In short: A global warming potential (GWP) value converts any greenhouse gas into an equivalent amount of CO₂, so different gases can be added together into a single tons CO₂e figure. The value changes because IPCC assessment reports revise it as atmospheric chemistry modelling improves — and because the answer depends on whether you use a 20-year or 100-year time horizon.
The problem GWP solves
A personal carbon footprint includes more than carbon dioxide. Methane escapes from livestock digestion, landfill decomposition, and natural gas leaks. Nitrous oxide is released from fertilised soils and some fuel combustion. Fluorinated gases appear in refrigerants and industrial processes.
Each gas traps heat differently, and they persist in the atmosphere for different lengths of time. Without a way to translate them into a common unit, you cannot add them together. Global warming potential is that translation mechanism.
Global warming potential (GWP) is the amount of heat a greenhouse gas traps in the atmosphere over a defined time period, expressed as a multiple of carbon dioxide. CO₂ has a GWP of 1 by definition. A gas with a GWP of 30 causes 30 times more warming per kilogram than CO₂ over the same period.
Multiplying the physical mass of a gas by its GWP gives the CO₂-equivalent mass — the tons CO₂e figure used in carbon footprint calculations. This is the only unit Decarb uses.
Why the time horizon matters
GWP is always quoted relative to a time horizon — most commonly 20 years (GWP20) or 100 years (GWP100). The choice makes a significant difference for short-lived gases like methane, which breaks down in the atmosphere roughly ten times faster than CO₂.
Over 20 years, methane is a far more potent warming agent than its 100-year average suggests, because most of its warming effect occurs in the near term. Over 100 years, as methane has largely degraded, its average impact per year is lower.
| Gas | GWP20 | GWP100 (AR6) | Atmospheric lifetime |
|---|---|---|---|
| Carbon dioxide (CO₂) | 1 | 1 | Hundreds to thousands of years |
| Methane (CH₄) — fossil | 82.5 | 29.8 | ~12 years |
| Methane (CH₄) — biogenic | ~80 | 27.0 | ~12 years |
| Nitrous oxide (N₂O) | 273 | 273 | ~109 years |
| HFC-134a (refrigerant) | 4,144 | 1,526 | ~14 years |
Source: IPCC Sixth Assessment Report (AR6), Working Group I, Annex II, Table AII.1.2 (2021). All values ± uncertainty range not shown here.
GWP100 is the convention used in national greenhouse gas inventories, the Paris Agreement framework, and most corporate emissions reporting standards. It is the basis for the emission factors Decarb applies.
How the methane value has changed across IPCC reports
GWP values are not fixed. Each IPCC Assessment Report revises them as scientists improve their understanding of atmospheric chemistry, aerosol interactions, and climate feedbacks. The methane GWP100 value illustrates how meaningful these revisions can be.
Methane (CH₄) GWP100 values across IPCC Assessment Reports. AR5 to AR6 reflects improved accounting of climate-carbon cycle feedbacks. Sources: IPCC SAR (1995), TAR (2001), AR4 (2007), AR5 (2013), AR6 (2021).
The rise from AR4 to AR5 reflected new understanding of indirect climate interactions — particularly methane’s effect on tropospheric ozone and stratospheric water vapour. The slight adjustment in AR6 came from improved modelling of carbon cycle feedbacks not previously included.
These revisions are not errors. They are the scientific process working as intended. A value that no longer changes would suggest the underlying research had stopped.
Which GWP values Decarb uses
Decarb applies GWP100 values from IPCC AR6 (2021) throughout the calculator. This is consistent with the current international standard for national inventory reporting and the GHG Protocol Corporate Standard.
Note on existing methodology sources: Some emission factor datasets — including earlier versions of EPA WARM, which Decarb references for waste categories — were developed using AR5 or AR4 GWP values. Where this is the case, it is documented in the Decarb methodology. We do not silently upgrade underlying datasets; we record the GWP basis used in each source.
