Photograph by Gerd Ludwig, Nat Geo Image Collection
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Every year, Amazon Basin rivers overflow their banks, turning forests into wetlands. Such trees can emit a large amount of methane, new research suggests.

Photograph by Gerd Ludwig, Nat Geo Image Collection

Trees release flammable methane—here's what that means for climate

There are more reasons than ever to conserve forests, but the surprising role of trees as a methane source adds a complication.

This article was created in partnership with the National Geographic Society.

In 1907, Francis W. Bushong, a chemistry professor at the University of Kansas, reported a novel finding in the journal Chemical and Physical Papers. He’d found methane, the main ingredient in natural gas, in a tree.

Years earlier, he wrote, he’d cut down some cottonwood trees and “observed the formation of bubbles in the sap upon the freshly cut trunk, stump and chips.” When he struck a match, the gas ignited in a blue flame. At the university, he replicated the flame test on a campus cottonwood and this time captured gas samples. The concentration of methane was not much below the level measured in samples from Kansas’s natural gas fields.

The finding was reported mainly as a novelty and faded into obscurity.

Tree methane is back, in a big way.

An expanding network of researchers has discovered methane flowing out of trees from the vast flooded forests of the Amazon basin to Borneo’s soggy peatlands, from temperate upland woods in Maryland and Hungary to forested mountain slopes in China.

Even as they strap $50,000 instruments to trees to record gas flows, more than a few of these researchers have been unable to resist using a lighter or match to produce the same blue flame that took Professor Bushong by surprise more than a century ago.

But the research now is driven by far more than novelty. Methane is second only to carbon dioxide in its importance as a greenhouse-gas emission linked to global warming. In a natural gas pipeline, methane is a relatively clean fossil fuel. But it is a powerful heat-trapping addition to the planet’s greenhouse effect when it accumulates in the atmosphere.

Methane released by a tree lights on fire. Credit: Daniel Yip & Christopher Schadt, Oak Ridge National Laboratory

The gas builds up as long as new emissions outpace the rate at which natural chemical reactions in the air or some forest soils break it down (that generally takes about a decade, compared to centuries for carbon dioxide). Since 1750, the atmospheric concentration has surged more than 250 percent (from around 700 parts per billion to more than 1,800 parts per billion). The main human sources linked to the rise are global agriculture—particularly livestock and rice paddies—landfills and emissions from oil and gas operations and coal mines.

Natural sources have always produced large amounts of the gas—currently on a par with those from agriculture. The main source is microbial activity in oxygen-deprived soggy soils and wetlands. (Increasingly, human-driven warming appears to be expanding wetlands, particularly in high latitudes, adding even more methane emissions.)

The full climate impact of methane from trees is nowhere near that of the tens of billions of tons of carbon dioxide released annually from smokestacks and tailpipes, or the methane from, say, humanity’s vast cattle herds or gas fields. But there is sufficient uncertainty in the estimates setting the “global methane budget” that trees could turn out to be a substantial source.

For the moment, this is a newly revealed frontier, said Kristofer Covey, a Skidmore College scientist focused on the chemistry and ecology of forests.

“At the global scale this could be huge”

“The emissions from an individual tree are small,” Covey said. “But there are several trillion trees. At the global scale this could be huge.” Covey organized an international workshop last spring to identify research priorities and just published a paper in New Phytologist that is, in essence, a call for help from a host of disciplines not yet focused on this issue. His coauthor is J. Patrick Megonigal, a tree researcher at the Smithsonian Environmental Research Center in Maryland.

New papers are being published month by month with remarkable rapidity, with each field measurement essentially constituting a new publishable finding.

“We’re very much still in the stamp collecting phase,” Covey said.

The findings are already challenging old norms. Dry upland forests were long assumed to be removing methane from the air through the action of a class of soil microbes called methanotrophs. But work by Megonigal and others is showing tree emissions can diminish or possibly exceed that methane-scrubbing capacity.

Misled by “a flat world”

How did this effect, measured by Bushong in 1907 and noted informally by forestry scientists for generations, stay hidden so long?

For decades, scientists studying flows of methane between terrestrial ecosystems and the air had set their instruments on the ground, never thinking trees might be involved, said Vincent Gauci, a professor of global change ecology at Britain’s Open University and an author of a string of recent papers on trees’ methane role.

What everyone had missed is that the stems and trunks and leaves of trees are surfaces, too, and the gas can flow there as well. “We’d been looking at a flat world,” Gauci said.

No more. Much of the methane now found to be escaping from trees in such wet conditions is thought simply to be microbial methane pumped up and out as oxygen flows down to the roots. But Gauci and other scientists are finding many instances in which trees produce their own methane—sometimes from microbes in the heartwood or other tissues and in other cases from a remarkable direct photochemical reaction thought to be driven by the ultraviolet wavelengths in sunlight.

The tree emissions measured in some regions are enormous, with an international team led by Sunitha Pangala of Lancaster University last year estimating in Nature that just the trees in the Amazon’s seasonally flooded forests were the source of between 14 million and 25 million metric tons of the gas annually—an amount similar to estimates for methane emissions from tundra all around the Arctic.

