The math isn’t adding up on forests and CO2 reductions

Big polluters can hide behind forest offsetsBy Justine Calma@justcalma  Apr 29, 2021, 5:12pm EDT

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Upward view of giant sequoias in Sequoia National Park,...
SEQUOIA NATIONAL PARK, CALIFORNIA, UNITED STATES – 2018/09/01: Upward view of giant sequoias in Sequoia National Park, California, USA.

More evidence surfaced this week that shows that forests are struggling to do humans’ dirty work when it comes to climate change. Although companies and countries are increasingly relying on forests to draw down their planet-heating carbon dioxide emissions, the math isn’t adding up to show big benefits.

California might have oversold the success of carbon offsets used in its cap-and-trade system, which is often billed as one of the world’s most successful market-based mechanisms to tackle climate change. The system appears to be failing because California is actually overcounting how much carbon dioxide forests keep out of the atmosphere, according to a new study by nonprofit CarbonPlan, that’s still under peer review, and reporting by ProPublica and MIT Technology Review. (One of the authors, James Temple, was previously a senior director at The Verge.)

That finding follows research published earlier this week in the journal Nature Climate Change that found a giant discrepancy between how much climate pollution countries officially reported and how much pollution independent models calculated for them. In this case, forests mucked up the numbers because countries are attributing more carbon reductions to their forests than independent models do.THAT LETS THOSE POLLUTERS OFF THE HOOK

Trees do provide a crucial service for people and the planet by “breathing in” and storing carbon dioxide. So protecting forests is important for their health and ours. But schemes devised to sell forests’ carbon storage to polluters as a way to cancel out their emissions haven’t always resulted in the CO2 reductions they’re supposed to achieve. Ultimately, that lets those polluters off the hook instead of pushing them to do more in the race to prevent a deeper climate crisis.

In the US, forest owners can sell their land’s ability to store carbon as “credits” to polluters. Since California sets a cap for how much CO2 pollution industries generate, companies can purchase those credits to cancel out some of their emissions and stay below the cap (with each credit representing one metric ton of CO2). But up to 39 million credits, nearly a third in the state’s program, didn’t actually provide the climate benefits they were supposed to, according to the new analysis by CarbonPlan.

That’s because the state was using averages to estimate how much CO2 each parcel of forest could hold. In reality, some pieces of forest can store more than others based on what kinds of trees are there and how dense the forest is. Forest managers also “gamed the system” by selling credits from parcels that inflated how much carbon they stored, ProPublica and MIT Technology Review reported.FOREST MANAGERS ALSO “GAMED THE SYSTEM”

The California Air Resources Board disputes the study’s findings, which are still undergoing peer review. “We were not given sufficient time to fully analyze an unpublished study and are not commenting further on the authors’ alternative methodology,” a spokesperson for the Air Resources Board wrote to ProPublica and MIT Technology Review. (This isn’t the first time California’s cap-and-trade system has come under scrutiny. Previous research found that some “economically disadvantaged neighborhoods” near regulated facilities experienced higher rates of pollution after the carbon trading system began.)

Discrepancies in carbon accounting due to forests show up globally, too, according to the Nature Climate Change study. There was a 5.5 billion ton difference between the amount of carbon dioxide emissions nations report each year and what independent models calculated. That’s a big gap, nearly as much as the US’s net emissions in 2019. It boils down to the way the US and other countries count up carbon caught by their forests, which doesn’t match up with the methods other researchers use. The lack of standardized reporting across the board for countries and scientists could throw a wrench in global efforts to curb greenhouse gas emissions. It’s hard to make progress if everyone’s taking different measurements.

What’s more, countries that are big polluters and that have a lot of forests, like the US, can lean on that asset more than countries without as much forest cover, The Washington Post writes. Countries like the US use forests to subtract emissions, and they end up reporting a smaller “net” footprint as a result. That can make it look like they’ve made bigger environmental gains even if they’re still polluting a lot.“A FREEBIE FROM NATURE”

If the US didn’t count on forests and other land ecosystems to offset some 12 percent of its emissions, its carbon footprint would actually be much higher. “We are lucky to have those natural carbon sinks,” Christopher Williams, a professor at Clark University, told The Washington Post. “However, that carbon uptake is a freebie from nature for which we do not really get to take credit in our battle against climate change.”

Despite the risks that come with relying on those freebies, it’s getting trendier than ever to invest in forest-based climate solutions. YouTubers and the World Economic Forum have launched feel-good tree-planting initiatives. Tech companies like Microsoft that have made commitments to become carbon “neutral” or carbon “negative” say that they’ll draw down at least as many emissions as they release — and are relying heavily on trees to do that. But trees can only do so much — and, if the results of these studies hold up, perhaps much less than people previously thought they could.

Planting trees doesn’t always help with climate change

Reforestation is seen as a way to help cool the climate, sucking excess warming carbon out of the atmosphere. But it’s not always that simple.

