Global warming potential is a score that compares how much heat a greenhouse gas traps with the heat trapped by carbon dioxide, which is how emissions of different gases are added into one CO2-equivalent total.
This page belongs to the climate change explained guide. It summarizes how public agencies define and apply the metric, and it is not advice.
What the metric measures
Greenhouse gases differ in two main ways. The US Environmental Protection Agency says that two key ways in which these gases differ from each other are their ability to absorb energy and how long they stay in the atmosphere. A gas that absorbs a lot of energy but disappears quickly can matter less over a century than a weaker gas that lingers.
Global warming potential, or GWP, folds both properties into one number. EPA describes it as a measure of how much energy the emission of 1 ton of a gas will absorb over a given period of time, relative to the emission of 1 ton of carbon dioxide. The larger the GWP, the more a given gas warms the Earth compared with CO2 over that period. The agency adds that the time period usually used is 100 years, and that CO2 has a GWP of 1 by definition because it is the reference gas.
The idea is not new. EPA notes that the Intergovernmental Panel on Climate Change has used the measure since 1990 to allow comparisons of the global warming impacts of different gases, and it has become the standard shorthand in national inventories and corporate reporting.
Lifetime is the other half of the story. EPA states that CO2 emissions cause increases in atmospheric concentrations of CO2 that will last thousands of years, whereas methane emitted today lasts about a decade on average but absorbs much more energy per tonne than CO2. The net effect of a shorter lifetime and higher energy absorption is what the methane GWP reflects.
From GWP to CO2 equivalents
A GWP is useful because it allows addition. EPA explains that GWPs provide a common unit of measure, which allows analysts to add up emissions estimates of different gases, for example to compile a national inventory, and lets policymakers compare reduction opportunities across sectors and gases.
The conversion is a multiplication. Eurostat’s glossary defines a CO2 equivalent as a measure that compares emissions by converting amounts of other gases to the equivalent amount of carbon dioxide with the same global warming potential. Its worked example uses older values: with a methane GWP of 25 and a nitrous oxide GWP of 298, one million tonnes of each gas counts as 25 and 298 million tonnes of carbon dioxide respectively.
The result is written as tonnes of CO2e or CO2-eq. It lets a single figure stand for a mix of gases, which is why a company or country can report one total even though its emissions include methane from livestock, nitrous oxide from fertilizer and carbon dioxide from fuel.
Values for the main gases
The GHG Protocol republishes the 100-year values from the IPCC’s Sixth Assessment Report (AR6) and notes that the latest values are recommended. The table shows a selection.
| Gas | Chemical formula | 100-year GWP (AR6) |
|---|---|---|
| Carbon dioxide | CO2 | 1 |
| Methane, non-fossil | CH4 | 27.0 |
| Methane, fossil | CH4 | 29.8 |
| Nitrous oxide | N2O | 273 |
| Sulfur hexafluoride | SF6 | 24,300 |
These values come from the GHG Protocol’s table of IPCC global warming potential values, and EPA’s overview of global warming potentials gives the same nitrous oxide figure.
Two features of the table stand out. Methane has two entries because the fossil value includes the extra warming from the CO2 formed when methane eventually oxidizes, while the non-fossil value does not, a distinction the GHG Protocol sets out in its instructions for choosing between them. And fluorinated gases such as sulfur hexafluoride score in the tens of thousands, which is why small leaks of them can matter in an inventory. EPA calls them high-GWP gases because, for a given amount of mass, they trap substantially more heat than CO2, and notes that some of them persist in the atmosphere for hundreds or thousands of years.
Why the same gas has different scores
A GWP always comes with a time horizon. EPA points out that for methane, which has a short lifetime, the 100-year GWP of 27–30 is much less than the 20-year GWP of 81–83. Methane breaks down after about a decade, so most of its effect lands early. A shorter window therefore makes it look far more potent.
The reverse happens for very long-lived gases. EPA’s example is CF4, which has a lifetime of 50,000 years, and for which the 100-year GWP of 7380 is larger than the 20-year GWP of 5300. Choosing a horizon is therefore a judgement about which timescale matters, and the 100-year convention is a convention rather than a law of nature.
Values also change when the science does. EPA says that updates can be due to updated scientific estimates of the energy absorption or lifetime of the gases or to changing atmospheric concentrations. The GHG Protocol table accordingly shows methane at 25, 28 and 27.0 in the fourth, fifth and sixth assessment reports, and nitrous oxide at 298, 265 and 273.
Which edition applies depends on the reporting scheme. EPA explains that UNFCCC guidelines now require the use of the GWP values from the IPCC’s Fifth Assessment Report for national inventories, while its own Greenhouse Gas Reporting Program generally uses the Fourth Assessment Report. When comparing two organizations’ totals, it is worth checking which GWP edition each used.
Limits and alternatives
GWP compares cumulative energy absorbed, not temperature. EPA describes the Global Temperature Potential as a measure of the temperature change at the end of that time period, again relative to CO2, though it needs modelling of the climate system and is harder to calculate. Both metrics simplify a complex system.
The term can also be confusing. EPA warns that the International Organization for Standardization community differs in its definition and use of the term Global Warming Potential from the IPCC, applying it to the embodied greenhouse gases of a specific product. A figure labelled GWP on a product label is therefore not always the same quantity as the one in the table above.
A concentration in the air is a different thing again. How much CO2 sits in the atmosphere is the subject of the page on atmospheric CO2 levels, while the page on climate change impacts covers what the resulting warming has already produced. A business that publishes CO2e totals or related climate claims should check the advertising and consumer rules that apply where it operates.
Frequently asked questions
Is CO2e the same as CO2?
No. CO2e expresses the combined warming effect of several gases as the amount of carbon dioxide that would have the same effect, using GWP values.
Why is 100 years the usual time horizon?
It is the convention most inventories and reporting schemes have adopted, although shorter and longer horizons such as 20 years are also published.
Does a higher GWP always mean more warming?
Per tonne emitted, yes, relative to CO2 over the chosen period. Total warming also depends on how many tonnes of each gas are emitted.
Which GWP edition should be used?
That depends on the scheme being followed. The GHG Protocol recommends the latest IPCC values, while some regulatory programs specify earlier editions.
