Is Methane From Cows the Same for the Climate as Methane from Fossil Fuels?
Sort of... but not entirely
Is methane from cows the same for the climate as methane from fossil fuels? This question comes up often. Usually someone makes the assertion that biogenic methane (methane from animals and plants) is part of a natural cycle, and so it is merely cycling carbon and not adding to warming, while fossil methane is ancient carbon that marks a break from that cycle and thus contributes to global warming.
A molecule of methane (CH4) emitted by an animal and from a fossil fuel project are very, very similar: One Carbon atom, four Hydrogen atoms. But they are not identical. We know this because they have slightly different carbon isotopic signatures. Scientists have used these isotopes to determine the source of methane in the atmosphere to see if it has come from a biogenic or fossil source.
*While these very similar but not identical molecules are in the atmosphere* the biogenic and fossil CH4 have the same warming influence on the planet, regardless of their isotopic signature. A CH4 molecule will absorb various wavelengths of solar radiation, just like other key greenhouse gases.
Nevertheless, the Global Warming Potential 100 (GWP100) of methane (aka: how much it heats the atmosphere over a period of 100 years) DOES vary between biogenic and fossil CH4. The following table from the IPCC's AR6 makes this clear [note different rows for CH4-fossil and CH4 non-fossil]:
This difference of ~9% in 100-year warming influence of biogenic and fossil CH4 is due to the originating source of the Carbon in the CH4 molecule: In the case of biogenic CH4 coming from a cow, that carbon originally came from Carbohydrates (Carbon and Hydrogen chains in the grass eaten by the cow), which in turn was originally removed from the atmosphere when it was in the form of CO2 (through photosynthesis). As such, the rumen in the cow is re-emitting that carbon back into the atmosphere, but this time as CH4, where it remains for about a decade before being oxidized back into CO2. So it is indeed a carbon-neutral cycle, but it is not a warming-neutral cycle.
However, in the case of fossil CH4, that resulting CO2 (after the CH4 'breaks down' into CO2) is 'additional' to atmosphere. Hence, fossil CH4 is warming the same amount as biogenic CH4 during its short atmospheric lifetime, but then the Carbon in the fossil CH4 continues to warm further over the 100 year period in the form of NEW CO2 added to the atmosphere (whilst in the case of biogenic CH4, the resulting CO2's warming influence is negated by the cooling influence of that CO2 having originated in the form of removed Carbon). [Watch this cool short animation showing the exchange of Carbon between the atmosphere, the ocean, the biosphere, and fossil fuel deposits]:
So it's true biogenic CH4 technically isn't 'adding' more C to atmosphere, as fossil CH4 does. BUT, that doesn't mean it can't warm the planet! If atmospheric CH4 emissions are *increasing*, they will have a warming influence, regardless of their Carbon origins. If we had a global herd of bison steadily increasing over time, their methane would heat the planet. If that herd was roughly stable in size over time, their methane would likely not heat the planet over the long run, as CH4 is a flow gas, and a similar amount of it would be breaking down each year as emitted by the herd. If the herd was declining in size over a period of 12 years or more, this would actually cause a *cooling effect* over all (even if some methane is still being emitted), as the atmospheric concentration of CH4 would be declining.
However, CH4 concentrations ARE increasing globally. And the global cattle herd is GROWING. Today we are way above the pre-industrial concentration of methane. Figure 5.4 from IPCC AR6 WG1 shows how dangerously high CH4 is today. A more recent assessment published by the Global Carbon Project notes that methane emissions are higher today than they likely have been for more than 800,000 years, and they continue to climb.
Over this time, both biogenic and fossil methane have contributed to the human-influenced increase in global CH4. One recent study estimates that CH4 from livestock is responsible for about 30% of anthropogenic CH4, and therefore about *12% of global warming to date*!
Now, there's a good debate to be had about whether the CH4 we now attribute to Livestock is greater than the CH4 that would be emitted by wild animals *had we not started farming* (or, if we *re-wild* lands and wild ruminants and herbivores or other species which generate methane like beavers and termites see a resurgence in numbers). The great bison herds of North America emitted a LOT of CH4 - one study claims they emitted the equivalent of 90% of what the present North American cattle herd emits!
Some ecologists argue that we underestimate the amount of pre-industrial herbivores, and so even if we got rid of livestock, the return of wild ruminants (or beavers) in wildlife-restored agricultural lands could result in about the same amount of biogenic CH4 as present - though of course that depends on specific geographic contexts (with savannah ecosystems being more likely to sustain larger wild herbivore populations).
Even so, we have a dangerous predicament on our hands, and limiting global warming to “safe” thresholds will require major CH4 emissions cuts (equivalent to ~150Tg/yr). Even cutting 100% of fossil CH4 would not get us there - as these equate to roughly 120Tg/yr (depending on how you’re doing the counting). This means that to address global warming, CH4 emissions reductions are likely required in the agriculture sector too.
Moreover, there is an ‘opportunity cost’ argument that could be made about methane emissions reductions: Any emissions of CH4 that aren’t reduced form a missed opportunity to cool the climate, because reducing CH4 emissions for a sustained period of time does indeed result in cooling influence (unlike CO2, which only cools when emissions are negative).
Luckily, methane reductions in the fossil fuel sector are often seen as a ‘win-win’ because companies can use (and profit from) methane that they capture as opposed to venting it off. Agricultural (biogenic) methane reductions are typically interpreted to be a bit harder to achieve in socio-technical terms, but there are plenty of scientific efforts seeking to do so, both on the demand-side (reducing the consumption of ruminant products) and production-side (innovating methane-abatement technologies like feed additives).
If you’ve enjoyed nerding out on the warming influence of CH4, stay tuned for Part 2 of this series: “On the Lifetime of Methane, and the Story of the Hydroxyl Radical Sink”