Suddenly we are all being told to plant trees. The hope is that they will save us from the worst effects of climate change.

The idea is everywhere. The Swedish climate activist Greta Thunberg has made a film arguing for extra protections for the world’s forests, and for the replanting of those that have been cut down. George Monbiot, a columnist in the UK’s Guardian newspaper, has founded a campaign called Natural Climate Solutions, which advocates restoring forests and other ecosystems.

This is not just talk. The UK government has planted millions of trees over the last decade, and has pledged another million between 2020 and 2024. Others have attempted far more dramatic feats: in 2016 one Indian state planted 50 million trees in one day, while in July last year Ethiopia claimed to have planted 350 million in a day. Even the UK’s Daily Mail, a right-wing newspaper not known for its climate activism, has just launched a campaign encouraging all its readers to plant a tree.

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Protecting existing forests and planting new ones are surely good things to do. However, scientists say we must not place too much faith in trees to save us. In particular, last year one research group claimed we can plant a trillion extra trees and remove a quarter of the carbon dioxide currently in the air. These figures have been widely criticised as overhyped and unreliable. Trees will definitely help us slow climate change, but they won’t reverse it on their own.

The underlying problem is that our society is releasing greenhouse gases, especially carbon dioxide (CO2), that are warming the Earth’s climate to levels we have never experienced before. As a result the great ice sheets are melting, contributing to rising seas, and extreme weather events like hurricanes and droughts are becoming more severe.

Trees have emerged as one of the most effective methods for drawing existing carbon out of the atmosphere (Credit: Getty Images)

Trees have emerged as one of the most effective methods for drawing existing carbon out of the atmosphere (Credit: Getty Images)

The solution is to stop emitting all greenhouse gases, for instance by replacing fossil fuels with renewable energy sources like solar power. Deforestation is actually one of the biggest sources of carbon dioxide, because when trees are cut down much of the carbon stored within them escapes into the air – especially if the wood is burned. For instance, in 2017 land use changes – mostly deforestation – contributed four billion tonnes of CO2 emissions to the global total of 41 billion tonnes of CO2. In other words, if we stopped cutting down trees we would cut our annual emissions by about 10%.

However, simply stopping all our emissions is no longer enough. At this point we have emitted so much CO2, and left emissions cuts so late, that we are almost certain to miss our targets of limiting warming to 1.5C or 2C. That means we must also find ways to actively remove CO2 from the air.

So long as a tree lives, that carbon stays within it – and trees can live for decades or centuries

All sorts of technological approaches have been proposed, but trees are an obvious contributor. New trees can either be planted in regions that have been deforested (reforestation) or in places that have never had them before (afforestation). As the trees grow they pull in CO2 through their leaves and convert it into carbohydrates, which they use to grow. So long as a tree lives, that carbon stays within it – and trees can live for decades or centuries. Trees are a natural “carbon sink”. It follows that we should both stop chopping down forests – especially tropical ones like the Amazon, which store huge amounts of carbon – and start planting more.

By some estimates, trees can be an enormous carbon sink. A study published in July 2019, led by Thomas Crowther of ETH-Zurich in Switzerland, estimated the world has room for an extra 0.9 billion hectares of forestOnce those trees had matured, they could store 752 billion tonnes of CO2. Planting trees, the team wrote, is “one of the most effective carbon drawdown solutions to date”.

This finding has had immediate, fierce pushback from other climate scientists. In October 2019, the journal Science published four highly critical comments. These argued that the researchers had overestimated the carbon trees could store – by a factor of five. They also highlighted multiple mistakes. For instance, much of the land Crowther described as “available” for tree planting already has plants growing on it, all of them storing carbon, many of which would have to be removed, according to Sonia Seneviratne of ETH-Zurich and her colleagues.

Replanting trees nearer the poles is not as effective at drawing back carbon as trees planted in the tropics (Credit: Getty Images)

Replanting trees nearer the poles is not as effective at drawing back carbon as trees planted in the tropics (Credit: Getty Images)

There are also deeper problems, because trees have more than one way to affect the climate.

The first issue is that trees are dark, at least compared to other things that might blanket the land, such as grass or snow. As a result, planting more trees typically makes the land darker. Since dark surfaces absorb more heat, a dark tree-covered surface will trap more of the Sun’s heat – and warm the local climate.

As a result, there is a delicate balance between trees’ ability to take in CO2, reducing warming, and their tendency to trap additional heat and thus create warming. This means planting trees only helps stop climate change in certain places.

Specifically, according to a 2007 study that has been repeatedly confirmed, the best place to plant new trees is the tropics, where trees grow fastest and thus trap the most CO2. In contrast, planting trees in snowy regions near the poles is likely to cause a net warming, while planting them in temperate climates – like that of the UK, much of Europe and parts of the US – may have no net effect on climate.

