GWP Time Horizon
A tonne of methane leaking from a gas pipeline can be counted as roughly 30 tonnes of carbon dioxide — or as 83 tonnes. Nothing about the leak changes between those two numbers. What changes is a single accounting choice: the number of years over which you compare methane’s warming to that of CO₂. Pick 100 years and you get the smaller figure; pick 20 years and it nearly triples.
That choice is the GWP time horizon — the period over which a gas’s warming effect is integrated and measured against carbon dioxide when converting it to CO₂-equivalent.
The GWP time horizon is the number of years over which a gas’s global warming potential is calculated. Because gases decay at different rates the horizon changes the answer — methane is about 30× on GWP100 but 83× on GWP20. GWP100 is the accounting standard.
Definition — What a Time Horizon Is
The global warming potential (GWP) of a gas measures how much heat one tonne of it traps in the atmosphere compared with one tonne of carbon dioxide. But a gas’s warming is not delivered all at once — it plays out over years or centuries as the gas gradually breaks down. To turn that into a single number, you have to choose a window of time over which to add up the warming. That window is the time horizon.
Formally, GWP over a horizon of H years is the total (integrated) radiative forcing from releasing one tonne of the gas over the next H years, divided by the same quantity for one tonne of CO₂. Carbon dioxide is the reference, so its GWP is exactly 1 at every horizon. The IPCC publishes GWP values over 20, 100, and 500 years; in practice only the first two are widely used, written GWP20 and GWP100.
The horizon is a convention, not a law of physics. There is no scientifically “correct” number of years — 100 years was chosen by early climate policy and has stuck ever since. Because it is a choice, it has to be stated and applied consistently, which is why every credible CO₂e figure is implicitly or explicitly tied to a horizon.
Definition at a glance
| What it is | The number of years over which a gas’s warming is integrated and compared to CO₂ |
|---|---|
| Common horizons | 20 years (GWP20) and 100 years (GWP100); the IPCC also publishes 500 years |
| Reference gas | CO₂ — its GWP is 1 at every horizon, by definition |
| Why it matters | Dramatically changes the CO₂e of short-lived gases such as methane |
| Published by | IPCC assessment reports (current: AR6) |
| Standard for accounting | GWP100 — mandated by the GHG Protocol and used under the UNFCCC |
Why the Horizon Changes the Answer
The horizon matters because gases have very different atmospheric lifetimes. Methane is a powerful but short-lived gas — most of it is gone within about a decade. Over a 20-year window, almost all of methane’s warming is captured, and it is compared against a CO₂ molecule that has barely begun to decay. Stretch the window to 100 years and methane’s contribution is long finished, while CO₂ keeps trapping heat for the rest of the century — so methane’s relative potency shrinks. That is why a shorter horizon makes methane look far more potent.
The IPCC Sixth Assessment Report (AR6) publishes GWP values over 20, 100, and 500 years, treating none as uniquely correct — the horizon is a policy choice, not a scientific one. Greenhouse gas accounting standards then fix that choice: the GHG Protocol Corporate Standard and national inventories reported under the UNFCCC use 100-year GWP values, and ISO 14064-1 follows the same convention. GWP20 appears in scientific and policy analysis to highlight near-term warming, but not as the basis for compliance inventories.
The direction of the effect depends on whether a gas is shorter- or longer-lived than CO₂. Short-lived gases (methane, many refrigerants) look more potent on a 20-year horizon. Very long-lived gases can look less potent — because over just 20 years, CO₂ (the denominator) has not yet decayed away, so the long-lived gas’s advantage has not fully built up.
GWP-100 vs GWP-20 Across Gases
The table below shows how each major greenhouse gas is valued under the two horizons. The GWP-100 column is live — drawn straight from the MasterBrain’s IPCC AR6 values — while the GWP-20 column is IPCC AR6 published data. Watch the final column: the horizon pulls short-lived and long-lived gases in opposite directions.
| Gas | GWP-100 (live, AR6) | GWP-20 (AR6) | Effect of the 20-year horizon |
|---|---|---|---|
| Carbon dioxide (CO₂) | 1 | 1 | Reference — always 1 |
| Methane (CH₄, fossil) | 29.8 | 82.5 | ~2.8× higher — short-lived |
| Nitrous oxide (N₂O) | 273 | 273 | Essentially unchanged — medium-lived |
| HFC-134a (refrigerant) | 1,530 | 4,140 | ~2.7× higher — short-lived |
| Sulphur hexafluoride (SF₆) | 24,300 | 18,200 | Lower — very long-lived |
Methane and HFC-134a — both short-lived — roughly triple on the 20-year basis. Nitrous oxide, with a lifetime of around a century, barely moves. And sulphur hexafluoride, which lingers for thousands of years, is actually valued lower at 20 years than at 100, because its enormous long-term advantage over CO₂ has not yet accumulated. The horizon is not a simple “make everything bigger” dial.
