Global warming potential is a critical metric used to translate climate impacts of superpollutants into tonnes of CO2 equivalence (tCO2e). In a carbon project, crediting methodologies use GWP to assess the number of credits associated with that project activity and that can be claimed and traded across the VCM. However, there is nuance to GWP values that is important to understand when investing in and supporting superpollutant carbon projects. A project’s climate impacts may look different based on the GWP being applied.

Superpollutants are gaining more attention in the carbon market because they offer some of the fastest pathways to slow near-term warming. Unlike CO₂, which can remain in the atmosphere for centuries, many superpollutants have much shorter atmospheric lifetimes, and reducing their emissions can therefore have an outsized impact on the rate of warming in the near-term.

This opportunity is helping move superpollutants from a niche conversation into a more prominent part of the carbon market. But as interest grows, so does the need to understand how the climate impact of these projects is measured. With a new report on superpollutant GWPs expected from the Intergovernmental Panel on Climate Change (IPCC) in 2027, this post aims to explain why this is a topic to watch.

What is GWP? A tool to measure and compare warming impact 

GWP is a way of comparing the warming impact of different greenhouse gases with carbon dioxide. Put simply, it asks: how much warming does a given amount of a gas cause over a specified period, compared with the same amount of CO₂?

CO₂ is assigned a GWP of 1. A different gas with a GWP of 400 therefore produces 400 times the warming impact of the same mass of CO₂ over the specified time horizon.

Two characteristics are particularly important to this calculation: how strongly a gas traps heat (heat absorption) and the time period over which the heat absorption is considered (time horizon). 

GWP is not a single, permanent number 

The most commonly used metric for greenhouse gas accounting is GWP100—the global warming potential over 100 years— but the impact of a gas can vary over different time horizons. Gases like methane have relatively short lifetimes after being emitted to the atmosphere and produce potent warming impacts while they remain. Over a 100-year time span, GWP100 for methane is generally around 27 to 30tCO2e, while the 20-year GWP20 is higher, around 80 to 83tCO2e. This difference reflects the fact that methane molecules start to disappear after 10 years or so, but the warming effect of the initial mass and any remaining molecules is averaged over the full 100 years.

While GWP100 is generally accepted as the standard for GHG accounting and corporate mitigation strategies, shorter time horizons can provide valuable insights into the near-term climate outcomes. As warming escalates, and the risk of crossing critical tipping points nears, consideration of shorter time horizons (indicated by GWP20) may become more relevant to mitigation strategies. We can do the carbon math for 100 years, but we may care deeply that the biggest impact is felt in the next 10 to 20 years.  

How will GWP values change over time? 

Today, corporate GHG accounting and projects in the carbon market commonly apply GWP values published by the IPCC. The most recent guidance on GWP values is provided in IPCC assessment report 6 (AR6). Each assessment report contains updates from the IPCC that account for the latest information on atmospheric lifetimes of specific greenhouse gases, radiative efficiencies, shifting atmospheric conditions and even improvements in measurements.

Notably, the latest GWP values may not be stipulated within all project methodologies. For instance, AR5 values are commonly accepted across corporate GHG accounting, and some carbon credit methodologies still use these values to calculate emission reduction claims even though they are not consistent with the latest data.

Let’s look at an example: CFC-12 shows how the numbers can change

Ozone-depleting substances (ODS) provide a useful illustration of the impact of GWP on superpollutant projects.

CFC-12 is a chlorofluorocarbon historically used in refrigeration and other applications. It is both an ozone-depleting substance and a powerful greenhouse gas.

Its GWP100 changed between successive IPCC assessment reports:

• CFC-12: AR5 = 10,200

• CFC-12: AR6 = 12,500

For example, a project might destroy two metric tons of CFC-12. With a GWP100 of 12,500 according to the most recent IPCC report, this translates to a climate impact of 25,000tCO2e. However, a project that uses GWP100 from AR5 results in an emission reduction of 20,400 tCO2e. Thus, under AR6, projects may claim nearly 5,000 more carbon credits for the same destruction activity. This example illustrates how changes in GWP can result in an outsized impact on credit claims for superpollutant projects.

Superpollutant accounting choices magnify over-crediting or under-crediting risks

For superpollutants, the effect of applying one GWP value over another (e.g., IPCC AR5 versus AR6 values) can be significant. 

If one tonne of a gas has a GWP of 10, one tonne of that gas represents 10 tonnes of CO₂e. But if the gas has a GWP of 1,000, the same physical tonne of that gas represents 1,000 tonnes of CO₂e. 

When evaluating a superpollutant credit, buyers will want to understand not only which GWP value is being used, but also the quantity of gas to which it is being applied, because these two pieces of the calculation work together. A project may accurately measure the amount of gas it destroys or avoids, but may apply an outdated GWP value if not otherwise specified in the crediting methodology. This will influence the resulting CO₂e and the number of credits claimed. 

Importantly, the choice of a specific GWP value does not by itself mean that a project is over-crediting. In our example above, using AR5 rather than AR6 values led to under-crediting. Rather, the fact that the values may differ over time highlights why the assumptions underlying the credit calculation deserve scrutiny.

Helpful considerations for buyers

As superpollutants become a larger part of the carbon market, buyers will want to understand the projects they support. Questions worth asking include:

• Which GWP values does the methodology require and which has the project applied (developers may opt to apply the most recent values, even when not required by a given methodology)?
• What time horizon is being applied?
• How much physical gas is the project claiming to destroy or avoid?
• How is that quantity measured or estimated?
• What other assumptions affect the conversion from the quantity of gas destroyed or avoided to credited tonnes?

These questions are relevant across the carbon market and are a key consideration for project developers. But they become especially important when dealing with superpollutant gases whose GWP can be thousands of times greater than CO₂. When the multiplier is that large, seemingly small differences in the underlying emission reduction calculation can have a large effect on the resulting number of tonnes claimed by a project.

New values for superpollutants are expected as the science evolves 

The science behind climate accounting will continue to evolve. As new IPCC assessments improve our understanding of greenhouse gases, their atmospheric lifetimes and their climate impacts, the values used in carbon accounting may evolve too. AR7 will be one to watch - while the full report is not expected until 2029, a report on superpollutants is expected to be published in 2027.

For buyers, understanding the climate benefit of a superpollutant credit means looking beyond the number of tonnes. It means understanding the science behind the multiplier and engaging with the project to learn how that multiplier is chosen and applied.

Superpollutants offer an important opportunity for near-term climate action. Making the most of that opportunity requires confidence that the credits used to finance it accurately represent the climate impact being claimed.