Trees’ emissions can also lead to warming if they react to form the greenhouse gas methane, or ozone

“You have to be careful where you do reforestation,” says David Beerling of the University of Sheffield in the UK.

Others say this problem is overblown. “They’re assuming that snow cover’s going to stay there with warming,” says Beverly Law of Oregon State University in Corvallis. She points out that the polar regions are warming faster than the rest of the planet, so much of the snow may melt in the coming decades – in which case planting trees will not make the ground that much darker. “That’s been kind of a red herring that’s held out there a lot,” says Law.

The other thing trees do is emit volatile chemicals into the air. “That’s the pine-y smell you get when you walk through a forest,” says Dominick Spracklen of the University of Leeds in the UK. These chemicals stick together to form tiny floating particles called aerosols, which have complicated effects.

For example, the aerosols create a faint haze. This scatters sunlight back into space, cooling the planet. “Probably the more important effect is those particles act as seeds for cloud droplets,” says Spracklen. This creates more low cloud, or thicker low cloud, which also bounces sunlight back to space.

Planting trees can play a part in reducing carbon in the atmosphere – but it cannot reverse global warming on its own (Credit: Getty Images)

Planting trees can play a part in reducing carbon in the atmosphere – but it cannot reverse global warming on its own (Credit: Getty Images)

However, the trees’ emissions can also lead to warming if they react to form the greenhouse gas methane, or ozone, which is a greenhouse gas at low altitudes. For Nadine Unger of the University of Exeter in the UK, this is a major problem. “The mutual relationships between forests and climate are actually really rather more complex and not fully understood,” Unger told the James Lovelock Centenary conference at the University of Exeter in July 2019.

In 2014 Unger calculated that, by chopping down forests from 1850 to the 2000s and thus preventing them emitting volatiles, we have created a cooling effect that slightly offset the warming from greenhouse gas emissions. Shortly afterwards she wrote an opinion piece for the New York Times headlined “To save the planet, don’t plant trees”.

However, other reforestation experts are critical of Unger’s findings. “The overall effect is quite small,” says Spracklen, who has studied the effects of aerosols. “Then the carbon storage blows all the rest out of the water.” Law agrees, saying the effects of aerosols are also “a red herring”.

Natural climate solutions could lock up the equivalent of 23.8 billion tonnes of CO2 per year

So how much can trees really help us solve our climate problem?

In a 2017 study, researchers led by Bronsom Griscomnow at Conservation International, estimated the full potential of “natural climate solutions”. This includes restoring wetlands and other ecosystems, and minimising emissions from farmland, but the biggest contributors by far were preserving existing forests and reforesting degraded areas.

The team estimated that the natural climate solutions could lock up the equivalent of 23.8 billion tonnes of CO2 per year. That is a little over half our annual emissions, but they emphasise that many of the strategies they studied would not be cost-effective: a more plausible figure would be 11-15 billion tonnes of CO2 per year. This implies natural climate solutions could mop up about 30% of the CO2 we need to deal with every year.

For Law, it is one of the best estimates published to date. The researchers “really did a pretty good job”, she says.

When trees are cut down, it is important that the carbon they contain is not released again into the atmosphere (Credit: Getty Images)

When trees are cut down, it is important that the carbon they contain is not released again into the atmosphere (Credit: Getty Images)

The UK’s Royal Society came to similar conclusions in a 2018 report on greenhouse gas removal technologies. They estimated that reforestation could remove three billion to 18 billion tonnes of CO2 per year. These are significant numbers.

Uncertainties do remain, however. For instance, the climate will keep changing for many decades, and this will affect trees’ behaviour and growth – but we don’t really know how yet. “There’s still a question mark,” says Beerling. “Will they be limited by nutrient availability or increased fire or increased drought?” Similarly, planting trees in dry areas can cause water scarcity because they suck up so much – as China has discovered.

However, there are also surprise benefits of planting trees. For instance, a 2018 study suggested that large-scale tree planting in dry tropical regions would cause a shift in weather patterns, leading to more rainfall on land – enabling more plant growth and therefore more carbon storage.

The real uncertainties are not scientific, but socio-political

Also, planting trees is not just about stopping climate change. “As well as the climate emergency, we’re facing a biodiversity crisis,” says Spracklen. Planting trees can help with both, he says, “but only if we do it right”.

At the moment a lot of the trees being planted are monocultures of fast-growing commercial species like acacia or eucalyptus. These have “virtually no biodiversity benefits and may even replace something that was better”. It would be better to restore species-rich forests, he says. In line with this, Law has highlighted that planting rich new forests can boost local biodiversity, as well as improving water availability.

Areas that are now used for farming – such as rearing sheep on hill country – can be difficult to reforest  (Credit: Getty Images)

Areas that are now used for farming – such as rearing sheep on hill country – can be difficult to reforest (Credit: Getty Images)

The real uncertainties are not scientific, but socio-political. Put simply, where will people and nations allow the large-scale planting of trees? “As soon as you get down onto the land, there’s people living there and they have aspirations for how they want to live their lives that maybe don’t involve tree-planting,” says Spracklen. “There’s virtually nowhere where land’s just lying idle and you can just come along and do that.”

