What is GWP*?

Greenhouse gases all contribute to global warming, but they differ in two keyways; how powerful their warming effect is, and how long they last in the atmosphere. Converting all these gases to one standardised measure (CO₂e: carbon dioxide equivalent) makes it easier to count and compare emissions. But the measures used to do this (GWP: Global Warming Potential) can also mean fundamental differences are lost.

Global Warming Potential (GWP) describes how much impact a gas will have on atmospheric warming over a period of time compared to carbon dioxide. Each greenhouse gas has a different atmospheric warming impact, and some gases remain in the atmosphere for longer than others. Carbon dioxide (CO₂) has the lowest warming potential, is the most abundant and lasts for thousands of years, so it is used as the baseline.

The GWP number for other gases shows the warming effect in CO₂ terms over time. For example, the GWP100 for nitrous oxide (N₂O) is 273. This means that 1 tonne of N₂O causes the same amount of warming as 273 tonnes of CO₂ (in this case over 100 years).

The most commonly used GWP measure is GWP100, meaning the average warming potential over 100 years. Alternatives include GWP20, modelling impact over 20 years, and GWP*, which models the impact of methane, accounting for its faster breakdown time, and provides a clearer picture of the actual warming impact of the short-lived gas.

GWP100 is the internationally accepted standard, and is used by countries for their emissions accounting, as agreed at the United Nations level. However, when using a blanket calculation such as GWP100 we need to be mindful of its limitations when making decisions around short-lived gases.

¹GWP100 factors from the IPCC’s 6^th Assessment Report (AR6). These are the most recent factors available. However, the UK’s national GHG accounting currently uses older factors.

The aim of GWP is to allow nations to see which activities are the biggest contributors to global warming across the various greenhouse gases. GWP should also allow nations to model the impact that various changes (mitigations) would have on overall warming. For gases that persist for over 100 years, like carbon dioxide (CO₂) and nitrous oxide (N₂O), GWP100 works well. However, because methane breaks down over just 12 years, GWP100 does not show the true impact of methane on global warming.

The United Nations recognises that GWP100 is inaccurate when measuring the warming impact of short-lived GHGs such as methane. In the UN IPCC’s 6^th Assessment Report (AR6), it acknowledges that other measures such as GWP* can more accurately model this. However, GWP100 remains the official standard for international reporting for the time being. Given the long timescales international decision making often works on, this is unlikely to change in the immediate future. 

GWP*

GWP* aims to better model the impact of methane and compare the warming effect it has relative to CO₂ over time. Under GWP*, methane emissions initially have a more potent impact than under GWP100, but then drop to a very low impact, well below that from GWP100.

Under GWP*, if the rate of emissions of methane stays constant over many years, then the additional warming effect of these emissions is relatively negligible. That is because the concentration of methane in the atmosphere will plateau as older methane breaking down balances out the warming impact of new methane. Furthermore, if methane emissions are falling, this can produce a net cooling effect. However, if methane emissions continually increase, this has a more dramatic increase under GWP* than GWP100.  

The charts below (from Lynch et al 2020) show how converting methane to CO₂ equivalent looks different using GWP100 and GWP*:

One aspect that GWP* doesn’t currently account for is the source of the methane. One of the products of methane breakdown is CO₂, which also has a warming effect. If the methane was from a non-cyclical (fossil) source, then this is new CO₂ being added to the atmosphere and bringing additional warming. However, if the methane is from a source in a natural cycle, such as from cattle, then the CO₂ it becomes is comparable to the CO₂ taken out of the atmosphere into the plants that fed the cattle just a few decades before. In this case, it is argued that the methane therefore provides no additional warming once it has broken down, and long-term stable methane emissions result in no additional warming.

In July 2023, eight internationally recognised scientists, from eight of the UK’s leading science institutes published an academic letter in support of using GWP* as part of assessing climate impacts. Click here to read our summary of the paper.

Measurement of emissions and sequestration must be consistent to allow fair global comparisons, as well as wider environmental impact measures and sustainability improvement.  At present measurement and comparisons are still evolving and this should be borne in mind when making policy decisions.

The critical question is which measure best reflects the actual impact on warming and on the environment and drives focus and reductions in the right areas. 

Pragmatically, in the meantime the industry should report emissions using both GWP* and GWP100 as echoed in UK industry Road Maps and by Beef and Lamb NZ. 

Regardless of what any evolving current measurement methodology says it is important to note that UK agricultural production has the potential to be fully sustainable and operate within a natural cycle.