What is Global Warming Potential (GWP)?

Quick Answer: Global Warming Potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a specific time period, relative to carbon dioxide. It is expressed as a multiplier: a gas with a GWP of 25 warms the atmosphere 25 times more than the same mass of CO2 over 100 years. GWP values are the basis for converting all greenhouse gas emissions into a single unit, CO2 equivalent (CO2e), which makes it possible to compare and total emissions across different gases.

What is Global Warming Potential?

Global Warming Potential is a standardised index that quantifies how much energy one tonne of a given greenhouse gas will absorb over a set time horizon, compared to one tonne of carbon dioxide. The higher the GWP, the more warming that gas causes per unit of mass.

The concept was developed by the Intergovernmental Panel on Climate Change (IPCC) and is now the standard tool used in climate science, carbon accounting, and emissions reporting worldwide. GWP values are periodically updated as scientific understanding improves, with the IPCC's Assessment Reports serving as the authoritative source.

The most commonly used time horizon is 100 years, referred to as GWP100. This is the default in most reporting frameworks, including the GHG Protocol, which underpins the majority of corporate carbon footprints.

How GWP Values Are Calculated

GWP is calculated by comparing the cumulative radiative forcing (the warming effect) of a gas over a chosen time period against that of CO2 over the same period. CO2 is assigned a GWP of 1 as the baseline.

Two factors determine a gas's GWP:

• Radiative efficiency: how effectively the gas absorbs infrared radiation
• Atmospheric lifetime: how long the gas persists in the atmosphere before breaking down

A gas that is highly efficient at trapping heat but breaks down quickly may have a high GWP over 20 years but a lower one over 100 years. This is why the choice of time horizon matters, and why some reporting contexts use GWP20 rather than GWP100 when the focus is on near-term climate impact.

GWP Values for Common Greenhouse Gases

The GHG Protocol requires companies to account for seven greenhouse gases, each with a distinct GWP100 value. These values come from the IPCC's Fifth Assessment Report (AR5), which most corporate reporting frameworks currently reference.

• Carbon dioxide (CO2): GWP = 1 (the baseline)
• Methane (CH4): GWP = 28-36 (AR5 value: 28 for fossil methane, 34 including climate-carbon feedbacks)
• Nitrous oxide (N2O): GWP = 265-298 (AR5 value: 265)
• Hydrofluorocarbons (HFCs): GWP varies widely by compound, ranging from 138 to over 12,000
• Perfluorocarbons (PFCs): GWP ranges from approximately 6,630 to 11,100
• Sulphur hexafluoride (SF6): GWP = 23,500
• Nitrogen trifluoride (NF3): GWP = 16,100

These figures illustrate why industrial gases like SF6 and certain HFCs represent significant risks even in small quantities. A single tonne of SF6 has the same warming impact as 23,500 tonnes of CO2.

Why Does GWP Matter for Carbon Accounting?

GWP is what makes carbon accounting possible at scale. Without it, a company would need to report separate totals for each greenhouse gas with no way to combine or compare them. By multiplying the mass of each gas by its GWP value, every emission can be expressed in CO2 equivalent (CO2e), creating a single comparable figure.

This matters in practice because:

• Scope 1 emissions often include gases beyond CO2, particularly methane from combustion or refrigerant leaks from HFCs. GWP converts these into the same unit as CO2 so they can be included in a total footprint.
• Scope 3 emissions from supply chains frequently involve agricultural processes (which produce methane and nitrous oxide) and industrial manufacturing (which may involve HFCs or PFCs). Accurate GWP application is essential for these categories to be reported correctly.
• Target-setting under frameworks like the Science Based Targets initiative (SBTi) requires a CO2e baseline. GWP is the conversion mechanism that makes that baseline meaningful.

For anyone building a GHG inventory aligned with the GHG Protocol, selecting the correct GWP values and applying them consistently is a methodological requirement, not an optional detail.

What Happens When GWP Values Are Updated?

The IPCC updates GWP values with each Assessment Report as scientific measurement improves. The shift from AR4 to AR5 values, for example, changed the GWP of methane from 25 to 28 (fossil) or 34 (biogenic, including feedback effects). The sixth Assessment Report (AR6, 2021) has since revised these figures further.

This creates a practical challenge for companies: if GWP values change between reporting years, historical footprints calculated using older values are no longer directly comparable to current ones. Most frameworks address this by requiring companies to apply a single consistent set of GWP values across all years and to restate historical data if they switch to a new version.

Seedling applies GWP values aligned with the reporting framework most appropriate to each client's context, with clear documentation of which IPCC Assessment Report values have been used. This means your footprint remains auditable and comparable year-on-year, without requiring you to track methodology changes yourself.

GWP20 vs GWP100: Why the Time Horizon Matters

Most corporate carbon footprints use GWP100 as the default, but GWP20 is increasingly referenced in scientific and policy contexts where near-term warming is the priority.

The difference is significant for methane. Under GWP100, methane has a value of approximately 28. Under GWP20, that value rises to around 80, because methane's warming effect is concentrated in the first two decades before it breaks down. For industries with high methane emissions, such as agriculture, oil and gas, or waste management, using GWP20 produces a substantially higher CO2e figure.

Neither time horizon is universally correct. GWP100 provides a long-term view that aligns with net zero targets set for 2050. GWP20 captures the urgency of short-lived but potent gases. The important thing is to state clearly which time horizon you are using, and to apply it consistently across all gases and all reporting periods.

Getting the GWP values right, applying them consistently, and documenting your methodology are what separate a defensible GHG inventory from one that will not hold up to scrutiny.