He points to the Welsh hills, which are severely deforested and consequently lacking in wildlife – but which are politically difficult to reforest because they are dominated by the sheep-farming industry. Similar conflicts over land use exist in all countries.

The message, then, is that trees can play a significant role in stopping dangerous climate change – provided we plant them in the right places. The challenge will be finding ways to fit huge new forests into our societies in such a way that people accept them.

Editorial: No, a trillion more trees and baby-step oil company reforms won’t fix global warming


A major ice sheet in western Antarctica is melting, and its collapse is predicted to raise the global sea level significantly.
(AFP photo / NASA / Handout)

Scientists at an Antarctic research station recently recorded a one-day air temperature of just under 70 degrees, a balmy afternoon in a region of the world unaccustomed to them. In fact, as far as researchers can tell, it has never been that warm in Antarctica before. The record was set against an increasingly scary global backdrop of rising temperatures and seas; more powerful storms, droughts and floods; a reduced Arctic ice cap, and accelerated melting and movement of glaciers around the globe — including Antarctica.

The culprit behind this crisis is the nearly 200 years that humans have spent burning fossil fuels — primarily coal and oil — for energy. So it was mildly heartening to see that BP, the London-based oil and gas giant, has promised to achieve “net-zero emissions” for its operations by 2050. That doesn’t mean BP is getting out of the oil-and-gas business. Rather, the corporation pledged to eliminate some emissions from its drilling, processing and business operations, and to compensate for others through investments in green technologies, reforestation projects and similar offset strategies. The announcement followed earlier pledges by such European-based oil companies as Royal Dutch Shell, Total and Equino to reduce emissions from their operations, though the BP pledge goes further.

None, of course, goes far enough. And new BP CEO Bernard Looney acknowledged the corporation had not settled on a strategy to achieve its net-zero emissions goal. Those details will come in September.

But at least the goal was set, which is far more than has been done by American-based oil companies like ExxonMobil and Chevron, which have acknowledged the role of greenhouse gas emissions in propelling climate change but have done little to address their contribution. Both are part of the corporate-driven Oil and Gas Climate Initiative, whose stated purpose is to reduce “our collective methane emissions by more than one-third” by essentially stopping leaks and moving the captured methane to where it could be burned.

Of course, baby steps by a handful of oil and gas companies aren’t going to do much to combat overall emissions. Similarly, the Trillion Trees Initiative, which President Trump touted in his State of the Union address, won’t do an awful lot, either. In fact, it’s one of those fig-leaf solutions that offers a pretense of significant action against global warming while ignoring the most pressing problem — the burning of fossil fuels in the first place.

Which is not to suggest that reforestation is a bad idea; in fact, continued forest clearing in the Amazon is exacerbating global warming and must stop. Because forests store carbon, restoring them could help capture and slow the accretion of carbon in the atmosphere, where it traps heat. One study found that the Earth’s ecosystems could handle an additional 25% of forests above what it holds now (though increased droughts and desertification related to climate change could whittle away at that), compensating for about 20 years of human-produced carbon. So large-scale reforestation falls in the category of “couldn’t hurt.”

Nevertheless, far, far more needs to be done, beginning with converting our global reliance on energy from fossil fuels to renewables as fast as is humanly possible. The best way to reduce carbon in the atmosphere is to not put it there in the first place.

So in that regard, the danger of the Trillion Trees Initiative is that pro-oil business conservatives will wave it around as a solution to global warming. But that’s like someone hoping to lose a lot of weight by taking daily walks while still eating the same calorie-rich foods.

The nation, and the world, need sober and aggressive policy changes if we are to stand any chance of mitigating the worst effects of global warming. Despite heightened awareness and national pledges under the 2015 Paris Agreement to try to limit global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels, global carbon emissions continue to rise. It will be expensive to adapt to the new climate reality and to fundamentally change the way humankind produces and uses energy, but it must be done before the supposedly most intelligent of the animal species manages through greed and willful ignorance to propel the collapse of global ecosystems.

‘A problem in every national forest’: tree thieves were behind Washington wildfire

Tree theft, which led to the deadly Maple Fire in Washington, may be costing the US Forest Service up to $100m each year

Hall of Mosses Trail at Olympic National Park in Washington.
 Hall of Mosses Trail at Olympic National Park in Washington. Photograph: Greg Vaughn/VW Pics/Universal Images Group via Getty Images

When two men discovered a rare and valuable 90ft-tall bigleaf maple tree in Washington State’s Olympic National Forest last year, they allegedly set about trying to steal it.

But there was a problem – the tree was home to a bee hive. The men reportedly tried to use a wasp killer to get rid of it. When that didn’t work, one allegedly poured gasoline on it, and lit it on fire.

