Prof Myles Allen is professor of geosystem science at Oxford University. Prof Piers Forster is professor of physical climate change at the University of Leeds.

The Paris Agreement on climate change, adopted in December and now officially signed by 177 countries, commits nations to substantially reducing their total greenhouse gas emissions, to below either current levels or a notional business-as-usual path up to 2030.

But for many, particularly developing, countries, exactly what “total emissions” means in practice depends on how much weight is given to CO₂ versus other gases, such as methane, in their national emissions accounts.

Our new study, published today in Nature Climate Change with our colleagues from Reading, Oslo and Wellington, proposes a new way of accounting for greenhouse gas emissions that would align commitments better with long-term temperature goals.

Exchange rates

For over 20 years, governments have used a “metric”, or exchange rate to compare the potency of different greenhouse gases, called the 100-year Global Warming Potential, or GWP100.

This is defined as the amount of heat trapped in the atmosphere over 100 years by a given mass of an emitted gas, compared to the same mass emitted of CO₂.

Our study shows that, despite its name, GWP100 actually indicates the relative impact on global temperature of different gases some 20 to 40 years after their time of emission.

If, therefore, temperatures approach their peak shortly after mid-century, as they need to if the Paris goal of a warming “well below 2C” is to be achieved, then GWP100 represents a rough-and-ready indicator of the relative importance of today’s emissions for peak warming.

But if they don’t, and temperatures continue to climb to the end of the century, GWP100 substantially over-values “short-lived climate pollutants”, such as methane and black carbon (soot). These substances are so-called because unlike CO₂, which sticks around in the atmosphere for many hundreds of years, they typically have lifetimes of years or even days.

Setting priorities

This matters, because governments and companies use GWP100 to decide on policy priorities.

For example, whether the benefit in terms of reducing CO₂ emissions that might come from replacing coal with shale gas is worth the extra methane emissions that large-scale fracking might cause.

We propose the simplest solution is to avoid pretending there is any kind of equivalence between “cumulative pollutants”, such as CO₂ and nitrous oxide, and any amount of short-lived pollutants, until emissions of cumulative pollutants are falling fast enough that it is possible to predict when they will reach net zero. This is required to stabilise global temperatures at 2C, or any other target.

A more radical option, our paper suggests, would be to use GWP100 to compare one-off emissions of a cumulative pollutant like CO₂ with a permanent change in the emission rate of a short-lived pollutant, such as methane.

The impact on future temperatures of releasing one tonne of CO₂ is similar in magnitude, but of opposite sign, to reducing methane emissions by 360g per year, forever.

We work out the size of the required reduction from the following formula: 1/GWP_H/H tonnes per year, where H is the GWP time horizon. So, for a GWP100 for methane of 28, this equates to 1/2800 tonnes per year, or 360g per year.

This highlights that while a small change in the rate of emission of short-lived pollutant can have a large effect, it must be sustained.

Forever is a long time. Reducing methane emissions this year, only to see them rise again in the 2020s, would make virtually no difference to future temperatures. But every tonne of CO₂ released contributes to peak warming, no matter what happens in the future.

“Faustian bargain”

It is far from clear that governments, under the UNFCCC process, would be able or willing to make this kind of “Faustian bargain”. That is, exchanging a temporary deferral of reductions in CO₂ emissions (which have to come down eventually anyway) for a permanent commitment to reduce short-lived climate pollutant emissions.

The other side of the coin, of course, would be that any country that allowed short-lived climate pollutant emissions to rise would immediately be heavily penalised.

Although the study doesn’t say so (it is a scientific paper), we (the authors of this guest article) strongly suspect that giving countries the option of wriggling out of reducing CO₂ emissions in exchange for making indefinite commitments to reduce rates of emission of other gases could be destabilising, to put it mildly.

So, the simplest option is for countries to specify, separately, how and when they propose to reduce emissions of CO₂ to net zero, which is ultimately needed to stabilise the climate, as well as how they propose to control the rates of emissions of all greenhouse gases in the meantime.

A handful of countries provided separate targets for methane, for example, in their Nationally Determined Contributions to the UNFCCC. This is clearly a good thing.

But most parties, including the United States and the European Union, gave only targets for total “carbon-dioxide-equivalent” emission reductions (defined using conventional GWP100).

Until it is clarified how much of these “equivalent” emission reductions will be made up of CO₂ itself, these represent ambiguous commitments to future climate.

This research was carried out with colleagues from the Centre for International Climate and Environmental Research (CICERO) in Oslo, the University of Reading and the New Zealand Agricultural Greenhouse Gas Research Centre.

Main image: Behind the scenes at the Paris agreement signing ceremony. Credit: United Nations Information Centre/Flickr.