If you have ever heard of the claylike mineral known as zeolite, chances are you share your home with a cat. You may also know that it comes in a powder, and that it is good at wicking out liquids and smells—ideal for concealing the minor indignities of being a feline. Desirée Plata, a civil engineering professor at MIT, uses zeolite for a different kind of molecular cleanup: Combine it with a metal catalyst—in Plata’s case, copper—add some heat, and it will trap and destroy methane, one of the most potent greenhouse gases.
Methane is a quixotic warming agent. Unlike carbon dioxide, which persists in the atmosphere for thousands of years, natural forces remove it within roughly a decade, mostly when it reacts with other molecules in the air. But for the brief time methane mixes aloft, it punches far above its weight, producing 80 times the warming effect of carbon dioxide over 20 years. By some estimates, it has been responsible for a third of anthropogenic warming so far, despite receiving far less attention. It is also notoriously difficult to track where the gas comes from. Some methane is trapped underground and then uncorked by natural fissures or by people boring into the ground for oil—or for methane itself, under the more anodyne name “natural gas.” But it can also be created anew by microbes wherever there’s a lot of biomass and very little oxygen: rice paddies, landfills, wetlands, or inside the digestive tracts of cows.
Over the past few years, the atmospheric concentration of methane has been spiking, puzzling and alarming climate scientists. According to the National Oceanic and Atmospheric Administration, the measurements from 2021 are poised to show the biggest increase since scientists started consistently measuring the gas. (The data takes a few months to catch up.) Is it a blip or a sustained rise caused by certain emissions sources? Or perhaps something else has changed in the cocktail of atmospheric gases, so that methane is destroyed less readily than before? “‘I don’t know’ is the honest answer,” says Rob Jackson, a climate scientist who studies methane at Stanford University. “The concentration increases are frightening. And if they continue, this is terrible news.”
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What’s clear is that the world’s first priority needs to be cutting methane emissions, Jackson adds. Sometimes that’s as simple as turning a screw on a leaky pipeline valve or plugging up a defunct gas well. But there are limits to that pinpointed strategy. With CO2, zeroing in on a so-called “super emitter” is as simple as scanning the horizon for the smokestacks of a coal-fired power plant. But comparable sources of methane emissions are often more sporadic—a pipeline leak here, a landfill plume there—a game of whack-a-mole for environmental watchdogs inhibited by limited surveillance. Accountability is also tricky: The methane emissions of a particular herd of cows can’t be measured as consistently as the CO2 spewed by a freeway full of cars.
Natural emissions, which are estimated to be about 40 percent of methane emissions, are even trickier, and they are likely to accelerate as the world warms, in part by firing up gas-emitting microbes that live in permafrost or underneath sea ice. “The problem with natural emissions is that there is not a lot we can do with them,” Jackson says. “It’s hard to estimate the emissions of the Chesapeake Bay, or more terrifyingly, measure what will happen if the Arctic starts melting. That’s letting the genie out of the bottle, and it’s impossible to get it back in.”
So perhaps, Jackson and other scientists suggest, it's time to think about removing methane from the atmosphere, in addition to cutting back on new emissions. It’s an idea that’s far more advanced for carbon dioxide—and perhaps for good reason, given that CO2 is the leading cause of warming and that humanity will be living with today's CO2 emissions for thousands of years. But with methane, proponents argue there’s a rationale for swift action—a chance to return to preindustrial levels within decades, thanks to its short life span. Jackson and other scientists have argued that the heating effects of methane are chronically undervalued, because current climate policies emphasize long-term temperature goals that extend far beyond the lifetime of a methane molecule. The value of reducing methane levels spikes when you factor in the benefits of preventing warming now.
But the idea presents a paradox: There may be too much methane coming from everywhere to do that with emission cutbacks alone, Jackson says. Yet perhaps there’s not enough of it in the air to feasibly take it out.
Destroying methane is, in a chemical sense, a relatively easy task. Nature does it constantly. Methane is a single carbon atom surrounded on four sides by hydrogen, and these bonds are broken apart by a process called oxidation—it involves oxygen atoms plus some goading by energy and chemical catalysts.
There are many ways to make that chemical reaction happen, explains Renaud de Richter, a scientific adviser to Methane Action, a nonprofit that advocates removing the gas from the environment. Most methane is oxidized naturally when it reacts with either chlorine atoms or hydroxyl radicals in the atmosphere. So one idea is to spray iron salts, perhaps with the assistance of cargo ships, that will coax more chlorine atoms out of briny ocean air. The idea is being tested by researchers at the University of Copenhagen inside a laboratory gas chamber. Another concept, favored by de Richter, would involve using thermal towers that passively suck in air and break down methane through photocatalysis—a process involving sunlight and metal catalysts. But neither idea has been tested in the real world yet.
