Global warming potential

• 1. Introduction

  ›

  Global warming potential is the invisible yardstick that lets scientists, governments, and companies compare the heat-trapping power of very different gases on a single common scale. It works by asking a deceptively simple question: if you released a given mass of some gas into the atmosphere, how much warming would it cause compared to releasing the same mass of carbon dioxide? Carbon dioxide is set at a value of exactly 1, and every other greenhouse gas is measured against it.

  What makes the answer complicated is time. A gas that traps heat intensely but breaks down within a decade looks very different from one that lingers for thousands of years. Methane, for instance, has a global warming potential of around 81 over a 20-year window, but that number falls to roughly 28 over a 100-year window. Sulfur hexafluoride, by contrast, stays in the atmosphere for about 3,200 years and its warming potential grows the longer you look.

  This documentary traces how that single dimensionless number became a cornerstone of international climate policy, what its formula actually measures, why the choice of time horizon can shift the apparent severity of an emission by a factor of three or more, and where the metric's critics believe it falls short.

• 2. The Physics Behind the Number

  ›

  Radiative forcing is the concept that makes GWP calculable. It measures the change in energy flux in the atmosphere, expressed in watts per meter squared, caused by a driver of climate change. When a greenhouse gas absorbs outgoing infrared radiation that would otherwise escape to space, it increases that energy flux and warms the surface below.

  Not every wavelength of radiation is equally useful for a gas to absorb. If the atmosphere already soaks up most radiation at a particular wavelength, adding more of a gas that absorbs there makes little additional difference. A gas has the most impact when it absorbs in a window where the atmosphere is relatively transparent, allowing it to capture energy that would otherwise be lost.

  The radiative forcing capacity of a gas can be broken down into contributions from narrow bands across the infrared spectrum, with each band contributing according to how strongly the gas absorbs in that interval and how large the existing energy flux is. Because GWP depends directly on a gas's infrared spectrum, infrared spectroscopy has become a central tool for studying how human activities affect the climate.

  For gases like methane and nitrous oxide, the relationship between atmospheric concentration and radiative absorption is not perfectly linear. As carbon dioxide concentrations rise, each additional unit has a smaller effect because those absorption bands are already partly saturated. Methane and nitrous oxide absorb at frequencies that are not as saturated, so rising concentrations of those gases have a proportionally larger effect. This non-linearity means that all GWP values are slightly lower than a more detailed calculation would produce, a point the IPCC has flagged explicitly.

• 3. Why Time Horizon Matters

  ›

  Methane offers the clearest illustration of why the choice of time scale is not a technicality but a consequential policy decision. Over 20 years, methane's GWP sits at roughly 81-83; over 100 years it drops to around 27-30; over 500 years it falls further to about 7-10. The reason is that methane has an atmospheric lifetime of roughly 12 years. It breaks down through chemical reactions in the atmosphere, eventually forming water and carbon dioxide. A shorter time horizon catches it at full force; a longer one reflects the fact that much of it has already decomposed.

  Sulfur hexafluoride moves in the opposite direction. Its GWP over 20 years is around 17,500, and over 100 years it climbs to about 23,500, because its 3,200-year atmospheric lifetime means it is barely diminished over either window. Tetrafluoromethane, with a lifetime of around 50,000 years, shows the same pattern even more starkly.

  The IPCC has consistently used 100 years as the standard time horizon for most reporting purposes, and regulators around the world have adopted that convention. But the choice is not scientifically mandated. New York State's Climate Leadership and Community Protection Act is one documented exception: it requires the use of 20-year GWP values rather than 100-year ones, making it distinct from all other jurisdictions participating in phase-downs of hydrofluorocarbons under the Kigali Amendment.

  The IPCC Fifth Assessment Report stopped including 500-year values in its standard tables, though it introduced a new variant that incorporates climate-carbon feedback, noted as carrying a large amount of uncertainty.