It might not seem so surprising to think of trees in Amazon forests as conduits for this gas, given that soggy soils, peat bogs, and other low-oxygen environments are the domain of microbes that generate this gas. But other studies have found trees generating substantial methane even in dry upland ecosystems—in some cases within the trunk of the tree, not the soil.

Such findings have spurred even more work, and it seems that everywhere someone looks, the more consequential, and confounding, the picture becomes.

At every scale, from whole forests to clusters of similar trees in a forest to the dynamics in individual trees, the one constant is variation, said Megonigal, at the Smithsonian research center in Maryland.

Covey described forests where similar trees in similar soils have been measured with a fiftyfold difference in methane emissions.

Some trees have been measured to be emitting methane near the base and absorbing it higher up the trunk.

But that’s not the least of it. Closer analysis has found that a single tree can be absorbing methane near the base through microbial processes and emitting it higher up the trunk.

Adding another, perhaps hopeful, twist, it appears that some trees actually sop up methane. The work has not yet been published, but was outlined last year at the European Geosciences Union meeting by Gauci, Pangala, and another colleague.

The study surveyed methane flows in trees in wet and dry soils from Central America and the Amazon to Britain and Sweden. Trees in wet soils uniformly were net emitters of methane but those in drier conditions in some regions actually were net absorbers of the gas.

Lessons for climate policy

The emerging findings on methane and forests are likely to stir discussions about next steps for climate policy related to forests, which has long focused on trees’ capacity to absorb and store carbon dioxide, with little attention to other properties.

“The thing we know about forests is that they sequester carbon,” Covey said. “That’s what you learn, what’s in a third grader’s cartoon drawing of a forest.”

The reality for climate is more complicated. “There is global warming but there’s no global forest,” he said.

The 2015 Paris Agreement on climate change supports forest projects as a way to draw down carbon dioxide emissions that countries have so far failed to constrain at the source. The United Nations has launched a Trillion Tree Campaign. There are a host of ways for companies and consumers to spend money on forest projects through “carbon offsets” to compensate for emissions from travel and the like.

In interviews, Covey and other researchers looking at the tree methane question stress they aren’t arguing that such efforts should pause, noting the many benefits of forest conservation, including carbon storage, resilience against floods, and safeguarding species-rich ecosystems.

Independent of climate diplomacy, countries around the world are working to accelerate forest conservation under a separate agreement, the Convention on Biological Diversity, to safeguard their value as home to vast arrays of species.

But the methane findings do highlight the importance of assessing the full range of climate impacts—for better or worse—of different forest and tree types in different regions. As with better understanding of forest ecology, this can then guide projects to maximize benefits and limit risks.

In recent years, other studies looking at the full impact of forests on the climate system have illuminated how a CO2-centered focus can miss significant additional cooling benefits of forests and—in some regions and forest properties—significant warming effects.

“For some forests all the arrows point in the same direction,” Covey said, describing the various ways trees can affect climate. “There are other places where the arrows don’t line up much.”

He and other researchers said a clearer view can improve climate models and also help insure that programs centered on the climate value of forests are as effective as possible.

In higher latitudes, the simple shift from light-reflecting open land to dark, rough-surfaced tree canopies can warm the local climate by absorbing more sunlight. Forests in the tropics are particularly valuable for local climate, cooling the air around them as their metabolic machinery results in enormous evaporation—and that also can result in more sun-blocking cloud cover and precipitation.

Other work has shown how a complicated array of volatile organic compounds emitted by trees react to create haze and clouds, influencing temperature and precipitation in a variety of ways. In 2014, debate erupted over over-distilled headlines implying that this work, particularly studies by the atmospheric chemist Nadine Unger, then at Yale, meant forests should not be saved.

No one interviewed for this story, including Unger, sees that as the case. Now at Exeter University, she said what’s needed are comprehensive assessments of forests and climate accounting for the full suite of properties.

What’s particularly notable now is that she and some of her past critics are all stressing that the prime focus of the world needs to be cutting emissions of carbon dioxide at the source, even as forests are saved for all the benefits they provide.

“Our best shot at achieving Paris Agreement global temperature targets is a laser focus on reducing CO2 emissions from energy-use in the wealthy mid-latitude countries,” Unger said.

Her point echoes a commentary by a range of scientists in the March 1 edition of Science on making sure “natural climate solutions”—including forest-focused projects—are not seen as an alternative to pursuing deep, prompt cuts in greenhouse-gas emissions. Both will be needed, they said.

William R. Moomaw, an emeritus professor of international environmental policy at Tufts University, said there will always be uncertainties in gauging the full mix of climate influences of forests. But that should not stand in the way of moving forward with programs to expand them or boost their carbon-holding capacity. The weight of evidence still points to forests as a key to maintaining a safe climate, Moomaw said.

“Given that forests were major factors in the stable carbon and temperature balance for the past 10,000 years until humans began cutting them down and also burning them, that suggests the balance of all factors was about right.”

Andrew Revkin is the Strategic Adviser for Environmental and Science Journalism at the National Geographic Society and has written on global warming for 30 years. He is the author of three books about climate, most recently Weather: An Illustrated History, from Cloud Atlases to Climate Change. He covered the environment for years at the New York Times.