Can Planting a Trillion Trees Stop Climate Change? Scientists Say it’s a Lot More Complicated

Compared with cutting fossil fuels, tree planting would play only a small role in combating the climate crisis.

Tree canopy. Credit: Alex Torrenegra/Flickr
President Donald Trump said in January that he backs an international “trillion trees” plan “to protect the environment.” Credit: Alex Torrenegra/Flickr

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It seems simple. Plant enough trees to soak up all the carbon dioxide released by burning fossil fuels and people can forget about global warming and get on with their lives.

The idea even resonates with President Trump who, in January, said he backs an international “trillion trees” plan “to protect the environment.” 

Trump’s endorsement, at the Davos World Economic Forum, grabbed headlines, coming from a president who has withdrawn from the Paris Agreement, dismantled environmental regulations aimed at reducing emissions and called climate change a “hoax.” In the same speech, he attacked young climate activists as “prophets of doom.” 

Since Davos, the tree planting plan has morphed on Capitol Hill into the so-called Trillion Trees Act, a proposed bill that would set targets for increasing wood growth to capture carbon, part of a growing global focus on nature-based climate solutions to complement greenhouse gas reduction efforts.


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Internationally, the WWF, BirdLife International and the Wildlife Conservation Society formed to “protect and restore” forests, while the Davos discussions spurred formation of as a global platform to mobilize funds that leans heavily on commercial forest interests. There’s even a tree-based cryptocurrency

How to plant a trillion trees is a wide-open question, however. Forest scientists warn that massive tree planting programs could have unexpected consequences, and that cutting fossil fuel emissions is far more important in fighting global warming. A more useful focus, they say, is to preserve and bolster the forests we already have. 

Why Trees?

Fossil fuel burning and fires emit about 11 gigatons of carbon per year; forests, fields, grasslands and oceans absorb about 6 gigatons. So every year, about 5 gigatons go into the atmosphere. More trees could reduce the excess carbon dioxide that is warming the planet. 

The idea that trees can help limit global warming is enshrined in the 2015 Paris climate agreement, with most countries including forest expansion as part of their plans to reduce emissions. Reports and studies from the Intergovernmental Panel on Climate Change estimate that nature-based solutions, including healthy forests, could provide up to one-third of the emissions reductions required by 2030 to meet the Paris Agreement targets.

A 2019 Swiss study, for example, estimated that globally, there was the potential to protect and regrow trees on about 3.5 million square miles of land (an area a little smaller than the United States). Doing so, the study projected, would increase the global forest cover by 25 percent and capture and store about a quarter of the carbon in the atmosphere when the forests matured.

Those figures are disputed, but the research has increased interest in the trillion trees idea, and the study is one of the supporting documents for the Trillion Trees Act in Congress.

Separate from the U.S. political process, the international Trillion Trees initiative promoted by the conservation coalition wants to protect and regrow one trillion trees around the world by 2050. The main focus is to protect key forests like the Amazon, and to support natural regrowth in logged areas.

Additionally, the platform, launched after the Davos conference, with major partners like Salesforce, Microsoft and Deloit, is aiming to accelerate forest restoration and conservation by finding financing, for example, with app-based crowdfunding. That could help pay for high-tech tools like satellite data and artificial intelligence to guide precision forestry and to monitor tree and soil carbon storage.

Forest Politics

In the United States, the trillion trees idea has taken the form of the Trillion Trees Act, a proposed bill that calls on the United States to take a leadership role in planting a trillion trees globally as a “pragmatic step toward addressing global carbon emissions.”

The measure was introduced by Rep. Bruce Westerman, an Arkansas Republican, on Feb. 12. At a Feb. 26 congressional hearing, House Resources Committee Chairman, Rep. Raúl Grijalva, (D-Ariz.), said the proposed bill could open the door to bipartisan progress on climate action. 

“For too long, my friends on the other side of the aisle denied that this was even a real issue,” Grijalva said during the hearing. “They would reject, or even mock, the overwhelming scientific consensus that the planet is warming, humans are responsible, and urgent action needs to be taken.” 

He added, “I welcome Republicans into what is hopefully a new chapter for their party focused on climate solutions, not climate denial.”

President Donald Trump and First Lady Melania Trump participate in a tree planting ceremony with French President Emmanuel Macron and his wife Brigitte Macron on the South Lawn of the White House on April 23, 2018. Credit: Jim Watson/AFP via Getty Images
President Donald Trump and First Lady Melania Trump participate in a tree planting ceremony with French President Emmanuel Macron and his wife Brigitte Macron on the South Lawn of the White House on April 23, 2018. Credit: Jim Watson/AFP via Getty Images

Democrats simultaneously introduced HR 5435, the American Public Lands and Waters Climate Solution Act of 2019, which would first set a net-zero emissions target and require public lands to be managed to reduce greenhouse gas emissions. Neither bill has advanced out of committee.

