British and New Zealand researchers say they have devised a new metric to help countries create more accurate greenhouse gas emission budgets for use in meeting the Paris Agreement targets.

They say their new metric - CO2-warming-equivalence - takes into account how different greenhouse gases warm the Earth's atmosphere over time and links greenhouse gas emissions directly with their warming outcomes.

The researchers say the current CO2-equivalence metric treats all gases equally, which means it overestimates the contribution of short-lived gases like methane.

Using their metric, the researchers say we'll have a more accurate and reliable way of linking emissions to global temperature rise.

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The study was published this week in the journal NPJ Climate and Atmospheric Sciences.

"The headline goal of the Paris Agreement is to keep global warming well below 2C, and to strive to stay below 1.5C," said Dr Michelle Cain of the Oxford Martin Programme on Climate Pollutants, who led the study.

"These are clear temperature-based targets. It therefore makes sense to set emissions reductions targets based on how much warming they will cause or avoid. We can do this using CO2-warming-equivalence."

Photo / File
Photo / File

The researchers said methane was a more powerful greenhouse gas per kilogram than CO2, but only about half of 2009's methane emissions remained in the atmosphere today and continued to contribute warming.

In contrast, almost all the CO2 from that year remained – and the CO2 would remain and continue to cause warming for a century or more.

They said the proposed metric unambiguously linked greenhouse gas emissions with their warming outcomes, no matter their lifespan. This meant the warming impact of all greenhouse gases could be calculated directly from reported emissions, allowing short-lived gases like methane to be effectively budgeted.

Countries' contributions under the Paris Agreement could also be assessed against the Paris goals easily and transparently using this metric, both individually and collectively.

"Setting targets in terms of temperature - not emissions - is what is so important," Dr Cain said, "because we can get to net-zero warming without net-zero emissions of every greenhouse gas."

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"Short-lived gases decay quickly, so as long as emissions are declining, warming from those sources is declining too. On the other hand, because CO2 lasts for centuries, even millennia, every tonne continues to add to warming, even as emissions decline.

"Understanding this we can see more clearly where we need to target efforts to mitigate climate change. Reducing methane emissions provides an immediate, but short term, benefit. Whereas to truly tackle the long-term damage we are doing to the climate, the focus needs to be on CO2 and other long-lived pollutants. Widespread use of this metric to set targets could ultimately be a game-changer in keeping keep global warming well below 2C."

Dr John Lynch was a co-author on the paper with the Livestock, Environment and People (LEAP) project, part of the Oxford Martin Programme on the Future of Food.

He said currently used climate metrics could not give the full picture of how agricultural greenhouse gas emissions contributed to global warming, or the impacts of changing what we eat or how we farm.

"Our new means of reporting warming-equivalent emissions provides a more reliable way of linking emissions and global temperature."

The researchers said this method built on previous work and was based on the direct relationship between methane emissions and the warming those emissions generated.

This method, called GWP*, produced a far better agreement between CO2-we emissions and resultant warming than CO2-e did. It provided a simple calculation to work out CO2-we emissions and was designed to be useful for informing policies that specifically aimed to limit global warming, as was required under the Paris Agreement.

The paper - Improved calculation of warming-equivalent emissions for short lived climate pollutants - published in npj Climate and Atmospheric Science, demonstrated the method for methane, although it could be applied to any short-lived climate pollutant.

The work will be presented at the Achieving Net Zero conference to be held in Oxford from September 9-11.