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Global warming potential expresses a gas's warming effect over 100 years relative to carbon dioxide. Methane's GWP100 of 28 means 5 kg is equivalent to 140 kg of CO2, while sulfur hexafluoride at 23,500 makes even tiny leaks significant. Methane's 20 year GWP is much higher at about 84, because it decays within roughly a decade.
Carbon dioxide equivalent
CO2e = mass x GWP
GWP100 values
methane 28, nitrous oxide 265, HFC-134a 1300, SF6 23500
Because methane breaks down in about a decade. Over 20 years its GWP is around 84, over 100 years only 28.
It is extremely infrared-absorbent and atmospherically inert, with a lifetime of about 3200 years, so emissions accumulate essentially permanently.