For food-related estimates, the distinction between fossil and biogenic methane sources matters. Livestock enteric fermentation is biogenic; natural gas leakage is fossil. AR6 assigns slightly different GWP100 values to each, and Decarb applies them accordingly.
How GWP affects your estimated footprint
For most people, the practical impact of GWP revision is concentrated in the food and waste categories. Transport and home energy are dominated by CO₂, which has a GWP of 1 and does not change.
A diet with regular beef and dairy consumption involves significant methane from enteric fermentation (in livestock). Applying AR6 methane GWP100 (27–29.8) rather than AR4 (25) increases estimated emissions from those sources by roughly 8–15%, depending on consumption levels.
Waste categories — particularly landfill — involve biogenic methane from decomposing organic matter. The same GWP revision applies, with similar proportional effects on estimated totals.
In absolute terms, the difference between AR4 and AR6 GWP values on an average personal footprint is likely 0.2–0.5 tons CO₂e per year — small relative to the total, but consistent in direction. Using the most current values available is the correct approach.
What this means for comparing calculators
If two carbon calculators give different totals for the same inputs, GWP values are one of several possible explanations. Other common sources of divergence include different emission factor databases, different scope definitions (what is and is not counted), and different assumptions about average consumption where direct data is not available.
Decarb documents all of these explicitly. The methodology page at decarb.co/methodology lists every emission factor source and the GWP basis applied — so the numbers are traceable, not just reported.
Frequently asked questions
What is global warming potential (GWP)?
Global warming potential is a standardised measure of how much heat a greenhouse gas traps in the atmosphere over a defined period, relative to CO₂. CO₂ has a GWP of 1 by definition. A gas with a GWP of 28 causes 28 times more warming per kilogram than CO₂ over the same time horizon.
Why does the GWP value for methane keep changing?
GWP values are revised in each IPCC Assessment Report as atmospheric chemistry modelling improves. The methane GWP100 rose from 21 (SAR, 1995) to 29.8 (AR6, 2021) as researchers improved accounting for indirect climate interactions. The value also depends on whether a 20-year or 100-year time horizon is used.
What time horizon does Decarb use for GWP?
Decarb uses GWP100 values — a 100-year time horizon — from IPCC AR6 (2021). This is the most common convention in national greenhouse gas inventories and corporate emissions reporting. Where source datasets use earlier IPCC report values, this is documented in the Decarb methodology.
How does GWP affect a personal carbon footprint calculation?
GWP values affect emissions estimates wherever non-CO₂ gases are involved — primarily food (livestock methane), waste (landfill methane), and some energy sources. The CO₂ majority of transport and home energy estimates is unaffected. Upgrading from AR4 to AR6 methane GWP values increases food-related estimates by roughly 8–15%.
Does CO₂e mean the same as CO₂?
No. CO₂e (carbon dioxide equivalent) converts any greenhouse gas into the amount of CO₂ that would cause the same warming over a given time horizon. It includes CO₂, methane, nitrous oxide, and fluorinated gases. A ton of CO₂e from methane represents far less physical gas than a ton of CO₂e from carbon dioxide.
Decarb methodology
See every emission factor we use — and where it comes from
The methodology page documents all GWP values, emission factor sources, and scope decisions behind the Decarb calculator.
Sources
- IPCC (2021). Sixth Assessment Report, Working Group I: The Physical Science Basis. Annex II, Table AII.1.2. ipcc.ch
- IPCC (2013). Fifth Assessment Report, Working Group I. Annex II, Table AII.1.2. Methane GWP100 = 28 (without climate-carbon feedbacks), 34 (with feedbacks).
- IPCC (2007). Fourth Assessment Report, Working Group I. Chapter 2, Table 2.14. Methane GWP100 = 25.
- IPCC (1996). Second Assessment Report. Methane GWP100 = 21.
- GHG Protocol (2023). Global Warming Potential Values. Based on IPCC AR6. ghgprotocol.org
- US EPA (2023). WARM Model. Emission factors for solid waste management categories. epa.gov/warm
