Getting the Measure of GHG Emissions

By Stephen Smith, Scientist at the CCC

If you work in the field of climate change then you are probably aware that CO2 isn’t the only greenhouse gas, or GHG. And you may even know that methane is considered 21 times more powerful than CO2. But where does the number 21 come from, and what does it really mean?

Climate policies such as the Kyoto Protocol and the UK carbon budgets address emissions of many different GHGs which are traded and aggregated on a ‘CO2-equivalent’ basis. For this to work there needs to be a way of putting different GHGs on to this common CO2-equivalent scale.

I have been working on a new method for comparing emissions with help from colleagues at the Met Office, University of Oxford and the Centre for Ecology and Hydrology. After several months’ work it is nice to see the final results published in Nature Climate Change.

The current policy tool is the 100-year Global Warming Potential (GWP100) which compares the total radiative forcing for 100 years after emitting a tonne of one GHG to that from CO2.Radiative forcing is a measure of the heating imbalance in the atmosphere. But given the recent global agreement to limit climate change to 2°C, a temperature-based metric – rather than one based on radiative forcing – would now seem to be more helpful.

Our proposal is to separate GHGs into two groups, according to how they influence peak temperature (see Figure). For long-lived GHGs like CO2 it is the total emissions over time that counts. For short-lived GHGs like methane the emissions rate is a better guide to peak temperature. The long-lived GHGs can be traded together on a CO2-equivalent scale and short-lived GHGs can be traded separately in terms of a methane-equivalent.

Viewing the different GHGs this way, instead of using the traditional GWP100, highlights some key messages from the latest climate science that are relevant for tackling climate change:

  • Today’s emissions of long-lived GHGs such as CO2 are just as important as those in future decades for determining the eventual peak temperature we reach.
  • For short-lived GHGs, such as methane, today’s emissions have less of a direct influence. However, there are some good reasons to tackle these emissions now, because it would slow the rate of warming (helping ecosystems to adapt, for instance) and improve air quality.
  • Staying below 2°C (or any other peak temperature limit) requires CO2 emissions to fall close to zero. In other words, the low-carbon economy will eventually need to become a zero-carbon economy.

I don’t know if our proposal will lead to a change in the official metrics, but I hope we have provided a simple framework for people to think clearly about the importance of different GHGs in preserving the 2°C limit. I am sure however that this won’t be the last word on comparing GHGs…

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