evidence: When hearing the words "greenhouse gas," most people think immediately of carbon dioxide. This is indeed the greenhouse gas that is currently producing the greatest impact on the Earth’s rapidly changing climate. But it is far from the only one making its mark, and for mitigating climate change it’s important to be able to compare the effects of the various gases that contribute to warming the planet. [...] Today, the number-two producer of human-caused greenhouse effects is methane, the main constituent of natural gas. When initially released, methane is about 100 times more potent than carbon dioxide, but its lifetime in the atmosphere is much shorter — about a decade, unlike carbon dioxide’s residence time of centuries. When averaged over a 20-year period, methane’s "greenhouse gas equivalency" is about 72 times that of carbon dioxide, but when looked at on a timescale of 100 years, that equivalency drops to just 25 times.