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Why do we compare methane to carbon dioxide over a 100-year timeframe? Are we underrating the importance of methane emissions?
This greenhouse gas is short-lived but has far greater heat-trapping potential than CO2. The more concerned we are about global warming over the next 10 or 20 years, the more emphasis we have to put on cutting methane emissions.
June 28, 2021
Methane is a colorless, odorless gas that’s produced both by nature (such as in wetlands when plants decompose underwater) and in industry (for example, natural gas is mostly made of methane). It is widely regarded as the second most important greenhouse gas, after carbon dioxide (CO2). However, methane is about 200 times less abundant in the atmosphere and lasts there for only about a decade on average—while CO2 can last for centuries. To put it another way: methane does its damage quickly but soon fades away, while CO2 traps a smaller amount of heat consistently, decade after decade.
Jessika Trancik, an MIT associate professor at the Institute for Data, Systems, and Society, says this interplay of different factors makes it hard to compare these two gases directly. Climate scientists often think about the issue like this: Exactly how many tons of CO2 would it take to warm the Earth as much as one ton of methane?
The trouble is that the answer changes depending on how far in the future you look. Let’s say a factory releases a ton of methane and a ton of CO2 into the atmosphere today. The methane immediately begins to trap a lot of heat—at least 100 times as much as the CO2. But the methane starts to break down and leave the atmosphere relatively quickly. As more time goes by, and as more of that original ton of methane disappears, the steady warming effect of the CO2 slowly closes the gap. Over 20 years, the methane would trap about 80 times as much heat as the CO2. Over 100 years, that original ton of methane would trap about 25 times as much heat as the ton of CO2.
Trancik says environmental organizations and climate models, including those used for major studies or international accords like the Paris Agreement, consider the warming effects of methane over a hundred years. (The EPA, for example, says methane is 25 times as potent as CO2, which is the 100-year statistic.1) Why this number, when methane is far more damaging in the short term? In part, Trancik says, it was an “accident of history.” Decades ago, when scientists began to tackle the complicated task of comparing different greenhouse gases, most climate projections were looking out to the year 2100—about 100 years in the future.
But that has begun to change as climate change has accelerated in the 21st century. “There's been a recognition that we have to bring those targets forward to more like 2050,” Trancik says, which is President Joe Biden’s target date for the U.S. to stop adding to the buildup of greenhouse gases in the atmosphere. Methane, like CO2, is increasing rapidly in the atmosphere, and we know that newly emitted methane will do most of its damage in the first few decades after its release. Trancik says more scientists are beginning to model the warming effects that today’s methane emissions will have over the next 20 or 30 years, to more accurately predict whether humanity can avoid overshooting targets such as stopping global warming at 1.5 degrees Celsius.
Choosing the right measurement for methane can have serious policy implications. If climate scientists start to use models that count each ton of methane as 80 or 100 tons of CO2, then the environmental impacts of new industrial projects suddenly look much different. Take energy plants that burn natural gas. Large amounts of methane leak into the air at various points in the natural gas supply chain. “If you assess the degree to which natural gas is a clean energy source, you can quickly see that the choice of metric as well as the amount of natural gas that's leaking can quickly affect the answer,” Trancik says. For example, the common claim that natural gas cuts greenhouse emissions by half compared to burning coal2 may not be true if researchers measure how much heat will be trapped by new methane emissions in the short term rather than over a century.
“The devil is really in the details here,” Trancik says. “It looks like a simple number, but there’s a lot more going on.”
Thank you to Tom Wells of Gainesville, Florida, for the question. You can submit your own question to Ask MIT Climate here.