How big is methane’s impact on the climate?

Methane accounts for about 30 percent of global warming above pre-industrial levels.  That number may well rise, because methane levels are rising fast – faster than carbon dioxide, and faster than at any time since record keeping began in the 1980s.  Since the pre-industrial era, atmospheric methane has increased almost threefold while atmospheric carbon dioxide has increased by about 50%.  

Historical methane emissions contributed around 0.5o C (±0.1o C) of present-day global mean warming above pre-industrial levels (1850–1900), which is around half of carbon dioxide’s contribution of 0.9o (± 0.2o C).  But if methane levels increase as expected in a business-as-usual scenario, this increase could contribute another 0.2o C (±0.1o C) of additional warming by 2050, or 0.9o C(±0.2o C) by 2100. 

Methane is an extremely potent greenhouse gas — many times more potent than carbon dioxide. Exactly how much more potent depends on the timeframe.  Ton for ton, methane has 120 times the immediate global warming potential (GWP) compared to carbon dioxide. Over 20 years, methane has a GWP about 84 times greater than carbon dioxide’s.

But over 100 years, methane’s GWP is only about 28 times greater than carbon dioxide’s.  It’s conventional to use the 100-year value for methane mainly because that’s how carbon dioxide GWP is measured, and keeping the values for the two gases the same is convenient for comparing them.  

But the comparison can be misleading, because in important ways, methane and carbon dioxide aren’t comparable. Unlike carbon dioxide, methane is a “short-lived climate pollutant” (SLCP) which means it persists in the atmosphere for a relatively short period.  It does most of its damage to the climate within 20 years of being emitted.   Using the same 100-year timeframe for both gases risks vastly understating methane’s climate impact.

study by Stanford University scientists Rob Jackson and Sam Abernathy found that using the 100-year timeframe leads to an 87% underestimation of methane’s importance to keeping warming to 1.5 degrees Celsius.  To best align GHG emissions metrics with that goal, they propose a 24-year time horizon as a more relevant way to compare methane’s climate impacts to carbon dioxide’s. That timeframe would give methane GWP of 75.

There is a range of scientific opinion on the likelihood of a large methane “burst” from thawing Arctic permafrost that might accelerate global warming.  Some scientists see a sudden burst as imminent and find evidence it may already be underway.  Others believe increases in methane emissions from the Arctic will be slow and steady.  

Whether the risk of a methane burst is currently high or low, its impacts are potentially so devastating that it makes sense to develop ways to mitigate it now.  Meanwhile, atmospheric methane is at record high levels and rising fast.  Developing methane removal capability could help on both fronts — lowering record-high atmospheric methane levels while also providing the means to mitigate a potential methane burst.

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