The result, according to a federal indictment unsealed this week, was an August wildfire that raged across the eastern half of the ancient forest, setting 3,300 acres of public land ablaze and costing $4.5m to fight.

Known as the Maple Fire, the smoke from the blaze also served to exacerbate an already bad summer for the region’s air quality. There were fires raging in Canada and Eastern Washington, and as smoke from these blazes struck Seattle, at some points the city reportedly had the worst air quality in the world.

The bigleaf maple’s wood was covered in a distinct pattern, which if harvested is extremely popular for woodworking and potentially worth thousands of dollars. Before the fire, the pair had allegedly spent months illegally harvesting these high-value maple trees and selling the wood, which is used to make furniture and musical instruments.

Wilke was also specifically charged with “setting timber afire” and “using fire in furtherance of a felony” – the latter comes with a mandatory 10-year sentence, according to Seth Wilkinson, the lead prosecutor on the case.

“Timber theft, which involves destruction of a public resource, is in itself a really serious crime in this area,” said Wilkinson. “But this one is magnified many many times because of the consequences here, which involved the destruction of thousands of acres of national forest.”

The upper portions of the winding Elwha River in the Olympic National Park south-west of Port Angeles, Washington.
 The upper portions of the winding Elwha River in the Olympic National Park south-west of Port Angeles, Washington. Photograph: Chris Wilson /The Washington Post/Getty Images

Hundreds of responders from a variety of agencies came together to fight the flames west of Seattle, but the combination of an extremely dry summer and the fact that this is a dense forest with a lot of flammable moss meant that it took at least four days to control.

Wilke pleaded not guilty during an initial court appearance this week and is being detained until his trial in December, according to Wilkinson. Williams was arrested a couple weeks ago and is in state custody in California, but is expected to be transferred to Washington for his arraignment.

Wilke’s attorney did not immediately respond to requests for comment, and court records did not name a federal attorney for Williams.

Tree theft has become a problem for the Olympic forest and other national forests across the country, said Susan Garner, spokesperson for the Olympic National Forest.

“We just don’t have enough law enforcement officers out there, and many of our roads are remote,” she said. “It’s a problem, I think, probably on every national forest.”

Tree theft may be costing the US. Forest Service up to $100m each year, according to a High Country News article published in 2017. Western Washington has been one of the areas most impacted.

The indictment states that Wilke and Williams, along with other unnamed individuals, spent the spring and summer on the hunt across the Pacific Northwest forest for maple trees. When they found a good candidate, the small team allegedly used a chainsaw to fell the tree, and later cut it into smaller pieces.

With the right permits, most national forests allow visitors to harvest trees for personal use. But Wilke and Williams were allegedly selling the blocks to a lumber mill in a small city in western Washington by presenting its owner with permits saying it had been harvested on private land, according to a news release from the US attorney’s office. By the time they found the maple in early August, they had allegedly sold thousands of dollars’ worth of wood to the mill.

After officials responded to the fire, Wilke was soon on their radar when he was found in the area. He was questioned by a Forest Service law enforcement officer, but denied cutting timber, according to the indictment.

More than one year later, the charred area, which had long been a popular place for hiking and sightseeing, remains closed to the public due to lingering safety concerns, such as falling trees, said Garner. And while there are spots where greenery has started to return, there are still many sections that have not shown signs of recovery.

“People need to be aware of their actions and to stop and think about what they’re doing and to treat public lands with the deference that they deserve because they belong to all of us,” said Garner.

Trump Order May Open California’s Giant Sequoia Nat’l Monument To Development

SAN FRANCISCO (CBS SF) — Environmentalists and the outdoor recreation industry aren’t standing for President Donald Trump’s new executive order that threatens to rescind, shrink or resize dozens of recent national monument designations, including seven in California.

Trump’s new executive order requires Secretary of the Interior Ryan Zinke to review national monument designations that are over 100,000 acres and created under Presidents Barack Obama, Bill Clinton and George W. Bush.

Trump argues that some national monument designations may “create barriers to achieving energy independence.”

But environmental groups and outdoor recreation companies see the review as the first step in an assault on public lands, with the ultimate goal being to open the land up for oil and gas drilling.

And they say they’re prepared to fight to keep these federal lands free from development.

San Francisco-based Earthjustice, a major nonprofit environmental law organization, says, “Any attempt to reverse or shrink a monument designation by the executive branch is unlawful under the Antiquities Act. Only Congress has the authority to modify a national monument. Earthjustice stands ready to defend the Antiquities Act and the national monuments protected under the law.”

According to Earthjustice, the seven national monuments in California that could be threatened are Giant Sequoia, Berryessa Snow Mountain, Carrizo Plain, Sand to Snow, San Gabriel Mountains, Santa Rosa & San Jacinto Mountains and Mojave Trails.

In Arizona, part of the Grand Canyon is also under review.

