GWP Horizon · Principle 6 — Energy Efficiency
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GWP Horizon.

Pick a gas, pick a time horizon (20, 100, or 500 years), and watch its CO₂-equivalent multiplier change.

Principle 6 guide
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What is GWPₕ, and why does the time horizon matter?

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Global Warming Potential over a horizon $h$ (GWP$_h$) answers one question: how many kilograms of CO₂ would need to be released to trap the same amount of extra heat, over $h$ years, as releasing 1 kg of some other gas? This tool compares the cumulative radiative forcings of different gases to each other, at whichever horizon you choose.

GoalPut the same gas's CO₂e side by side at 20, 100 and 500 years, and see how much its ranking against other gases can shift depending on which horizon is chosen.
WhyEvery carbon footprint, product label and national inventory you've seen reports emissions in CO₂e as if it were one objective figure. Nearly all of them default to GWP100 without saying so, a convention from the 1997 Kyoto Protocol, not a measurement, and a different horizon can tell a different story.
HowPick one or more gases below, enter a mass, and toggle between 20-year, 100-year and 500-year horizons.

The definition

$$\text{GWP}_h(X) = \frac{\displaystyle\int_0^h a_X\,[X](t)\,dt}{\displaystyle\int_0^h a_r\,[\text{CO}_2](t)\,dt}$$
SymbolTermUnits
\(h\)Chosen time horizon: 20, 100, or 500 yearsyears
\(a_X\)Radiative efficiency of gas \(X\): extra heat trapped per unit increase in its atmospheric concentrationW m⁻² ppb⁻¹
\([X](t)\)Mass of gas \(X\) remaining in the atmosphere \(t\) years after a 1 kg pulse emission (its decay curve)kg
\(a_r,\,[\text{CO}_2](t)\)The same two quantities for the reference gas, CO₂as above
\(\text{GWP}_h(X)\)Result: kg CO₂ needed to match gas \(X\)'s cumulative forcing to time \(h\)kg CO₂e / kg gas

The numerator and denominator are both integrals of radiative forcing over time, not emissions or masses directly. Short-lived, powerful gases like methane front-load almost all of their warming into the first two decades, so their multiplier is much larger at $h=20$ than at $h=100$. Long-lived gases decay slowly, so their multiplier barely changes, or even grows, at longer horizons.

Why 100 years?

There is no physical reason to prefer 100 years over 20 or 500. IPCC AR6 says so explicitly: it "does not recommend an emission metric because the appropriateness of the choice depends on the purposes for which gases or forcing agents are being compared." GWP100 became the default because the Kyoto Protocol adopted it for treaty accounting in 1997, and the convention stuck.

Strengths and limitations

Strengths

  • A single, internationally standardised way to compare gases with wildly different atmospheric lifetimes
  • Directly traceable to physical radiative forcing, not an economic or political judgement
  • Makes the true cost of long-lived, ultra-potent gases such as SF₆ and PFCs impossible to hide behind low emitted mass
  • Choosing a shorter horizon (GWP20) foregrounds near-term methane mitigation, relevant to staying below 1.5 to 2°C this decade

Limitations

  • GWP is a static multiplier for a one-off pulse emission; it says nothing about the rate or timing of warming from an ongoing emissions stream
  • It does not capture non-linear atmospheric chemistry beyond the chemical adjustments AR6 already folds into CH₄ and N₂O; higher-order feedbacks are not included
  • The time horizon is a policy default, not a measurement; changing it changes which gases look worst, without any new science
  • GWP100-based net-zero targets can still permit rising temperatures if short-lived and long-lived gases are not sequenced correctly, a point IPCC AR6 raises directly (Box 7.3)
  • Newer metrics such as GWP* and Combined-GTP (CGTP) are designed to track actual temperature response better than any single-horizon GWP, but are not implemented in this tool
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Gas & time-horizon selector

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Choose a time horizon, then search the library to add gases — or use "Other" for one not listed — and enter a mass emitted for each. The active horizon's column is highlighted below, and the CO₂e result recalculates live.

Source: GWP values are compiled from openclimatedata/globalwarmingpotentials (gwp_values.csv, downloaded 08/2026), covering every IPCC assessment report — SAR (1995) through AR6 (2021) — for ~105 species. Not every report published every horizon (20- and 500-year values only exist where TAR or AR6 did), so each gas's dropdown offers only the reports it actually has, e.g. HFC-134a's GWP100 has shifted from 1300 (SAR) to 1530 (AR6). "Other" rows use your own values instead, and are marked "user-supplied" throughout. See References.
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Gas & source Mass emitted (g) GWP20 GWP100 GWP500 CO₂e at active horizon (kg)
Σ CO₂e of selected gases at active horizon — kg CO₂e at GWP100
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Results

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CO₂e of selected gases at the active horizon

Same emission, three time horizons

The right-hand chart is the point of this tool: the same entered mass of each selected gas, priced in CO₂e at all three horizons side by side. A gas whose bars fall steeply from left to right is short-lived and front-loads its warming; a gas whose bars stay flat, or rise, is long-lived and its impact does not fade with time.

