Global Warming Potential

Global warming potential (GWP) measures how much heat a greenhouse gas traps over a specified period compared with carbon dioxide, when equal masses of the gases are emitted. More precisely, it compares their cumulative radiative forcing, or influence on Earth's energy balance, following a single emission pulse. Carbon dioxide is the reference gas, with a GWP of 1 for every time horizon.

What the comparison measures

GWP compares the warming influence of equal masses, not equal volumes or equal numbers of molecules, of different greenhouse gases.

• A gas with a GWP of 100 has 100 times the cumulative radiative forcing of the same mass of carbon dioxide over the stated period.
• The value depends mainly on the gas's ability to absorb radiation and how long its warming influence persists in the atmosphere.
• GWP is a relative index, not a direct measurement of the temperature rise caused by a gas or its total contribution to global warming.

Why the specified period matters

The time horizon is essential because greenhouse gases differ in how long they remain in the atmosphere. A GWP value is therefore incomplete unless its assessment period is stated.

• GWP100, which uses a 100-year period, is widely used for comparing greenhouse gas emissions.
• A 20-year period gives greater relative weight to short-lived gases such as methane than a 100-year period does.
• Carbon dioxide's warming influence persists much longer than methane's, so the relative comparison changes with the chosen period.

Use in emissions accounting

GWP allows emissions of different gases to be expressed in a common unit called carbon dioxide equivalent (CO2e).

• The mass of a gas emitted is multiplied by its GWP for the chosen period to calculate its carbon dioxide equivalent.
• This enables greenhouse gas inventories and mitigation assessments to combine and compare emissions of different gases.
• CO2e is an accounting equivalence and does not mean that different gases have identical atmospheric behaviour.