The order draws special attention to the latest designation, the 1.3 million-acre Bear’s Ears National Monument in Utah. Republican Governor Gary Herbert and the Utah legislature has asked Trump to rescind the designation of Bears Ears as a national monument.

Ventura-based outdoor retailer Patagonia has not only been a staunch supporter of Bears Ears, but with Wednesday’s executive order, it has threatened to sue.

“We’re watching the Trump administration’s actions very closely and preparing to take every step necessary, including legal action, to defend our most treasured public landscapes from coast to coast,” Patagonia said in a statement.

Trump’s executive order calls for a preliminary report with suggested legislative acts on Bears Ears be provided to the president within 45 days after the executive order was issued. A final report on suggested actions on all national monuments under review is to be provided within four months.

Zinke tried to reassure the public as he discussed the executive order stating, “nobody loves public lands more than I do. You can love them the same. But not more.”

He argued that in some cases, the designation of the national monuments may have resulted in loss of jobs, but when pressed, he didn’t list any specific communities that lost jobs as a result of the monuments. He said that would be looked at in the review process.

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.

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 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.

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.

How much can forests fight climate change?

15 JANUARY 2019

Trees are supposed to slow global warming, but growing evidence suggests they might not always be climate saviours.

Gabriel Popkin
Kapur trees (Dryobalanops aromatica) in Malaysia avoid overlapping crowns as they grow, which creates a jigsaw-puzzle pattern when viewed from below. Credit: Stuart Franklin/Magnum
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When it comes to fighting global warming, trees have emerged as one of the most popular weapons. With nations making little progress controlling their carbon emissions, many governments and advocates have advanced plans to plant vast numbers of trees to absorb carbon dioxide from the atmosphere in an attempt to slow climate change. But emerging research suggests that trees might not always help as much as some hope.

Forest schemes got a big boost from the 2015 Paris climate accord, which for the first time counted all countries’ efforts to offset their carbon emissions from fossil-fuel use and other sources by planting or protecting forests. China aims to plant trees over an area up to four times the size of the United Kingdom. California is allowing forest owners to sell credits to CO2-emitting companies, and other US states are considering similar programmes, which could motivate projects that establish new forests and protect existing ones. The European Union is moving towards allowing countries to include forest planting in their plans to fight climate change; some nations in the bloc have also pledged billions of dollars to tropical forest programmes.

Many scientists applaud the push for expanding forests, but some urge caution. They argue that forests have many more-complex and uncertain climate impacts than policymakers, environmentalists and even some scientists acknowledge. Although trees cool the globe by taking up carbon through photosynthesis, they also emit a complex potpourri of chemicals, some of which warm the planet. The dark leaves of trees can also raise temperatures by absorbing sunlight. Several analyses in the past few years suggest that these warming effects from forests could partially or fully offset their cooling ability.

Such concerns have prompted vigorous debate among scientists about how forests in different regions have warming or cooling effects. Nobody denies that trees are good for the environment; after all, forests provide a host of benefits, and harbour much of the world’s terrestrial biodiversity. And no researchers are suggesting cutting down existing forests or curtailing efforts to combat deforestation. But as governments, corporations and non-profit organizations advance ever-more ambitious programmes to slow climate change, some scientists warn against relying on forests as a solution to global warming until a better understanding emerges. Researchers are involved in major campaigns to collect data using aeroplanes, satellites and towers in forests to sample the full suite of chemicals that trees emit, which can affect both climate and air pollution.

At the same time, some researchers worry about publishing results challenging the idea that forests cool the planet. One scientist even received death threats after writing a commentary that argued against planting trees to prevent climate change.

The questions are multiplying as more scientists enter the debate. At the same time, increasingly dire warnings about climate change — and the potential for huge amounts of money to go towards planting forests — have made working out how trees affect climate a matter of urgency. “People want an answer; they want to be able to say, ‘this is what we should do’,” says Gordon Bonan, a geoscientist at the National Center for Atmospheric Research in Boulder, Colorado. When it comes to forests and their ability to cool the climate, he says, “there are a lot of misstatements or overplaying of what can be done.”

Carbon sponges
If tree-planting programmes work as advertised, they could buy precious time for the world to reduce its reliance on fossil fuels and replace them with cleaner sources of energy. One widely cited 2017 study1 estimated that forests and other ecosystems could provide more than one-third of the total CO2 reductions required to keep global warming below 2 °C through to 2030.

Although the analysis relies on big assumptions, such as the availability of funding mechanisms and political will, its authors say that forests can be an important stopgap while the world tackles the main source of carbon emissions: the burning of fossil fuels. “This is a rope that nature is throwing us,” says Peter Ellis, a forest-carbon scientist at The Nature Conservancy in Arlington, Virginia, and one of the paper’s authors.

The first inkling that plants suck CO2 from the air dates back to the 1780s, when Swiss pastor Jean Senebier grew plants under different experimental conditions. He suggested that plants decompose CO2 from the air and incorporate the carbon, an idea corroborated by subsequent discoveries about the mechanisms of photosynthesis.

