I’ve always been a little confused as to why so much attention is paid to methane emissions. It’s short-lived, so isn’t it maybe somewhat less important that CO2 emissions, which are long-lived? One reason it is quite prominent is because it is regarded as having a large global warming potential; one tonne of methane is regarded as having about 28 times the impact of one tonne of CO2. The global warming potential (GWP) is defined as the time-intergrated climate forcing due to a one-off pulse of methane when compared to a one-off pulse of CO2 of the same mass.
However, as this paper by Myles Allen, and colleagues, highlights, the actual impact of a pulse of methane is very different to that of a long-lived greenhouse gas like CO2. The figure on the right shows warming due to equivalent pulses of short-lived, and long-lived greenhouse gases. A pulse of a long-lived greenhouse gas like CO2 will lead to sustained warming that will persist for a long time. An equivalent pulse of a short-lived greenhouse gas, like methane, will have a large impact initially and then decay away. After a few decades, the impact will be largely negligible. Hence, simply comparing the global warming potentials of short- and long-lived greenhouse gases can be somewhat misleading. However, a sustained emission of a short-lived greenhouse gas can be equivalent to a single pulse of a long-lived greenhouse gas. The figure on the left shows the sustained emission of short-lived greenhouse gases compared a single pulse of long-lived greenhouse gases, with the total emission of the short-lived greenhouse gases over a century being equivalent to the single pulse of the long-lived greenhouse gas.
All of this implies that one should be careful when comparing short-lived and long-lived greenhouse gases. When considering long-lived greenhouse gases, stabilising temperatures requires getting emissions to zero. When considering short-lived greenhouse gases, it requires getting emissions to stabilise. Constant emission of a long-lived greenhouse gas is equivalent to increasing emissions of a short-lived greenhouse gas. Finally, reducing emissions of a long-lived greenhouse gas leads to continued warming until emissions get to zero, while reducing emissions of a short-lived greenhouse gas can lead to cooling. Also, if emissions of short-lived greenhouse gases are reduced to zero, their overall warming impact will eventually become negligible.
As a consequence of this, there are a couple of papers that have suggested a modification to the global warming potential metric, called GWP*. It still uses the original GWP metric, but depends on how the emission rate of the short-lived GHGs changes (essentially because one can relate a single pulse of a long-lived greenhouse gas to an increase in the emission rate of a short-lived greenhouse gas). This allows one to more properly compare changes in the emission of short-lived and long-lived greenhouse gases.
The reason this is important is because if we simply use the original GWP metric, we could end up committing a lot effort to reducing emissions of a short-lived greenhouse gas (that only has a warming impact for decades) at the expense of emission reductions of a long-lived greenhouse gas, that can have a warming impact that lasts for centuries, or longer. Similarly, the original GWP metric would suggest that reducing the emissions of short-lived greenhouse gases should still lead to warming, as it does for long-lived greenhouse gases. This, however, is wrong. Reducing the emissions of short-lived greenhouse gases should lead to cooling. Hence, we could end up over-estimating what we need to do to achieve some target.
On the other hand, because short-lived greenhouse gases have a large impact on decadal timescales, they are still important if we are to achieve some of our near-term targets. So, they certainly can’t be ignored. There are also other factors associated with them that are also important (for example, land use change and excess water use associated with livestock farming).
There are also some potential issues with the new global warming potential metric, as pointed out in this paper. Under the new metric, a region with a high, but constant, level of methane emission, could argue that they’re no longer contributing to global warming and, hence, don’t need to do anything. However, reducing their methane emissions could still account for some of the impacts that they’ve already had, and would clearly make it easier to achieve some of our targets.
It’s clear that this isn’t a simple issue, but it does seem that there is still some confusion about how short-lived greenhouse gases, like methane, compare to long-lived greenhouse gases, like CO2. I hope this post clarifies some of the issues. I hope I have indeed explained things properly. Feel free to correct me if I have got something wrong. I’ve also provided links, below, to the papers I’ve mentioned, plus links to a couple of other articles about this topic, including a very nice Carbon Brief guest post by Michelle Cain (one of the authors of the papers I mentioned). If you want to learn more, I can also recommend this podcast with Michelle Cain.
New use of global warming potentials to compare cumulative and short-lived climate pollutants – Allen et al., Nature Climate Change, 2016.
A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation – Allen et al., NPJ climate and atmospheric science, 2018.
Improved calculation of warming-equivalent emissions for short-lived climate pollutants – Cain et al., NPJ climate and atmospheric science, 2018.
Unintentional unfairness when applying new greenhouse gas emissions metrics at country level – paper by Joeri Rogelj about an unintentional issue with the new GWP* metric.
Guest post: A new way to assess ‘global warming potential’ of short-lived pollutants – Carbon Brief guest post by Michelle Cain.
Why we’re still so incredibly confused about methane’s role in global warming – Chris Mooney article about this topic.