Global warming potential (GWP) is a number that tells you how much heat a greenhouse gas traps in the atmosphere compared to carbon dioxide. CO2 is the baseline, with a GWP of 1. Every other greenhouse gas gets a number relative to that. Methane, for example, has a 100-year GWP of 27 to 30, meaning one ton of methane traps roughly 28 times more heat than one ton of CO2 over a century.
How GWP Is Calculated
Two factors determine a gas’s GWP: how efficiently it absorbs heat, and how long it stays in the atmosphere. Scientists call the first factor “radiative efficiency,” which is essentially how good the gas is at catching infrared radiation (the heat that rises from Earth’s surface). Some gases absorb heat at wavelengths that CO2 misses entirely, making them far more potent per molecule.
The second factor is atmospheric lifetime. A gas that lingers for thousands of years keeps trapping heat the entire time, while one that breaks down in a decade stops contributing relatively quickly. GWP combines both factors into a single number by calculating the total heat energy a pulse of gas absorbs over a set time period, then dividing that by the total heat energy the same mass of CO2 would absorb over the same period.
Why the Time Horizon Matters
GWP always comes with a time window attached, usually 100 years. The 100-year version (GWP100) is the international standard used in climate treaties and national emissions reporting. But the choice of time horizon changes the numbers dramatically, especially for gases that don’t stick around very long.
Methane is the clearest example. It’s a powerful heat-trapper but breaks down in roughly a decade. Over 100 years, its GWP is 27 to 30. Over 20 years, its GWP jumps to 81 to 83, because most of its warming punch lands in that first couple of decades. The 100-year window dilutes that impact by averaging over 80 additional years when the methane is already gone.
The reverse happens for extremely long-lived gases. CF4, a synthetic compound used in electronics manufacturing, persists for about 50,000 years. Its 100-year GWP is 7,380, but its 20-year GWP is only 5,300. Because it keeps absorbing heat for millennia, a longer time window captures more of its total impact.
This isn’t just an academic distinction. Choosing GWP20 over GWP100 makes methane look roughly three times worse relative to CO2. That shifts policy priorities: a 20-year lens puts more urgency on cutting methane from agriculture and natural gas systems, while a 100-year lens emphasizes long-lived gases like CO2 itself.
CO2 Equivalents: Putting It Into Practice
GWP exists so that policymakers and analysts can compare apples to apples. When a country reports its total greenhouse gas emissions, it doesn’t just list tons of each gas separately. Instead, it converts everything into a single unit called CO2 equivalents (CO2e). The math is simple: multiply the mass of any gas by its GWP, and you get the equivalent amount of CO2 that would cause the same warming.
If a farm releases 100 metric tons of methane in a year, you multiply 100 by methane’s GWP of roughly 28, giving you 2,800 metric tons of CO2e. That number can then be directly compared to, say, 2,800 metric tons of actual CO2 from a power plant. This conversion is what makes national greenhouse gas inventories possible and what allows international agreements to set reduction targets across different sectors of the economy.
Which GWP Values Are Used Officially
International climate reporting under the UN Framework Convention on Climate Change currently relies on GWP values from the IPCC’s Fourth Assessment Report, published in 2007. That standard was locked in by a 2013 decision requiring countries to use those specific numbers in their annual emissions inventories, based on the 100-year time horizon. The IPCC has since published updated values in its Fifth and Sixth Assessment Reports, with some numbers shifting as scientists refine their models of atmospheric chemistry. But international reporting rules take time to update, so the official accounting still uses the older figures.
GWP Values for Major Greenhouse Gases
CO2 is always 1, by definition. Methane (from livestock, wetlands, landfills, and natural gas leaks) has a 100-year GWP of 27 to 30. Nitrous oxide, released by agricultural fertilizers and industrial processes, has a 100-year GWP of about 273. It persists in the atmosphere for over a century, making it a long-term concern even though it’s emitted in smaller quantities than CO2 or methane.
The synthetic fluorinated gases occupy the extreme end of the scale. Sulfur hexafluoride (SF6), used as an insulator in electrical equipment, has a 100-year GWP of roughly 25,200, making it the most potent greenhouse gas that the IPCC tracks. CF4 comes in at 7,380. These gases are emitted in tiny volumes compared to CO2, but even small leaks carry outsized climate consequences because of how long they persist and how efficiently they absorb heat.
Limitations of GWP
GWP is useful, but it has a fundamental limitation: it measures total accumulated heat over a time window, not the actual temperature change at any given point. For long-lived gases like CO2 that build up in the atmosphere, total accumulated heat is a reasonable proxy. For short-lived gases like methane, it can be misleading. A steady stream of methane emissions doesn’t keep adding new warming the way a steady stream of CO2 does, because old methane is breaking down as new methane is released.
This gap led researchers at the University of Oxford to propose an alternative called GWP*, which accounts for changes in emission rates rather than just total mass emitted. Under GWP*, stable methane emissions look very different from rising methane emissions. If a farm keeps its methane output flat year over year, GWP* treats that as contributing little additional warming, while conventional GWP100 still assigns the same large CO2-equivalent number every year. The distinction matters most for setting “climate neutral” targets in sectors like agriculture, where methane dominates the emissions profile. GWP* hasn’t replaced GWP100 in official reporting, but it’s increasingly part of the scientific conversation around how we measure warming fairly across different gases.