More than two centuries later, Senebier’s insights form a key component of plans to combat the accumulation of CO2 in the atmosphere. The rationale is that trees can lock up carbon in their wood and roots for decades or even centuries. The 1997 climate treaty known as the Kyoto Protocol allowed rich countries to count carbon storage in forests towards their targets for limiting greenhouse-gas emissions. In practice, few nations did so because of the agreement’s unwieldy accounting mechanisms and other factors. Later negotiations laid out a framework for enabling wealthy countries to pay poorer tropical countries to reduce emissions from deforestation and to increase carbon in forests. The framework was formalized under the 2015 Paris agreement, which required countries to commit to reduce greenhouse-gas emissions; more than 50 nations have pledged to add tree cover or protect existing forests (see ‘Where are the trees?’).

Source: X.-P. Song et al. Nature 560, 639–643 (2018).
Such schemes required firm data on how much carbon is locked up in forests. In the past few decades, scientists have worked to create national estimates of carbon loss and gain from vegetation by studying field plots and by combing through satellite data. In 2011, an international group led by researchers at the US Department of Agriculture’s Forest Service concluded that forests globally are a large carbon sink, taking more carbon out of the air through photosynthesis and wood production than they release through respiration and decay2.

That doesn’t mean that all forests cool the planet, however. Researchers have known for decades that tree leaves absorb more sunlight than do other types of land cover, such as fields or bare ground. Forests can reduce Earth’s surface albedo, meaning that the planet reflects less incoming sunlight back into space, leading to warming. This effect is especially pronounced at higher latitudes and in mountainous or dry regions, where slower-growing coniferous trees with dark leaves cover light-coloured ground or snow that would otherwise reflect sunlight. Most scientists agree, however, that tropical forests are clear climate coolers: trees there grow relatively fast and transpire massive amounts of water that forms clouds, two effects that help to cool the climate.

More-recent studies have branched out to include other ways in which forests can influence climate. As trees live, grow and die, scientists have learnt, they are in constant conversation with the air, swapping carbon, water, light and a bewildering array of chemicals that can interact with the climate.

Atmospheric chemist Nadine Unger, then at Yale University in New Haven, Connecticut, conducted one of the first global studies examining one part of this exchange: the influence of volatile organic compounds, or VOCs, emitted by trees. These include isoprene, a small hydrocarbon that can warm the globe in several ways. It can react with nitrogen oxides in the air to form ozone — a potent climate-warming gas when it resides in the lower atmosphere. Isoprene can also lengthen the lifetime of atmospheric methane — another greenhouse gas. Yet isoprene can have a cooling influence, too, by helping to produce aerosol particles that block incoming sunlight.

Unger ran an Earth-system model that estimated the effects of chemical emissions from forests. Her results suggest that the conversion of forests to farmland throughout the industrial era might have had little overall impact on climate3. Clearing forests liberated carbon stored in trees, but increased Earth’s albedo (leading to cooling) and decreased emissions of VOCs that can both cool and warm.

As a corollary, Unger suggested that reforestation would also have uncertain climate effects. Trees in tropical and temperate zones emit huge quantities of isoprene that is not accounted for in most forestry schemes. Higher-latitude boreal forests emit mostly terpenes, which help to cool the climate by forming aerosols that can block sunlight and promote the formation of cloud particles — although Unger didn’t attempt to quantify this cloud-seeding effect. She acknowledged that her study was a first step, and called for increased monitoring of forest chemicals and their atmospheric interactions.

She followed up on her research paper by writing an opinion piece in TheNew York Times entitled ‘To Save the Planet, Don’t Plant Trees’, which argued that the large uncertainties around the extent to which forests cool or warm the climate made tree planting a risky strategy for fighting climate change. The article, and especially the headline (which Unger did not write), triggered a tsunami of complaints from researchers, who disputed the science and said the piece threatened to undermine years of research and advocacy. A group of 30 forest scientists wrote a responseon the environmental news website Mongabay, saying, “We strongly disagree with Professor Unger’s core message.”

At 304 metres high, the Zotino Tall Tower Observatory measures gases and aerosols above taiga forest in central Siberia. A similar tall tower in the Amazon makes measurements above the tropical rainforest.Credit: Michael Hielscher/MPI
Unger says she received death threats, and that some colleagues stopped speaking to her. Some scientists, however, agreed that it was important to look at the impacts of forest VOCs. Subsequent studies have both supported and contradicted Unger’s 2014 analysis. A team led by Dominick Spracklen and Catherine Scott, atmospheric chemists at the University of Leeds, UK, ran a model that included how aerosols from forests can seed clouds, which reflect sunlight. They concluded that the net effect of VOCs from forests is to cool the global climate4.

Unger, in turn, questions some of Scott and Spracklen’s assumptions. Unger, who is now at the University of Exeter, UK, and Spracklen are discussing using a common experimental design to try to resolve their differences.

