Measuring global warming potential of greenhouse gases
Different greenhouse gases have different warming effects over different timescales
Carbon dioxide and methane are the 2 main greenhouse gases (GHGs) that cause climate change by warming our planet's atmosphere.
Carbon dioxide stays in the atmosphere until it is absorbed by vegetation, soils, oceans or rocks, with a large part of it remaining in the atmosphere for centuries.
Methane doesn't stay in the atmosphere anywhere near as long. Once emitted, around 90% of it breaks down through chemical reactions with a half-life of around 12 years. But in that short time the warming effect from methane is much greater. At any given time, one tonne of methane traps around 100 times as much heat as one tonne of carbon dioxide.
The warming we see today is from a cumulative buildup of carbon dioxide since the Industrial Revolution, combined with present-day methane emissions from the last decade.
GWP100, GWP20 and GWP* are all ways to show the warming effects of different GHGs
Because all greenhouse gases behave differently in how much they warm and for how long, the Global Warming Potential (GWP) metrics were introduced as a way to compare all greenhouse gases to 1 tonne of carbon dioxide.
GWP100 measures the warming effect of a gas over 100 years, and GWP20 over 20 years. Methane’s GWP100 is equal to 28, meaning 1 tonne of methane causes 28 times more warming than 1 tonne of carbon dioxide over 100 years. Methane’s GWP20 is equal to 86, meaning 1 tonne of methane causes 86 times more warming than 1 tonne of carbon dioxide over 20 years.
GWP100 was used as the metric for comparing different GHGs and setting reduction targets in the Paris Climate Agreement and IPCC reports. It's used in the Nationally Determined Contributions which are each country's reduction targets presented at each annual COP climate talks.
Methane heats the atmosphere 86 times more than carbon dioxide over 20 years, and 28 times more over 100 years because it is both short-lived and more potent compared to carbon dioxide. The largest sources of methane from human activities are from fossil fuel extraction (40%), livestock (30%), and landfill and agricultural waste (19%).
GWP* measures differences not total amounts
GWP* is different from both GWP100 and GWP20. For short-lived GHGs like methane, it accounts for the differences between them and carbon dioxide, and therefore does a better job than GWP100 and GWP20 in estimating impacts of current emissions on future warming. But in doing so, GWP* also masks the warming that has already taken place. Because of this loophole, it needs to be treated as a climate model rather than an accounting metric.
This means that if 2 different companies emitted the same amount of methane in a year, they could have very different GWP* scores. If Company A emitted 10 tonnes of methane in 2004 and reduced this to 5 tonnes by 2024, its GWP* will have a large negative value (-560 tCO2e) because of the reduction of 5 tonnes. Conversely, Company B emitted 0 tonnes of methane in 2004 and increased this to 5 tonnes by 2024, so its GWP* will have a far higher value than for Company A (+640 tCO2e) because of the increase of 5 tonnes.
In this example, GWP* doesn’t show that the actual emissions of methane from both companies in 2024 are exactly the same. What GWP* does show is that company A is on the right trend in terms of reducing its methane emissions and contributes less to climate warming than 20 years ago, while company B is moving in the wrong direction by increasing its emissions.
Our example shows that, if properly used, GWP* has the effect of emphasising, the need for rapid methane reductions, much more so than GWP100.
GWP* and the problem of misinterpretation
The main problem with GWP* is that it's open to misinterpretation, especially when used at a country, company or landscape level. By masking the scale of past methane emissions, it heavily favours historically high emitters, over those with historically low emissions. It doesn't work with the concept of reaching “net zero” which was based on GWP100, and it can't be used with other GWP metrics. You can’t mix different metrics together, just like you can't measure your speed in kilometres per hour when the limit is set in miles per hour.
Worryingly, some elements within the livestock industry are now lobbying for the use of GWP* as the metric to be used for country-level emissions reporting. We strongly object to this. Agriculture is the largest global emitter of the potent greenhouse gas methane which needs to be cut by 30% by the end of this decade and by 50% by 2050 if we are to stay within 1.5⁰C of warming limit. GWP* would make even modest methane reductions appear to be far more beneficial than they really are.
That's why we are supporting the call of over 60 NGOs urging global governments not to adopt GWP* as a climate metric at a country or company level.
When we calculated the carbon footprint of the Cairngorms National Park, we used separate targets for carbon dioxide and methane emissions, from both human activities and land-use.
We never use GWP* with our land management clients
As well as working with private companies and charities, we also work with protected landscapes, private estates and council areas across England, Wales and Scotland. So far, we've calculated the total carbon footprints and set decarbonisation targets for around 20% of the land area of the UK. We never use GWP* with any of our clients.
For our landscape clients, we use both GWP100 and GWP20 for reporting GHG emissions. We also estimate the actual contributions of emissions from a given geographical area to date to climate warming, which can be done separately for carbon dioxide, methane and nitrous oxide, and distinguishes between fossil- and land-based sources.
Similarly, we set separate decarbonisation targets for different GHGs and different sources of emissions, which removes the need to use any GWP metric. By design, this approach also removes the need to use a “net zero” concept, which can also be misinterpreted and misused.
We're supporting the call of over 60 NGOs urging global governments not to adopt GWP* as a climate metric at a country or company level.
For more details on GWP* and the how it can be misused, see the policy briefings from the Small World team and from Feedback Global.