Global Warming Potential Calculator

Global warming potential expresses the radiative energy absorbed by a pulse emission of one kilogram of a gas, integrated over a chosen time horizon, divided by the same integral for one kilogram of CO2. Two properties set the value: radiative efficiency, meaning how strongly the molecule absorbs infrared radiation in the atmospheric window, and atmospheric lifetime. Methane absorbs strongly per molecule but is oxidized by hydroxyl radicals with a perturbation lifetime near 12 years, which is why its AR6 values fall from 82.5 at 20 years to 29.8 at 100 years and 7.6 at 500. Nitrous oxide, with a lifetime near 109 years, changes little between the 20 and 100 year horizons, sitting close to 273 at both. Very long-lived synthetic gases behave in the opposite direction. Because the CO2 reference is itself partly removed by ocean and land uptake over centuries while SF6 and CF4 persist essentially undiminished, their GWP rises with horizon: SF6 goes from 18,300 at 20 years to 25,200 at 100 and about 34,000 at 500. The 100-year value is the reporting convention under the UNFCCC and the Greenhouse Gas Protocol, but it is a policy choice rather than a physical constant, and that choice materially changes how methane-intensive sectors such as livestock and gas production rank against others. GWP also treats a pulse of a short-lived gas as though it were equivalent to a permanent CO2 addition, which is why alternative metrics exist for sustained methane flows.

The metric was created for the IPCC First Assessment Report in 1990, which needed a way to compare a basket of gases in a single index for negotiators drafting what became the UN Framework Convention on Climate Change. The Kyoto Protocol in 1997 fixed the 100-year horizon and adopted values from the IPCC Second Assessment Report of 1995, under which methane carried a GWP of 21. Successive assessments revised the numbers as spectroscopy, lifetime estimates, and background concentrations improved: methane moved to 23 in 2001, 25 in 2007, 28 in 2013, and 29.8 in AR6 in 2021, that last increase reflecting the first inclusion of climate-carbon cycle feedbacks. Reporting practice lags the science deliberately, since changing factors mid-period breaks comparability; UNFCCC inventory reporting shifted to AR4 values during the 2010s and later toward AR5, and many corporate standards still use them. Fluorinated gases became a GWP story of their own after the Montreal Protocol of 1987 phased out ozone-depleting CFCs and their HFC replacements proved to be potent greenhouse gases; the Kigali Amendment, agreed in 2016 and in force from 2019, commits parties to phase down HFC production and consumption. A newer metric, GWP*, proposed by Myles Allen and colleagues in 2018, expresses sustained methane emission rates in warming-equivalent terms and remains actively debated.