In this blog, Myles Allen, Professor of Geosystem Science in the School of Geography and the Environment and Department of Physics at the University of Oxford, discusses separating the contributions of long-lived and short-lived greenhouse gases in the context of setting global emission targets.
It is important for the academic community to distinguish scientific disputes over matters of fact from disagreements over their policy implications. An example is the “metric problem”, how to compare emissions of long-lived climate forcers (LLCFs) such as carbon dioxide and nitrous oxide with emissions of short-lived climate forcers (SLCFs) such as methane. Strong disagreements over how best to relate emissions targets to warming goals in the context of the Paris Agreement and corporate net zero strategies gives the impression of much greater scientific uncertainty over this topic than is actually the case. To address this, a small group of us set out to convene a short paper setting out what was agreed, with co-authors invited from all the groups we could find (through a highly imperfect citation search) who had published on the topic of greenhouse gas metrics since 2015.
The process of establishing what everyone agreed on took significantly longer than anticipated,1 but we ended up with a simple outcome, best summarised as unpacking the following statement from the IPCC 6th Assessment Report: “expressing methane emissions as CO2-equivalent emissions using GWP100 overstates the effect of constant methane emissions on global surface temperature by a factor of 3–4, while understating the effect of any new methane emission source by a factor of 4–5 over the 20 years following the introduction of the new source.” We showed that this statement applies to any SLCF, so the change in global average surface temperature ∆T over a multi-decade period ∆t resulting from a combination of global LLCF and SLCF emissions is given by ∆T≈KE[E̅L∆t + 85∆ES + 0.28E̅S∆t] ,# where E̅L and E̅S are average rates of aggregate LLCF and SLCF emissions, respectively, over that time-interval, and ∆ES is the change in SLCF emission rates between the beginning and end of that time-interval, all emissions are expressed as CO2-equivalent using 100-year Global Warming Potentials, and KE is the Transient Climate Response to Emissions, or 0.45±0.18 °C per trillion tonnes of CO2. This is essentially just a quantification of the IPCC statement. For ∆t=20 years following the introduction of a new SLCF source, ∆ES=E̅S so warming due to that SLCF source is (85/120+0.28) x KEE̅S∆t, or 4–5 times higher than is implied by treating that SLCF as an LLCF like CO2. For a constant SLCF source, ∆ES=0, warming is 0.28 x KEE̅S∆t, or 3–4 times smaller than is implied by treating that SLCF as CO2.
Everyone agreed this was how the global average surface temperature responded to global LLCF and SLCF emissions, either new or long-established, and we provided a simple demonstration with a standard reduced-complexity climate model to confirm it. A clear and important implication is that LLCFs and SLCFs must be indicated separately in emission targets for their implications for global temperatures to be unambiguous.
Readers will no doubt be interested to know what the co-authors could not agree on, which is less clear from the paper. Probably the starkest point of disagreement was how the above statement applies to sub-global sources and sinks, such as emissions from an individual country, company or farm. Unlike CO2, which has a very simple cumulative impact on global temperature, both changes in SLCF emission rates and ongoing constant SLCF emissions affect global temperatures, and by different amounts per tonne of SLCF emitted. This leaves a number of options: (1) not attempting to quantify the warming impact of individual sub-global sources and sinks of greenhouse gases at all; (2) not expecting the warming impacts of sub-global activities to add up to their global impact; (3) allowing the warming impact of a sub-global emission source to depend on other emissions, so an individual methane source is deemed to have a greater warming impact per tonne of methane emitted when global methane emissions are rising than when they are constant or falling, regardless of what that individual source is doing (which precludes even an approximate scenario-independent evaluation of the warming impact of any activity); or (4) simply using this expression to calculate the individual warming impact of a sub-global source, so someone increasing their emissions of an SLCF is deemed to be causing more warming per tonne of SLCF emitted than someone holding their emissions constant. Option (4) means that an SLCF emitter is held responsible not only for their emissions, but also for how they contribute to changing global SLCF emission rates. Although attractively simple, it implies that current SLCF emitters would be able to take credit for reducing global temperatures simply by reducing their SLCF emission rates, while LLCF emitters can only do the same through active LLCF removal, an implication that some co-authors found unacceptable.
The discussion continues.
Described by the BBC as "the physicist behind net zero", Myles has been studying how human activities and natural drivers contribute to changes in global climate and risks of extreme weather since the early 1990s. In 2005, Myles first proposed the concept of a global carbon budget: that peak warming is largely determined by the total amount of carbon dioxide we release into the atmosphere, not the rate of emissions or the atmospheric concentration in any given year. Prompted by this finding, he has long been a proponent of fossil fuel producers taking responsibility for cleaning up after the products they sell, rather than placing the onus on relatively powerless consumers: https://go.ted.com/mylesallen. In 2010 he was awarded the Appleton Medal and Prize from the Institute of Physics and in 2022 a CBE for services to climate change prediction, attribution and net zero. Myles is a Professor of Geosystem Science at the Environmental Change Institute, School of Geography and the Environment and the Department of Physics, University of Oxford, and Director of the Oxford Net Zero initiative.