Global Warming Potential

The Global Warming Potential (GWP) is a unit of measurement developed to compare the atmospheric global warming impacts of gases. It tells us how long a gas remains in the atmosphere, and how much energy it absorbs over a given time period, relative to 1 ton of carbon dioxide (CO₂). These are the two most important characteristics of a greenhouse gas (GHG) in terms of their impact on the climate. The higher a gas’s GWP value, the more energy and infrared radiation it absorbs per pound, the longer it stays in the atmosphere, and the greater its contribution to global warming. GWP inventories allow analysts and policymakers to identify opportunities to reduce emissions from different industries.¹ The United Nations Framework Convention on Climate Change (UNFCCC) requires use of GWPs for inventory reporting.²

All GWPs are calculated in comparison with CO₂, which at a GWP of 1, is the metric’s baseline reference point. CO₂ emissions released into the atmosphere remain there for thousands of years, though the GWP is usually calculated using a time period of 100 years.³ This means that GWP values are typically used for gases with a long atmospheric lifetime, but not for short-lived gases. In order to calculate a GWP value, gases must last long enough in the atmosphere to mix and spread out evenly across the planet. Short-lived gases that aren’t able to concentrate globally in the atmosphere, and therefore don’t have a GWP, include water vapor, carbon monoxide, tropospheric ozone, some ambient air pollutants such as NO_x and NMVOCs, and tropospheric aerosols like SO₂ products and black carbon. However, GWPs can also account for indirect emissions, such as those created through indirect radiative forcing. This process occurs when a gas undergoes a chemical reaction that produces other greenhouse gases, or otherwise affects processes such as the atmospheric lifetimes of other gases.⁴

GWP values are calculated by the Intergovernmental Panel on Climate Change (IPCC), and published in its Fourth Assessment Report (SAR).⁵ The IPCC presents multiple methods for calculating GWPs, taking into account the influence of future warming on the carbon cycle. For this reason, GWPs are often presented as ranges, rather than a single value.⁶ They are applied to units of mass, like kilograms, pounds, and metric tons, rather than units of volume.⁷ Sometimes it is useful to calculate a gas’s GHG emissions in terms of how much CO₂ would be needed to produce similar warming effects over a given period of time. This metric, calculated by multiplying a quantity of gas by its corresponding GWP, is called the CO₂ equivalent (CO₂ eq).⁸

Gases like CO₂, with relatively long atmospheric lifetimes and global average concentrations, include methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF₆), and nitrogen trifluoride (NF₃).⁹ N₂O’s 100-year GWP is 265-298; on average, its emissions remain in the atmosphere for more than 100 years. CH₄‘s GWP is lower, between 28-36 over 100 years, and its average lifetime is ten years. Although its lifetime is much shorter than CO₂‘s, it absorbs much more energy, and its GWP reflects this net effect. Some of the highest-GWP gases, with values in the thousands or tens of thousands, include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), HFCs, PFCs, and SF₆.¹⁰

1. Environmental Protection Agency
2. Government of Canada
3. Environmental Protection Agency
4. Greenhouse Gas Management Institute
5. Government of Canada
6. Environmental Protection Agency
7. Greenhouse Gas Management Institute
8. Government of Canada
9. Greenhouse Gas Management Institute
10. Environmental Protection Agency