A “controversial” methane metric?
Posted on 27 October 2025 by Ken Rice
This is a re-post from And Then There’s Physics
There’s a recent Carbon Brief article about a supposedly controversial methane metric. The metric in question is GWP*, which I’ve actually written about before. Methane emissions are typically compared to CO2 using a metric known as Global Warming Potential (GWP). These are often measured over periods of 20 years (GWP20) or 100 years (GWP100). For methane GWP20 has a value of about 80, while GWP100 has a value of about 30.
As the Carbon Brief article says, these are often interpreted as suggesting that
one tonne of methane causes the same amount of warming as around 80 tonnes of CO2, when measured over a period of 20 years…….. When calculated over 100 years, methane’s shorter lifetime means it causes around 30 times more warming than CO2.
These metrics highlight that methane is a potent greenhouse gas that can contribute substantially to global warming. The problem is that the interpretation of these metrics is not actually correct. These metrics are computed by integrating the radiative forcing of a pulse of emissions over the relevant time period. However, this doesn’t necessarily correctly represent the warming due to this pulse of emission.
Unlike CO2, methane has an atmospheric lifetime of about 10 years. This means that half of a pulse of methane will be gone after 10 years, three-quarters after 20 years, and only a few percent will be left after 50 years. Hence, it’s really not correct to suggest that over 100 years, methane will cause around 30 times more warming than CO2.
The figure on the right (from Allen et al. 2016) illustrates it very nicely. It shows the warming due to equivalent pulses of different greenhouse gases, using GWP100 to determine the equivalence. It’s certainly the case that an equivalent pulse of methane will cause more warming than CO2 initially, but after about 40 years the warming is about the same. The warming due to a pulse of CO2 levels off after about 20 years and persists well beyond 100 years, at which point there is virtually no warming from the equivalent pulse of methane.
The GWP* metric was introduced to more accurately represent methane-driven warming, and it does a pretty good job. It accounts for key differences between warming due to long-lived greenhouse gases (CO2) and short-lived greenhouse gases, like methane. Stopping CO2-driven warming requires getting CO2 emissions to (net) zero. This isn’t the case for methane. As this Carbon Brief article highlights, methane-driven warming will actually stabilise if methane emissions stabilise and will actually fall if methane emissions are reduced.
Figure 7.22 from the IPCC AR6 WGI report.The Figure on the left also compares GWP100, GWP20 and GWP* in two different emission scenarios. GWP20 (light blue) almost always, over-estimates the warming. GWP100 (purple) does pretty well when emissions are increasing, but starts to diverge when emissions start to decrease. GWP* (dark green) follows the actual warming (black line) in both emission scenarios.
The reason GWP* is seen as controversial is because some may use GWP* to argue that we don’t need to reduce methane emissions, or don’t need to reduce them as much as might be suggested if using GWP20, or GWP100. I think this is a valid concern, but I don’t think it’s a good reason to keep using metrics that don’t actually represent what many think they do, over one that does.
One key point to make is that GWP20, GWP100 and GWP* are just metrics that are used to link methane emissions with warming. None of them actually tells us what we should do. Those are policy decisions policy makers make based on information provided.
One advantage of reducing methane emissions is that it will actually reverse some past warming, which is not the case when it comes to reducing CO2 emissions. I think there are perfectly good reasons for doing this, but I do think we should be clear about this and use a metric that properly represents this, rather than ones that will suggest we need to do this or else methane-driven warming will continue.
We also have to be careful of thinking that we can offset some CO2 emissions through reductions in methane emssions. This is simply not the case, as illustrated by this 2010 Realclimate post called Losing time, not buying time. I think it’s really important to not treat methane and CO2 as equivalent, so it seems odd to characterise a metric that does this as being controversial. I’ll stop there and will put some links below to other posts that might be worth reading.
Links:
What the ‘controversial’ GWP* methane metric means for farming emissions – Carbon Brief article that motivated this post.
Methane – one of my earlier posts about methane emissions and GWP*.
Methae, again – another of my posts about methane.
Losing time, not buying time – 2010 Realclimate post about why it’s important to not treat methane and CO2 as equivalent.
Understanding methane – another post by me about understanding methane emissions.
A new way to assess ‘global warming potential’ of short-lived pollutants – Carbon Brief article by Michelle Cain introducing GWP*.
Agricultural emissions – a post by my about emissions from agriculture.
The definitive CO2/CH4 comparison post – Realclimate post comparing methane and CO2.