implication is that we should accept legacy gas power stations in our grid as we replace coal while we wait for cheaper long-term storage options to become available, including power-to-gas (green methane).