A measure of a greenhouse gas's ability to absorb heat and warm the atmosphere over a given time period. It is measured relative to a similar mass of carbon dioxide, which has a GWP of 1.0. So, for example, methane has a GWP of 25 over 100 years, the metric used in the Kyoto Protocol. It is important to know the timescale, as gases are removed from the atmosphere at different rates.