Which reference gas has a global warming potential of 1 by definition?

Which reference gas has a global warming potential of 1 by definition? Carbon dioxide has a global warming potential of 1 by definition.

Global warming potential, or GWP, compares the accumulated radiative effect of emitting a given mass of a gas with the effect of emitting the same mass of carbon dioxide. Because carbon dioxide is the reference gas, its GWP is assigned the dimensionless value 1.

The comparison is not simply a measure of how powerful a gas is at one instant. GWP also reflects how long the gas remains in the atmosphere and the time horizon selected, commonly 20 or 100 years. A gas can therefore have different GWP values under different horizons.

Methane and nitrous oxide are often confused with the reference gas because they have higher GWPs. Their values depend on the assessment and time horizon, while carbon dioxide remains the baseline. “CO₂-equivalent” emissions are calculated by multiplying an emission’s mass by the relevant GWP.