CO2 eq (also written CO2e or CO2-equivalent) is a universal unit for measuring greenhouse gas emissions. It expresses the climate impact of any greenhouse gas in terms of how much carbon dioxide would produce the same amount of warming. Because different gases trap different amounts of heat and linger in the atmosphere for different lengths of time, CO2 eq gives us a single number to compare them all on equal footing.
Why a Common Unit Exists
The atmosphere contains several heat-trapping gases, not just carbon dioxide. Methane, nitrous oxide, and various industrial gases all contribute to warming, but they do so at vastly different strengths. One ton of methane, for instance, traps far more heat than one ton of carbon dioxide. Without a shared yardstick, there would be no practical way to add up total emissions from a factory, a country, or a supply chain that releases a mix of gases.
CO2 eq solves this by converting every gas into the equivalent amount of carbon dioxide that would cause the same warming. The international standard, used by the IPCC, the U.S. EPA, and corporate reporting frameworks like the GHG Protocol, is to express all greenhouse gas emissions in CO2 equivalents.
How the Conversion Works
Each greenhouse gas is assigned a number called its Global Warming Potential, or GWP. This number captures two things: how effectively the gas absorbs heat, and how long it stays in the atmosphere before breaking down. Carbon dioxide is the baseline, so its GWP is always 1. Every other gas is rated relative to CO2.
The formula is straightforward: multiply the mass of the gas by its GWP. If you release 1 ton of a gas with a GWP of 273, that equals 273 tons of CO2 eq. The result tells you how much warming that emission causes, stated in the language of carbon dioxide.
GWP Values for Major Gases
The most widely used GWP values come from the IPCC’s Sixth Assessment Report (AR6), published in 2021. These are calculated over a 100-year time horizon, meaning they reflect the total heat a gas traps over a century compared to CO2.
- Carbon dioxide (CO2): GWP of 1, by definition.
- Methane from fossil sources: GWP of 29.8. One ton of fossil methane has roughly the same warming effect as 30 tons of CO2 over 100 years.
- Methane from non-fossil sources (such as livestock or wetlands): GWP of 27.
- Nitrous oxide: GWP of 273. This gas comes largely from agriculture and industrial processes, and it persists in the atmosphere far longer than methane.
- Sulfur hexafluoride (SF6): GWP of 24,300. Used mainly in electrical equipment, it is one of the most potent greenhouse gases known, though released in relatively small quantities.
- Various hydrofluorocarbons (HFCs): GWPs range from around 135 to over 14,600 depending on the specific compound. These are commonly found in refrigeration and air conditioning systems.
A gas like SF6 illustrates why CO2 eq matters so much. Even a tiny leak of SF6 creates a climate impact equivalent to releasing thousands of times that weight in carbon dioxide.
The 100-Year vs. 20-Year Question
GWP values change depending on the time window you choose. The 100-year horizon (GWP100) is the international default used in climate treaties and corporate reporting. But some scientists and policymakers also look at a 20-year horizon (GWP20), which gives more weight to gases that are powerful in the short term but break down relatively quickly.
Methane is the clearest example of why this matters. Over 100 years, methane’s GWP is 27 to 30. Over 20 years, it jumps to 81 to 83, because methane is an intense heat-trapper that breaks down within about a decade. Using GWP20 makes methane look roughly three times worse than the standard 100-year figure suggests. This is why some climate advocates argue that the 20-year lens better captures the urgency of cutting methane emissions now.
The reverse is true for extremely long-lived gases. A perfluorocarbon called CF4, which persists for about 50,000 years, actually has a lower GWP on the 20-year scale (5,300) than on the 100-year scale (7,380), because its warming effect accumulates slowly over millennia.
How GWP Values Have Changed Over Time
The IPCC updates its GWP estimates as climate science improves. Between the Fifth Assessment Report (AR5, 2013) and the Sixth (AR6, 2021), most values shifted modestly. Methane’s 100-year GWP actually dropped slightly, from 28 to 27 for non-fossil sources. Nitrous oxide rose from 265 to 273. Many industrial gases saw larger increases: HFC-143a, used in some refrigerant blends, jumped 21% from 4,800 to 5,810, and SF6 edged up from 23,500 to 24,300.
These updates matter because organizations that track their emissions over time need to decide which set of GWP values to use. Switching from older to newer values can change a company’s reported carbon footprint even if actual emissions stay the same. The GHG Protocol, which sets the most widely used corporate reporting standards, currently references both AR5 and AR6 values depending on the reporting context.
Where You’ll See CO2 Eq in Practice
The GHG Protocol’s Corporate Standard requires companies to report seven greenhouse gases in CO2 equivalents: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride. This covers everything from the natural gas a company burns (which releases CO2 and some methane) to refrigerant leaks from its HVAC systems (which release HFCs).
You’ll also encounter CO2 eq on carbon footprint labels for consumer products, in national emissions inventories submitted to the United Nations, and in carbon offset markets where a credit represents one metric ton of CO2 eq avoided or removed. When a country pledges to cut emissions by a certain percentage, that target is stated in CO2 eq so it captures all warming gases, not just carbon dioxide itself.
The concept is simple at its core: CO2 eq is a translation tool. It takes the messy reality of multiple gases warming the planet at different rates and gives us one number to work with. That single number is what makes it possible to set targets, compare industries, and track whether global emissions are going up or down.