• 4. Carbon Dioxide Equivalent

  ›

  Once a gas has a GWP value, its emissions can be converted into a carbon dioxide equivalent mass, abbreviated as CO2e or CO2eq. The calculation is direct: multiply the mass of the gas by its GWP. Two tonnes of a gas with a GWP of 100 produce a CO2e of 200 tonnes; nine tonnes of the same gas produce 900 tonnes CO2e.

  This single conversion makes it possible to add together the climate impacts of methane leaks from a gas pipeline, nitrous oxide from agricultural soils, and hydrofluorocarbons from refrigeration units into one total expressed in a familiar unit. On a global scale, CO2e is typically expressed in gigatonnes, the symbol being Gt. At industrial and national reporting levels, million metric tonnes of CO2 equivalent, abbreviated MMTCDE, is the common unit.

  The concept extends beyond mass to concentration. An atmospheric CO2e concentration of 500 parts per million means the actual mix of all greenhouse gases and aerosols in the air would warm the Earth as much as 500 parts per million of CO2 alone would. Converting a mixed-gas atmosphere to that figure requires knowing the atmospheric concentration of each gas, its GWP, and the ratio of its molar mass to the molar mass of CO2.

  For transportation, a further derived quantity expresses CO2e per distance travelled, with standard units of grams per kilometer, written as gCO2e/km, allowing direct comparison of different vehicles or fuels.

• 5. Policy and International Agreements

  ›

  In 1997, the Conference of the Parties to the Kyoto Protocol made a foundational decision, recorded as decision 2/CP.3, that GWP values from the IPCC Second Assessment Report would be the standard for converting different greenhouse gas emissions into comparable equivalents for international reporting.

  That standard was updated in 2013 at the Warsaw meeting of the UN Framework Convention on Climate Change, recorded as decision 24/CP.19. The new requirement drew on GWP values from the IPCC Fourth Assessment Report, which had been published in 2007. Those 2007 figures remained in use for international comparisons through 2020, even as the latest scientific research had produced revised estimates.

  The Kigali Amendment to the Montreal Protocol extended this framework to hydrofluorocarbons, a group of high-GWP synthetic compounds. Countries party to the amendment are required to use GWP100 values from the IPCC Fourth Assessment Report for their phase-down calculations, creating a single fixed reference that does not shift as new assessment reports appear. This standardisation means countries are comparing emissions against the same baseline rather than using figures from different assessment cycles.

  One consequence of this architecture is that countries and companies often continue using older GWP values from the Second and Fourth Assessment Reports even when newer research exists, because consistency across time and between reporting entities is valued over scientific currency.

• 6. Alternative Metrics and Their Critics

  ›

  The global temperature change potential, or GTP, offers one alternative framing. Rather than estimating how much infrared radiation a gas absorbs, GTP estimates the actual rise in average surface temperature the gas would cause over a chosen horizon, relative to the temperature rise the same mass of CO2 would cause. Because oceans absorb heat at complex rates, calculating GTP requires modelling how the whole climate system responds, making it a more demanding computation.

  A more contested proposal is GWP*, sometimes written GWP-star. Its proponents argue it gives a better account of short-lived climate pollutants such as methane. The reasoning is that a sustained increase in the rate of emitting a short-lived gas has a similar effect on radiative forcing as a one-time pulse of CO2, because both produce a lasting increase in atmospheric warming. GWP-star therefore assigns an increase in emission rate of a short-lived gas an equivalent number of tonnes of CO2.

  Critics have raised two objections to GWP-star. The first is a technical dispute about whether it is a suitable metric at all. The second is an equity concern: developing countries whose methane emissions are still growing are penalised under GWP-star, while developed countries such as Australia or New Zealand that have steady methane emissions are not penalised for those emissions, though they may still be held accountable for their CO2.

  Water vapour illustrates another edge case. It absorbs infrared radiation across a broad and potent spectrum, broader and with more absorption bands than CO2. But its 100-year GWP is negligible, estimated somewhere between -0.001 and 0.0005. The reason is that the GWP definition is based on emissions, and human emissions of water vapour from sources such as cooling towers and irrigation are removed by precipitation within weeks, well before they can accumulate.