Researchers from the Department of Physics at Oxford developed a new formula to accurately measure the warming impact of methane emissions – essential to informing climate policy.

Climate policy since the early 1990s has relied heavily on the notion of ‘CO2-equivalent emissions’ that equate one tonne of methane with a certain number of tonnes of CO2. This is problematic because CO2-induced warming is proportional to cumulative CO2 emissions, CO2 being a long-lived climate pollutant (LLCP). Methane-induced warming, however, does not depend on cumulative emissions, as methane is a short-lived climate pollutant (SLCP). Being able to accurately measure the impact of methane is essential to defining climate policies in pursuit of a long-term temperature goal, as methane is the second most important anthropogenic greenhouse gas after CO2. Methane accounts for 25% of CO2’s contribution to warming to date, and 11% of the current CO2-induced warming trend; a science-based treatment of methane emissions has long proved a challenge for climate policy.  

Recalculating impact of methane emissions

An international research collaboration, led by Professor Myles Allen at the Department of Physics, developed a simple formula that uses well-established constants to more accurately measure the impact of methane on global warming. Traditionally, methane emissions are simply multiplied by 28 to give CO2-equivalent emissions; this is the method used for greenhouse gas reporting by the United Nations Framework Convention on Climate Change (UNFCCC), in climate policy in the UK and EU, and ubiquitously in “carbon footprint” calculations. The Department of Physics has shown that the actual rate of ‘CO2-warming-equivalent’ emissions, which generate approximately the same amount of warming whether emitted as CO2 or methane, can be calculated more accurately using a different formula: multiply the current methane emissions rate by 128 and subtract the methane emission rate of 20 years ago multiplied by 120.

The traditional metric therefore overstates the impact of constant methane emissions by a factor of 3-4 which is an issue for traditional livestock farmers and in quantifying the environmental impact of supermarket supply chains. Equally, the traditional method understates the impact of changes in methane emission rates, by a factor of 4-5 over the first 20 years after the change. This is particularly an issue for regulation of new methane sources such as fracking.

Emission metrics

This work bridges the gap between contemporary climate physics and the ‘emission metrics’ commonly used by policymakers, industry, environmental scientists and the general public to quantify impact on climate. It provides a new conceptual framework to compare the impacts of different activities on global temperatures, with especially important implications for the assessment of agricultural sustainability.

The team’s findings built on previous work in 2016 relating the impact of LLCPs such as CO2 and N2O and of SLCPs such as methane on global temperature by equating a permanent change in the emissions rate of an SLCP with a one-off emission of a fixed quantity of CO2. The key contribution of this work was to point out that it was possible to equate the impacts of LLCPs and SLCPs using existing reported quantities. The group further refined this thinking to get to the formula proposed in 2019, and updated in 2021.

In practice in New Zealand

Methane emissions make up around half of New Zealand’s greenhouse gas emissions under conventional accounting, and it has the highest per capita methane emission rate in the world (6 times the global average), as, unusually for a developed country, a large part of its economy is based on agriculture. Research produced by the Department of Physics has been heavily cited in the debate over the New Zealand Zero Carbon Act, and has influenced one of its key policies – to treat biogenic methane separately to other greenhouse gases, and to set reduction, not elimination, targets for methane emissions.