A Worked Example: One Tonne of Methane
Take a single tonne of fossil methane — a routine figure for a gas leak or a landfill vent — and convert it to CO₂-equivalent under each horizon:
The same one tonne of methane. On the 100-year basis (green) it counts as its live AR6 GWP-100 value; on the 20-year basis (amber) it counts as 82.5 tonnes of CO₂e (IPCC AR6). Nothing physical differs between the bars — only the span of years chosen. This is why a report’s headline “tonnes of CO₂e” is meaningless without stating the horizon.
Report the same methane under GWP20 instead of GWP100 and your methane-heavy footprint nearly triples overnight — without a single molecule changing. The horizon is an accounting lens, not a measurement.
Which Horizon Should You Use?
For almost all greenhouse gas accounting, the answer is GWP100. It is the value mandated by the GHG Protocol, used in national inventories under the UNFCCC, and assumed by every mainstream disclosure framework and carbon market. If you are building a corporate inventory, calculating a product footprint, or reporting to a regulator, use 100-year values — and cite the assessment report they come from (AR5 or AR6), because those revise over time.
GWP20 has a legitimate but narrower role: it makes the near-term warming of short-lived gases visible, which matters for methane-heavy sectors — oil and gas, agriculture, waste — where near-term action has outsized value. It is a useful analytical lens, not a reporting standard. The one rule that never bends: do not mix horizons within a single inventory. Every gas in a footprint must be converted on the same basis, or the total is meaningless. You can quote a company’s methane on GWP20 to make a point, but you cannot add GWP20 methane to GWP100 CO₂ and call the sum a footprint. To see the effect for yourself, the GWP calculator and the methane-to-CO₂e converter both apply the 100-year AR6 basis.
Common Confusions
- Quoting a CO₂e figure without stating the horizon. A “tonne of CO₂e” is ambiguous until you say whether it is GWP100 or GWP20 — for methane the two differ by nearly 3×.
- Mixing horizons in one inventory. Every gas must use the same basis. GWP20 methane plus GWP100 CO₂ does not add up to a valid total.
- Assuming a shorter horizon always means a bigger number. It raises short-lived gases but lowers very long-lived ones like SF₆, because CO₂ has not yet decayed at 20 years.
- Confusing the horizon with the assessment report. GWP100 vs GWP20 is the time window; AR5 vs AR6 is which IPCC report the value comes from. Both must be stated, and they are independent choices.
- Thinking 100 years is scientifically “correct”. It is a policy convention. The IPCC treats 20, 100, and 500 years as equally valid physically; accounting standards simply picked 100.
Frequently Asked Questions
The GWP time horizon is the number of years over which a greenhouse gas’s warming effect is integrated and compared to carbon dioxide when calculating its global warming potential. The two common choices are 100 years (GWP100) and 20 years (GWP20); the IPCC also publishes 500-year values. Because gases break down at different rates, the horizon changes the answer: a gas is compared against a CO₂ reference that decays over the same window. Carbon dioxide’s GWP is 1 at every horizon by definition. The 100-year horizon is the standard used in greenhouse gas accounting.
GWP100 measures a gas’s warming over 100 years relative to CO₂; GWP20 measures it over 20 years. The shorter horizon captures more of a short-lived gas’s warming before it decays, so it makes those gases look far more potent. Under IPCC AR6, one tonne of fossil methane is worth 29.8 tonnes of CO₂e [GreenCalculus gwp.CH4_fossil.ar6_100] on the 100-year basis but 82.5 on the 20-year basis — nearly triple. GWP100 is the accounting standard used by the GHG Protocol and the UNFCCC; GWP20 is used to highlight near-term warming, especially from methane.
Because methane is short-lived — most of it is gone from the atmosphere within about a decade. Over a 20-year window almost all of methane’s warming has happened, and it is compared against a CO₂ molecule that has barely started to decay, so methane looks very potent. Over 100 years methane’s warming is long finished while CO₂ keeps trapping heat, so methane’s relative potency falls. That is why methane’s value swings from about 30 tonnes of CO₂e per tonne on GWP100 to roughly 83 on GWP20. Long-lived gases like nitrous oxide show almost no difference between the two horizons.
GWP100. The 100-year horizon is mandated by the GHG Protocol Corporate Standard, used in national inventories under the UNFCCC, and assumed by mainstream disclosure frameworks and carbon markets. Companies should use 100-year values and state which IPCC assessment report they come from (AR5 or AR6). GWP20 is a legitimate analytical lens for highlighting near-term warming in methane-heavy sectors, but it is not a reporting basis. The essential rule is never to mix horizons within one inventory — every gas must be converted on the same basis or the total is invalid.
No. A shorter horizon raises the value of gases that are shorter-lived than CO₂, such as methane and many refrigerants — often roughly tripling them. But for very long-lived gases it can do the opposite. Sulphur hexafluoride (SF₆), which persists for thousands of years, has a GWP of 24,300 [GreenCalculus gwp.SF6.ar6_100 · IPCC AR6 WGI Ch 7 Table 7.SM.7 (2021) — AR6 GWP-100] over 100 years but only 18,200 over 20 years — lower, because over just two decades CO₂ (the reference) has not yet decayed, so SF₆’s enormous long-term advantage has not fully accumulated. The horizon shifts gases in different directions depending on how their lifetime compares to that of CO₂.
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