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Compared to those methods, Plata’s zeolite-based removal is old science. The idea emerged from the industry that produces methanol—a liquid chemical that’s used as an antifreeze, among other things, and can be converted into a variety of fuels like ethanol. Microbes do this naturally. Sitting in the seabed, they get a little methane from the ground below, a little oxygen from the air up above, and produce methanol. Industrial players have tried to replicate this using a powdered zeolite as a sort of “molecular sieve” that traps the methane in its pores, then oxidizes the molecules with heat, oxygen, and metal catalysts.
For methanol producers, the problem with this setup is that the reaction is difficult to control precisely. The concoction tends to over-oxidize, turning the precious methanol into carbon dioxide and water vapor. “Engineers have obsessed over how to prevent that from happening,” Plata says. Their solution is to try out funky modifications, like alternately flooding the reaction with methane and oxygen, although this makes the whole process inefficient.
But Plata’s goal isn’t to produce any methanol; it’s simply to get rid of methane. Hence her solution: Don’t sweat the carbon dioxide. “People freak out when I say that,” Plata says. Yes, it’s a little odd to suggest turning one greenhouse gas into another. But, she says, because of CO2’s substantially lower warming effect, its relative effect on the climate is “a minuscule little blip” compared to letting methane hang around. In the lab, Plata dried up a copper and zeolite mixture and placed it in a tube with various mixtures of atmospheric gases, including methane at varied concentrations. “It works. I’ll say that. We can convert low levels of methane,” Plata says. “The question is about how quickly you can make it work.”
Last month, Plata’s team received a $2 million grant from the US Department of Energy intended to quickly move the technology out of the lab. The next step is converting their powdered catalyst into a zeolite-based filter that’s easier to push air through—a process she compares to the catalytic converter at the back of a car. Plata wants to install the filters in places where methane is concentrated, but there’s not enough of it to burn—a process known as flaring that’s commonly used to get rid of methane leaking from natural gas and oil wells. Flaring and other thermal techniques can destroy methane at concentrations as low as 2,000 parts per million. She imagines the zeolite-based filtration being used in less concentrated environments, like mine shafts or indoor dairy farms.
Jackson, who is involved in a team working on similar technology, says he likes the zeolite strategy because it occurs “inside a box.” Unlike spraying chemicals into the open air from cargo ships, it’s easier to count how much methane is being destroyed and whether there are knock-on effects—more reactions happening in the air producing byproducts that you might not want. But he acknowledges that “zeolites aren’t magic.” Among the key concerns are producing a material that allows as much air as possible to flow through it, and bringing down the temperature of the reactions—both in an effort to conserve energy. (Though an improvement over other methods, Plata’s lab process works best at a balmy 300 degrees Celsius.)
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But Jackson feels the concept is getting close to viability for what he describes as a sweet spot—for deployment to places where there isn’t good technology to destroy or pull back on high volumes of methane emissions. Over time, Jackson says, the idea is to reduce the concentration of methane that can be viably filtered to about 2 parts per million—the background level of the gas in the atmosphere.
Getting there will be tough, says Klaus Lackner, a geophysicist at Arizona State University and a pioneer in CO2 capture technology. Lackner says the challenges of removing either gas from the atmosphere are similar, but in his view, far more daunting to scale for methane. Atmospheric concentrations of methane are relatively low—CO2, by comparison, is at 412 parts per million—and it would be almost impossible to push enough air through enough filters to trap a worthwhile amount, he says. What’s more, even if you could, “you have to pull a lot out to make a difference,” because natural processes may compensate to replace the artificially removed methane on a global scale. The oceans release a little extra, the microbes munch a little less. “You have to be as big as nature,” he says.
The barrier to being as big as nature is the potential cost. It would require either a tremendous number of passive machines to process enough of the atmosphere to make a dent in overall concentrations, or energy-hogging fans that are “barely affordable” even for CO2 capture. (By one estimate, humanity would need to build some 10,000 direct air capture facilities by the end of the century to actually reduce CO2 levels.) But, he adds, it’s useful to keep studying the various strategies. “At the end of the day, you could say we have an insurance policy,” Lackner says—especially if methane emissions start to get truly out of control, such as if a warming Arctic causes a runaway release of the gas.
Jackson acknowledges the challenges—and that reducing methane emissions, as well as CO2, is priority number one. That means tighter rules for emitters and better surveillance to identify where the gas is escaping. Currently, “it’s not clear who would pay” for methane removal, he says, even in cases where the technology would be sited at businesses with high emissions, like at a dairy farm or a coal mine.
But that could change, he adds. The idea of methane removal is getting more attention, including an analysis of various technologies in the 2021 report from the Intergovernmental Panel on Climate Change. He also points to the recent signing of more than 100 nations to the Global Methane Pledge, a US- and EU-led effort to cut emissions by 30 percent from 2020 levels by the year 2030. That may encourage nations to consider more stringent rules for facilities that produce methane, and perhaps put a higher value on the carbon equivalence of methane emissions, translating into incentives for removal or taxes for emissions. The point, Jackson says, is that we should do everything we can to stop warming now: “Methane is the strongest lever we have for that.”
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