Environmental Science
Global Warming Potential (GWP)
Quick fact
The GWP of methane over 100 years is 28 times that of CO2, but over 20 years it jumps to 84 times—showing how the time horizon dramatically changes comparisons.
Why this is interesting
Why is a pound of methane considered 28 times more potent for global warming than a pound of carbon dioxide?
Read the full explanation
Understanding Global Warming Potential (GWP)
Imagine you have different types of blankets: CO2 is like a thin sheet, while methane is a much thicker wool blanket for the same weight. But methane only stays on your bed for a decade before it disappears, while the cotton sheet lasts centuries. Global Warming Potential (GWP) is a scoring system that combines two factors: how much each gas traps heat (its 'radiative efficiency') and how long it remains in the atmosphere (its 'lifetime'). To compare them fairly, scientists choose a standard time period—usually 100 years—and calculate how much heat each gas would trap over that time relative to the same mass of CO2. This gives a number like 28 for methane, meaning it traps 28 times more heat per pound over 100 years.
A deeper explanation
The mechanism behind GWP lies in the physics of radiative forcing. Each greenhouse gas absorbs infrared radiation at specific wavelengths; CO2 has moderate absorption, while methane and nitrous oxide have stronger absorption bands. Additionally, gases have vastly different lifetimes in the atmosphere: CO2 persists for centuries (some fractions remain for millennia), methane only about 10–12 years, and nitrous oxide around 114 years. GWP integrates the time-dependent radiative forcing over a chosen horizon (e.g., 100 years) relative to CO2. Mathematically, the GWP of a gas is the ratio of the time-integrated radiative forcing from a pulse emission of that gas to that from an equal pulse of CO2. This metric is critical because it enables conversion of emissions into 'CO2 equivalents' (CO2e), the standard unit for carbon footprints and emission inventories. Policymakers rely on GWP to set reduction targets—for instance, curbing methane offers a powerful short-term win against warming, while CO2 cuts are essential for long-term climate stability. However, GWP values are updated as science advances, and the choice of time horizon can change priorities, making it a subject of careful debate.