An index (i.e., a relative measure) of how much heat a greenhouse gas traps in the atmosphere over a chosen time horizon, relative to that of carbon dioxide. The GWP represents the combined effect of the differing times these gases remain in the atmosphere and their relative effectiveness in absorbing outgoing thermal infrared radiation. Although the most common time horizon is 100 years, GWP have been reported also for time horizons of 20 years and 500 years.