GWP Horizon · Principle 6 — Energy Efficiency
The Green Chemistry Portal
DodecaGreen The Green Chemistry Portal

GWP Horizon.

This tool does not calculate a process's carbon footprint. It teaches what "kg CO₂e" actually means: pick a gas, pick a time horizon (20, 100, or 500 years), and watch its CO₂-equivalent multiplier change. The 100-year convention used everywhere, including elsewhere on this site, is a policy choice, not a physical constant.

Principle 6 guide
01

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

−

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? It is a ratio of two cumulative radiative forcings, not a footprint. This tool does not compute kg CO₂e per kg of product, per experiment, or per anything else; it compares gases to each other, at whichever horizon you choose.

GoalShow that "kg CO₂e" is not a single fixed number for a given gas. It depends on a choice of time horizon, and that choice changes which gases look most important.
WhyEvery other GWP-based tool on this site, and almost every carbon accounting standard in the world, quietly picks GWP100. That is a defensible convention, inherited from the Kyoto Protocol, but it is a choice with consequences, not a law of physics.
HowPick one or more gases below, enter a mass, and toggle between 20-year, 100-year and 500-year horizons. Watch methane's multiplier roughly triple at 20 years compared to 100, while CO₂ stays fixed at 1 by definition.

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. Every kg CO₂e figure elsewhere on this site, and in almost every corporate and national emissions report, is a GWP100 figure, whether or not it says so.

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

Related to the Carbon Footprint tool

The Carbon Footprint Estimator on this site, like almost every carbon calculator anywhere, uses GWP100 without saying so. That is not wrong, GWP100 is a reasonable default, but it is worth knowing it is a default. If your process emits a lot of methane and you are reporting against a near-term climate target, a GWP20-weighted figure would look meaningfully worse. This tool exists to make that choice visible.

02

Gas & time-horizon selector

−

Choose a time horizon, then tick the gases you want to compare and enter a mass emitted for each (defaults to 1 kg). The GWP factor for the active horizon is highlighted in the table; the CO₂e result recalculates live. Can't find a gas you need? Add your own at the bottom of the table with your own GWP20 / GWP100 / GWP500 values.

Source: GWP20 / GWP100 / GWP500 values below are the assessed central estimates from IPCC AR6 WG1 Chapter 7 (2021), Table 7.15 (CO₂, CH₄ fossil/non-fossil, N₂O, HFC-32, HFC-134a) and Table 7.SM.7 (HFC-23, SF₆, NF₃). See References.
Gas Mass emitted (g) GWP20 GWP100 GWP500 CO₂e at active horizon (kg)
Found your own values? Use "Add custom gas" to enter a name and your own GWP20 / GWP100 / GWP500 figures, for example from a more recent assessment, a different gas not covered by AR6's headline tables, or a value from a peer-reviewed source. Custom rows are marked "user-supplied" throughout the tool, in the charts, the interpretation, and the CSV export, so it stays clear which numbers are IPCC AR6 and which are yours.
Σ CO₂e of selected gases at active horizon — kg CO₂e at GWP100
03

Results

−

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.
04

Caveats & limitations, read before citing a number

−
01
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.
02
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.
03
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.
04
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.
05
This has real consequences for the rest of this site. The Carbon Footprint Estimator and the (now archived) process-level GWP calculator both use GWP100 without stating that as a choice. This tool exists to make that choice explicit and defensible, not to replace either.
06
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.
05

Export

+

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.

06

Where can I read more?

+

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. DOI. — Underpins the GWP* metric referenced in the caveats section.
  2. 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. — GWP20 / GWP100 / GWP500 / GTP / CGTP values for CO₂, CH₄ (fossil and non-fossil), N₂O, HFC-32, HFC-134a, CFC-11 and PFC-14 used in this tool.
  3. 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. — GWP20 / GWP100 / GWP500 values for HFC-23, SF₆ and NF₃, which are not in the Table 7.15 headline list.
  4. 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.
07

Contributors

−

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: 13/08/2026

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.

08

How do I cite this page?

+

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

Reference
BibTeX