There was a recent Conversation article about methane called Climate explained: methane is short-lived in the atmosphere but leaves long-term damage that caused a bit of a stir on Twitter. One way people assess the significance of different greenhouse gases, is to use what is called the Global Warming Potential (GWP). This is determined by integrating the radiative forcing due to a pulse of a particular greenhouse gas over some time interval and then comparing that to the equivalent calculation for a pulse of CO2 of the same mass.
A greenhouse gas like methane has a lifetime of only about a decade, but the initial radiative impact is so large that even if you calculate the GWP over a period of 100 years, it is still has a much larger GWP than CO2. This calculation is correct, but people then interpret this as suggesting that the climate impact of a pulse of methane after 100 years will be much greater than the impact of a pulse of CO2 of the same mass. This is not correct, because most – if not all – of the pulse of methane will be gone after 100 years.There’s a really nice Carbon Brief article, by Michelle Cain, about a new way to assess the global warming potential of short-lived pollutants. It also includes the figure on the right that illustrates the issue really nicely.
If you consider CO2, then if emissions are increasing, warming accelerates. If emissions are constant, then we continue warming at a constant rate. If emissions are going down, then we continue to warm until emissions get to zero.
Methane, on the other hand behaves quite differently. Methane emissions can increase and we could still end up warming at a constant rate. If methane emissions are constant, then we’d relatively quickly reach a state where methane-driven warming stabilised. If methane emissions start going down, then the impact of methane would actually lead to cooling, and we could eventually reverse all the methane-driven warming. By comparison, the only way to reverse CO2-driven warming is to actively remove CO2 from the atmosphere.
There are, however, some complications. If we’ve had increasing methane emissions for a long time, then this could have led to warming of the deep ocean, which will persist for a long time. If the source of methane is fossil-fuel-related, then when it decays to water and CO2, this will be a new CO2 molecule, which will contribute to long-term warming. However, relative to the long-term impact of direct CO22 emissions, these impacts are probably still quite small.
One reason why I think this is important is highlighted in this Realclimate post by Ray Pierrehumbert. If we think that there is some long-term benefit to rapidly reducing methane emissions and we do so at the expense of reductions in CO2 emissions, then we could end up reducing emissions of a species that will have little long-term impact while failing to reduce the emissions of one the impact of which will persist for generations.
This is not, though, to argue that we shouldn’t be looking at reducing methane emissions. All I’m suggesting is that we should be properly comparing the impact of the different greenhouse gas species when deciding what we should do and should not be basing these decisions on a metric that probably over-esimates the impact of short-lived greenhouse gases, like methane.
There’s also a potential fairness issue. When we expect a CO2-emitting industry to reduce their emissions, we’re effectively asking them to limit how much they contribute to future warming. When we expect a methane-emitting industry to reduce their emissions, we’re effectively asking them to reverse some of their past warming. There may be good reasons for doing this, but I would argue that it’s better to be clear about this than to suggest an equivalence that isn’t actually correct.
Climate explained: methane is short-lived in the atmosphere but leaves long-term damage – The Conversation article by Zebedee Nicholls and Tim Baxter.
Guest post: A new way to assess ‘global warming potential’ of short-lived pollutants – Carbon Brief article by Michelle Cain.
Losing time, not buying time – Realclimate post by Ray Pierrehumbert.