The IPCC’s AR5 estimated the global warming caused by a tonne of livestock methane would be 28 times that of a tonne of carbon dioxide. New research destroys that estimate.
The FAO calculates that cattle generate up to two-thirds of the greenhouse gases from livestock, and are the world’s fifth largest source of methane. If cows were a country, the United Herds of Earth would be the planet’s third largest greenhouse-gas emitter.
These calculations are based on figures supplied by the IPCC’s AR5, which contends that the global warming potential (GWP) of methane over 100 years is no less than 28 times the global warming it expects to be caused by an equivalent weight of carbon dioxide. This estimate is up from the GWP of 21 put forward in the IPCC’s previous report.
All this is now challenged by a new and authoritative research paper, Allen et al (2017): “A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants, under ambitious mitigation”. This paper finds that conventional GWPs misrepresent the impact of short-lived gases (such as methane) on global temperature – and recommends the adoption of a new metric, denoted as GWP*.
This is a big advance. The abstract observes that, “measured by GWP*, implementing the Paris Agreement would reduce the expected rate of warming in 2030 by 28% relative to No Policy”. And who would know this better than lead author Myles Allen, who was also a co-author of the IPCC’s SR1.5 in 2018.
Currently visiting New Zealand, Professor Allen has recommended that enteric methane be entirely omitted from that country’s cap-and-trade scheme (ETS) because a steady-state herd of cattle can add very little to global warming. Methane has a half-life in the atmosphere of only about six years – so that every new molecule added is offset by the expiry of a molecule emitted by that herd a few years earlier.
Traditional greenhouse gas accounting ignores the impact of changing methane emission rates while grossly exaggerating the impact of steady methane emissions”. And –
Climate policy the world over has traditionally treated every tonne of methane as supposedly “equivalent” to 28 tonnes of carbon dioxide… It isn’t.
To find the carbon dioxide emissions that would actually have a similar impact on global temperature as methane emissions, you need to multiply those methane emissions by seven (not 28), and add the rate of change of methane emissions (measured in tonnes of methane per year per year), multiplied by 2100.”
If there is no “rate of change” (ie the quantity of emissions by weight is constant over time) then there is a one-off impact of only seven times the equivalent weight of CO2. Note that this should only be counted once – there is no accumulation as is the case for CO2 and other long-lived gases.