Interpretation

Select at least one gas and enter a mass above to generate an interpretation.
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Caveats & limitations, read before citing a number

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GWP is a static multiplier, not a warming curve. It is calculated for a single pulse emission, at one point in time. It says nothing about the rate at which warming actually happens, or about a continuous emissions stream rather than a one-off release.
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It does not fully capture non-linear atmospheric chemistry. AR6 folds in known chemical adjustments for CH₄ and N₂O (their indirect effects on ozone and stratospheric water vapour), but higher-order feedbacks and interactions between gases are not modelled here.
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The 100-year horizon is a policy default, not a physical law. IPCC AR6 explicitly declines to recommend a single metric, precisely because the right horizon depends on what question you are asking. GWP100's dominance comes from the Kyoto Protocol's 1997 accounting rules, not from any measurement.
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Horizon choice changes which gases look worst. Methane's GWP20 is roughly triple its GWP100. A process, product, or country that looks fine on a GWP100 basis can look very different at GWP20, and neither number is "more correct" than the other; they answer different questions.
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Better metrics exist for some questions. GWP* and Combined-GTP (CGTP) are newer approaches designed to relate short-lived gas emissions to actual temperature outcomes more faithfully than any single-horizon GWP. They are referenced in IPCC AR6 Table 7.15 but are not implemented in this tool.
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Export

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Export the current gas comparison as a CSV data file, or the charts as PNG or SVG image files. Everything runs in your browser; nothing is sent to a server.

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Where can I read more?

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References are sorted alphabetically by first author.

  1. M. R. Allen, J. S. Fuglestvedt, K. P. Shine, A. Reisinger, R. T. Pierrehumbert and C. J. Forster, "New use of global warming potentials to compare cumulative and short-lived climate pollutants," Nat. Clim. Change, 2016, 6, 773–776. Article. — Underpins the GWP* metric referenced in the caveats section.
  2. P. Balcombe, J. F. Speirs, N. P. Brandon and A. D. Hawkes, "Methane emissions: choosing the right climate metric and time horizon," Environ. Sci.: Processes Impacts, 2018, 20, 1323–1339. Article. — A worked case study (LNG as shipping fuel) of exactly the horizon-choice problem this tool visualises.
  3. IPCC, "The Earth's Energy Budget, Climate Feedbacks and Climate Sensitivity," in Climate Change 2021: The Physical Science Basis. Sixth Assessment Report (AR6), Working Group I, Chapter 7, Table 7.15, Cambridge University Press, 2021. PDF. — Primary source for this tool's AR6 GWP20 / GWP100 / GWP500 figures, reached via the openclimatedata compilation below.
  4. IPCC, "Chapter 7 Supplementary Material," in Climate Change 2021: The Physical Science Basis. Sixth Assessment Report (AR6), Working Group I, Table 7.SM.7, Cambridge University Press, 2021. PDF. — Primary source for the AR6 figures of species not in the Table 7.15 headline list.
  5. D. A. Lashof and D. R. Ahuja, "Relative contributions of greenhouse gas emissions to global warming," Nature, 1990, 344, 529–531. Article. — One of the papers that established comparing gases via a single time-integrated multiplier, the idea GWP formalises.
  6. G. Myhre, D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, "Anthropogenic and Natural Radiative Forcing," in Climate Change 2013: The Physical Science Basis. Fifth Assessment Report (AR5), Working Group I, Chapter 8, Cambridge University Press, 2013. — Established the pulse-emission AGWP methodology that AR6 Table 7.15 builds on; primary source for this tool's AR5 figures.
  7. Open Climate Data, "globalwarmingpotentials," GitHub repository, accessed August 2026. GitHub. — The compiled dataset (gwp_values.csv) this tool's gas library and per-gas assessment-report picker are built from; collates SAR, TAR, AR4, AR5 and AR6 GWPs for ~105 species into one table.
  8. UNFCCC, "Report of the Conference of the Parties on its Third Session, Addendum: Kyoto Protocol to the United Nations Framework Convention on Climate Change," FCCC/CP/1997/7/Add.1, 1997. PDF. — Kyoto Protocol decision text; source of the GWP100/100-year accounting convention referenced throughout this tool.
  9. U.S. Environmental Protection Agency, "Understanding Global Warming Potentials." Web page. — A shorter, plain-language explainer of GWP; a good starting point alongside this tool.
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Contributors

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Roles follow the CRediT taxonomy (Contributor Roles Taxonomy), adapted for educational software. Hover a contributor's name for a summary, or a column header for the definition of that role.

Contributor

© 2025– DodecaGreen Project. All rights reserved. · Last updated: 17/08/2026

This portal was built with the assistance of a large language model (Claude, Anthropic), which was used to generate and refine code, articulate and structure contributed ideas within the defined page format, and support iterative design decisions. All scientific content, conceptual frameworks, pedagogical choices, and final outputs were directed, reviewed, and verified by the contributors listed above.

Whilst every effort has been taken to ensure accuracy, mistakes can happen. If you notice something that doesn’t look quite right, kindly reach out to the DodecaGreen team via the Contact page.

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How do I cite this page?

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If you use this tool in teaching or published work, please cite the DodecaGreen portal as the source.

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