Different gases persist in the atmosphere for different lengths of time, so the comparison must also be based on a time frame. While CO 2 can remain in the atmosphere for centuries, methane is more quickly removed from the atmosphere by natural processes. As a result of oxidization and a few other processes that remove much smaller amounts of methane from the atmosphere, methane stays in the atmosphere for, on average, about 12 years. One helpful way to think about atmospheric life is this: if a proposal for a fracked gas project has an expected life span of 60 years, the plant’s methane emissions would live in the atmosphere for approximately 72 years. Atmospheric processes would convert these emissions to carbon dioxide on a rolling basis.

There are many atmospheric processes that impact methane, and methane, in turn, impacts other climate-changing processes, so analyzing the precise global warming impact depends on which of several complex atmospheric processes are considered. In a 2009 study by scientists at NASA and Columbia University, researchers evaluated a range of atmospheric processes and compositions and estimated that methane’s global warming potential could be 79 to 105 times greater than CO2 over a 20-year time frame. Analyzing the GWP of methane over 100 years, they concluded methane’s GWP was 26 to 41 times higher than carbon dioxide.

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The most recent Intergovernmental Panel on Climate Change (IPCC) report values methane’s 20-year GWP value at 86 and 100-year GWP at 34. The IPCC’s figures are widely accepted by experts and are the most commonly cited when discussing the climate impacts of greenhouse gases. As we learn more about methane and feedbacks in the climate system, methane’s GWP has continually increased. The IPCC’s 1995 report, for example, put methane’s 100-year GWP at 21. The GWP of methane increased by about 20 percent between the IPCC’s fourth and fifth assessment report, when the IPCC first included multiple feedbacks in the atmosphere that can cause methane to produce other gases that trap heat, like ozone and water vapor. The GWP of methane is also changing because our understanding of the warming potential of CO 2 has improved over the last 20 years, as has our understanding of how long methane typically stays in the atmosphere before being converted into CO 2 .