How big is methane’s impact on the climate?

Overall, methane in the atmosphere contributed about one-third of global warming over the past decade, with carbon dioxide contributing most of the rest.  Reducing current methane levels in the atmosphere could lower that contribution, generating tangible climate benefits over the next decade or two.  Some scientists also worry about the potential for a large methane “burst” from Arctic permafrost that could accelerate global warming.  Developing a way to reduce methane levels could be an important safeguard against that possibility.

Methane is a potent greenhouse gas with 120 times the immediate global warming potential (GWP) per ton compared to carbon dioxide.  Current practice evaluates GWP over a 100-year time horizon.  Over 100 years, methane’s GWP is 28 times that of carbon dioxide.  But this understates its harmful impact on the climate because methane’s persistence in the atmosphere is relatively short, which means it does most of its damage to the climate within 20 years of being emitted.  Over 20 years, atmospheric methane has a GWP of about 84 times that of carbon dioxide.  That’s the most relevant way to compare it as a greenhouse gas with carbon dioxide.

Methane accounts for at least 25% of global warming above pre-industrial levels, though that’s the net effect on absolute global-mean temperature after some cooling from certain other gases which mask methane’s full warming impact.

Historically, methane pollution has caused about half the warming carbon dioxide has (0.5 ◦C for methane vs. 0.9 ◦C for carbon dioxide).  But atmospheric methane concentrations are rising fast, and by the end of the century, if no further actions are taken to reduce methane levels, it would contribute 0.9 ◦C of warming.

Since methane is such a powerful warming agent over a 20-year timeline, a sudden burst of methane emissions, for example from thawing permafrost in the Arctic, would have the potential to accelerate global warming even more dramatically.  While there is no scientific consensus on whether or not this could happen soon, i.e. in the coming decades vs. over centuries, some scientists believe we may be seeing signs of it already.  But whether the risk is currently high or low, the impacts of a methane burst would be so devastating that action is needed now.  Developing and deploying ways to cut atmospheric methane levels would lower global warming, reducing the risk of a methane burst. At the same time, it would also equip us with an emergency response tool to manage a methane burst if one occurs.

There is broad scientific consensus that limiting global warming over the next century is critical for the future of societies and ecosystems, because otherwise the pace of unchecked warming may outstrip their ability to adapt.  Given methane’s high GWP and its potential for accelerating warming, it is critical to lower methane emissions and atmospheric methane levels quickly. 

Whether seen in terms of shrinking and reversing methane’s already large and fast-growing contribution to global warming, or in terms of insuring against a dramatic acceleration of global warming in the event of a methane burst, cutting atmospheric methane levels represents “a powerful opportunity to slow down the rate of warming and limit temperature rise by midcentury if we act now,” leading scientists say.

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