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by Kristen Minogue

Frozen methane bubbles in an Alaska lake. When icy permafrost thaws, microbes are able to consume the carbon stored there and turn it into methane gas. (Miriam Jones/USGS)

We’ve underestimated greenhouse gases. Not carbon dioxide, arguably the most famous greenhouse gas except water. But others, like methane, are less abundant but more powerful in terms of trapping heat. And our figures about that have probably skewed low.

Carbon dioxide (CO₂) comprises a staggering three-fourths of global greenhouse gas emissions, making it a major driving force behind climate change. But methane (CH₄), long locked in Arctic permafrost, is escaping as ice thaws. Methane also enters the atmosphere via natural gas, livestock, coal mining, oil and even wetlands.

For years scientists and policymakers have reported that methane is roughly 30 times more powerful than CO₂ over a century. This fall, two biogeochemists tested a more accurate model and discovered the true figure is far higher – more like 45 times more powerful than CO₂.

The good news? Taking methane out of the atmosphere makes an even bigger difference than putting it in.

Reality Check

Climate scientists usually calculate the heat-trapping ability of a greenhouse gas by comparing it to CO₂. But trying to compare two greenhouse gases is somewhat like comparing sugars and saturated fats. They’re both dangerous, but in different ways. Getting a figure involves more than measuring how much heat each gas traps. There’s also the issue of how long each gas remains in the atmosphere. Methane lasts about 12 years before getting destroyed; carbon dioxide can stay indefinitely unless something removes it. To put them on equal footing, scientists use something called the global warming potential (GWP).

“You have an apple and an orange and a pear, and what the GWP does is convert all three of those to apples,” said Pat Megonigal, biogeochemist at the Smithsonian Environmental Research Center and co-author of the new study, published in Ecosystems.

Once in the atmosphere, methane lasts about 12 years before being transformed into carbon dioxide. (NASA)

To find a gas’s global warming potential, climate scientists pick a timeframe, typically a century. They calculate how much heat energy 1 ton of CO₂ emitted in a single burst would absorb over the following century, and compare that to what a 1-ton burst of methane would absorb over the same time.

The trouble is, greenhouse gas emissions don’t work that way in the real world.

“You don’t just emit one year and then stop,” Megonigal said. Sites releasing greenhouse gases today—whether coal-burning power plants or methane-emitting wetlands—do it continuously.

However, the figure’s simplicity is appealing. In 1997 it was adopted by the Kyoto Protocol, the first international agreement to reduce greenhouse gas emissions. Its influence has only risen since. Today it helps determine the effectiveness of everything from cap-and-trade programs to United Nations efforts to preserve forests.

Megonigal and the study’s lead author, Scott Neubauer, thought the research community could create something better. They took a different approach. Instead of looking at a single pulse of carbon dioxide, methane or another greenhouse gas, they looked at repeated pulses –the “sustained-flux global warming potential.” They imagined a scenario in which a new burst of gas entered the atmosphere five times a year and calculated the result over 500 years.

Under the old model, methane emerged as 32 times powerful than CO₂ after 100 years. Under the new sustained-flux model, methane clocked in at 45 times more powerful than CO₂—a 40 percent increase.

But that wasn’t their strangest discovery.

Nature’s Imbalance

After examining emissions, Neubauer and Megonigal flipped the switch and tracked what happened when methane was removed from the atmosphere, what they termed “sustained global cooling potential.” Something odd happened when they ran the process backwards. After a century, the effect of removing methane wasn’t 45 times higher than removing CO₂. It was 200 times higher.

Credit: SERC

It’s as though Mother Nature made a checkbook-balancing error. However, there’s a reason taking methane out makes more of a difference that putting it in.

Emitting methane is like putting water into a leaky sink. When methane enters the atmosphere, it naturally gets destroyed after about 12 years. Like water in a sink, its levels will rise at first, but eventually things will even out.  There’s a continuous flow of give and take.

But sites that pull methane out of the atmosphere don’t put it back in. The give-and-take flow no longer exists—it’s similar to removing the drain from a sink without running the faucet.

Sites like upland forests can do that, said Megonigal. “There’s bacteria in these soils that consume, that take methane and essentially burn it for energy, like miniature little gas stoves.” Granted, those bacteria turn methane into CO₂ and emit that instead. But since CO₂ is so much weaker than methane, they’re still decreasing the system’s total greenhouse gas firepower.

The lesson: Where methane is concerned, removing may be better than reducing. Instead of only worrying about cutting methane emissions, a more powerful strategy might be finding ways to take out what’s already there.