The importance of reducing methane emissions

Since I’ve written quite a lot about methane, I thought I would promote Realclimate’s definitive CO₂/CH₄ comparison post. It highlights that the large relative rise in atmospheric methane concentrations means that methane has made a significant contribution to modern warming. It also points out that when you also include the non-direct effects of increases in methane, it is responsible for about 0.6⁰C of the warming since 1750, compared to about 1^oC for CO₂.

The Realclimate post also discusses the difference between CO₂, which is long-lived and hence a stock pollutant, and methane, which is short-lived and hence a flow pollutant. This means that if we stop emitting CO₂, the CO₂-driven warming will stabilise, while if we stop emitting methane, the methane-driven warming will mostly reverse. One should be careful, though, of assuming that if we stopped emitting methane we’d cool by ~0.6^oC, because the cooling impact of other short-lived forces is also comparable to the warming impact of increases in methane. Hence, our committed warming is roughly comparable to the warming due to CO₂.

I have for a long time shared the concerns expressed in this Realclimate post; we should be careful of thinking that reducing methane emissions can buy us time, because if we do so at the expense of reductions in CO₂ emission we’d be committing future generations to more warming than if we’d prioritised CO₂ emission reductions.

However, I have come to realise that it is also important to also aim for reductions in methane emissions. One simple reason is that if we do continue to increase methane emissions, then it could contribute significantly to future warming. Given that methane is relatively short-lived in the atmosphere, we could aim to stabilise methane emissions, which would lead to little further methane-driven warming.

However, this would then mean that we would essentially be sustaining methane’s contribution to the warming since 1750 (~0.6^oC), making it extremely difficult to achieve some of our stated targets, for example limiting warming to < 2^oC. Also, since reductions in CO₂ emissions would probably also result in reductions in the emissions of the short-lived species that have a cooling effect, we could actually end up with a period of quite rapid warming.

So, if we do start reducing methane emissions, we would reverse some of the methane-driven warming, counter-acting the impact of reductions in the short-lived pollutants that have a cooling influence. It would also probably give us a better change of limiting warming without overshoots. As the Realclimate post highlights, there are also other benefits (air pollution, crop yields, public health). However, since long-term warming depends mostly on how much CO₂ we emit, it is key to do this in conjuncion with reductions in CO₂ emissions, rather than instead of reductions in CO₂ emissions.

Links:

..and Then There’s Physics – methane, post I’ve written about methane.
The definitive CO₂/CH₄ comparison post – recent Realclimate post comparing CO₂ and methane.
Losing time, not buying time – 2010 Realclimate post warning against thinking that methane emissions could buy us time.