They and other researchers say that such studies are hamstrung by sparse data sets on forest emissions. “In my opinion, we still don’t know enough” to say what effect forest VOCs have, says Alex Guenther, an atmospheric scientist at the University of California, Irvine.

The latest findings are piling on even more complexity. Ecologist Sunitha Pangala at Lancaster University, UK, spent much of 2013 and 2014 in the Amazon rainforest, where she placed gas-measuring chambers around the trunks of more than 2,300 trees. “What we were really surprised about was the magnitude at which these trees are emitting methane,” says Pangala. She and Vincent Gauci at the Open University in Milton Keynes, UK, and their colleagues reported in 2017 that trees account for around half of the Amazon’s total methane emissions5. Researchers had previously assumed that methane leaked into the air directly from the soil, where it is produced by microbes. The new work suggests that trees could be another conduit for that microbial methane, potentially explaining why more methane has been detected above tropical wetlands than has been measured emanating from soil alone.

In a study first published last October, Gauci and other colleagues added another wrinkle when they found both methane and nitrous oxide, also a greenhouse gas, leaking from trees in upland forests6.

The global significance of these findings is still unclear. Pangala and Gauci both estimate that the cooling effect of trees taking up carbon greatly outstrips the warming from tree emissions of methane and nitrous oxide. But Kristofer Covey, an environmental scientist at Skidmore College in Saratoga Springs, New York, has found methane leaking from non-wetland trees in temperate forests7, and argues that such emissions could, in some places, diminish the climate benefits of trees more than researchers and environmentalists realize. “That’s a really painful message,” he says.

The recent explosion of results underscores the need for a full account of the impacts of forests, says Unger. “As long as we understand that tropical trees are taking carbon dioxide out of the atmosphere, we must also accept that they’re putting methane and VOCs into the atmosphere.”

Action stations
Scientists who champion forests say that although more research is always good, existing results are mature enough to support the use of forests to fight climate change, especially given the urgency of the problem. “We can’t necessarily afford to hold off on those things; we have to begin taking some action,” says Jason Funk, an environmental scientist in Chicago, Illinois, who served as an adviser and observer to the Paris agreement.

Researchers are now turning to sophisticated computer models and using larger and more-comprehensive data sets to nail down exactly what forests in different places do to the climate. In some cases, the results have been sobering. Last October, a team led by ecologist Sebastiaan Luyssaert at the Free University of Amsterdam modelled a variety of European forest-management scenarios8. The researchers concluded that none of the scenarios would yield a significant global climate impact, because the effects of surface darkening and cloud-cover changes from any added forests would roughly eliminate their carbon-storage benefits.

To estimate the climate impact of planting forests in different parts of the United States, ecologist Christopher Williams at Clark University in Worcester, Massachusetts, is combining global satellite data collected over more than a decade with carbon-sequestration figures based on data from the US Forest Service. He has found in preliminary work that adding trees to the US west coast and to regions east of the Mississippi River makes sense, climatically speaking. But albedo changes make forest planting in the Rockies and the southwestern United States a bad deal for the climate in most cases, because the conifers that thrive in those regions are dark and absorb more sunlight than do underlying soils or snow. He hopes to turn this research into a standardized methodology that forest managers can use to assess a project’s climate impact.

Getting planners to adopt such methods could prove challenging, however. Williams has found that some resist considering albedo effects, including representatives of companies hoping to sell carbon credits for forest projects. “Even other scientists sometimes have disbelief in the magnitude of the albedo effect, or even its existence,” he says.

“I have heard scientists say that if we found forest loss cooled the planet, we wouldn’t publish it.”

More data about the climate impacts of forests could come from long-term studies that track the gases and chemicals that trees emit and absorb. Researchers are using a 325-metre tower in the Amazon to monitor carbon, water and other chemical fluxes over a roughly 100-square-kilometre area of intact rainforest northeast of Manaus in Brazil. A companion tower in Siberia does the same.

Teams have erected smaller research towers to collect similar samples at hundreds of sites around the globe amid different types of forest; a tower in Norway, for example, will soon be the first in that country to start taking data in a forest. But many important areas have not yet been covered. Two NASA instruments launched in the past year — the Global Ecosystem Dynamics Investigation and the Ice, Cloud, and Land Elevation Satellite-2 — should soon provide a more consistent global picture of forests’ carbon stores.

Scientists who debate the climate impacts of forests are eager to get their hands on these data. And even those who are firmly convinced that forest projects can fight climate change welcome the added rigour of more-comprehensive studies. Ellis, for one, acknowledges that the analysis he co-authored1 considered albedo effects only crudely; the team did not consider VOCs and methane emissions from trees.

“We need to more honestly account for these other effects and be more careful about how we strategize,” says Ellis. “We’re using a blunt tool, when it would be much more preferable to use a sharper one.”