Another measure that could encourage tree planting has been floated in the Senate by Colorado Democrat Michael Bennet (D-Colo.), with tax credits for “farmers and ranchers, state and local governments and tribes, to sequester carbon in agriculture, forestry, rangelands and wetlands.”

Westerman called trees the “ultimate carbon sequestration device,” and said his bill turns science into practical solutions. Support for the measure came from wood industry lobbyist Steve Marshall, who said the bill could support efforts to use more wood for building material as a way to store carbon long-term.

“We have an obligation to conserve our resources and make them available to future generations, and I challenge anyone to find a better climate solution than taking care of our forests.”

Just Planting Trees is Not Enough

Climate scientists and many Democrats on the House committee greeted the proposed tree planting legislation skeptically, saying that the only real climate solution is to cut greenhouse gas emissions to zero as soon as possible. 

Forests can only be part of a long-term plan to curb global warming after that happens, Yale evolutionary biologist and ecologist Carla Staver testified at the Trillion Trees Act hearing.

“Our primary focus must be reducing our dependence on fossil fuels,” she said, adding that any plausible attempt to limit global warming within our lifespan must also include forest protection and reforestation. “However, it is also crystal clear that tree planting alone will not fix our ongoing climate emergency,” she said.

In February, a coalition of 95 environmental groups sent a letter to Congress opposing the Trillion Trees Act as the “worst kind of greenwashing and a complete distraction from urgently needed reductions in fossil fuel pollution.”

As written now, the proposed law would count biofuel from forests as carbon neutral, a claim that’s contested by climate advocates and scientists, who have said the push to burn wood for fuel actually threatens global climate goals. It would also limit public and judicial reviews of forest management and even create incentives for more logging of existing old-growth forests, which are the best existing carbon sinks, said Alexander Rudee, a forest policy analyst with the World Resources Institute.

“Requiring harvests to increase annually will likely cause a net loss of trees, at least in the short-term, since natural regeneration isn’t 100 percent effective, and could increase emissions from burning or decomposing harvest residues,” Rudee said.

Magical Thinking

One reason the trillion-tree meme caught on may be that the world wants a simple solution to climate change and is ready for a positive message. Everyone can picture the act of placing a seedling in the ground and helping it grow, a nurturing symbol that must be part of our earliest collective memories as a species.

But Colorado State University atmospheric scientist Scott Denning, who studies how carbon moves through the global climate system, said the basic numbers of the trillion trees story don’t add up.

“No doubt, if you replaced every area of non-forest with forest, you could sequester a lot of carbon,” Denning said. “But very little of the world is available for planting a trillion trees. Most of the land that might be suitable is in use for farms and cities. Most of the places that can support forests, like the Amazon, Congo, Indonesia and Southeast Asia, already have forests.” 

He added, “You have to fix global warming by stopping burning oil and gas. To think you can just plant trees and keep burning oil and gas doesn’t make sense. We have to to get away from magical thinking.”

And while trees might help the planet survive in the long run, scientists say, first we have to save them. Global warming is a threat-multiplier for drought, fires and pests that have killed trees across millions of acres in the last 20 years. And forests all over the world are already in the full grip of the climate crisis, said University of Arizona ecohydrologist David Breshears

The forests around Breshears’ lab in Tuscon, near the shrinking southern end of the western North American forest belt, have been one of the mortality hotspots for trees. Planting a trillion trees around the world wouldn’t change that, he said.

“We’d like more trees to slow down the warming. But the warming we’re trying to slow is killing the trees. There’s real concern that we’re going to be losing a lot of trees, with more frequent die-offs.

“We need to hold on to the trees that we have,” Breshears said.

University of Montana scientist Diana Six says that widespread tree planting, like everything else we do with nature, requires caution.

“People think, what could be bad about planting more trees, but few have the background to understand the ecological issues,” she said, adding that any major tree-planting program needs to look closely at many related factors, including water and potential social impacts like the dislocation of communities.

“People think they’re going to put these trees in and they’ll grow and live happily ever after,” Six said. “A lot of places that don’t have trees are just too dry—you can’t just stick them in the ground and poof, have a new forest. And they don’t think about the time frame of trees.

“These trees are going to have to grow well and live a long time and not be prone to disease and insects, to capture a lot of carbon,” she said. “This is a multigenerational commitment to take care of these forests. They can’t be harvested, and you can’t just plant them and walk off.”

Future of the Amazon

The rapid forest changes underway might be a sign that it’s time to develop plans to save existing forests before trying to engineer the climate with tree plantations, a panel of international scientists said last week during a streamed discussion at this year’s virtual European Geosciences Union assembly.

Scientists on the panel said limiting global warming damage and stopping the deforestation of the Amazon Rainforest should be the highest item on the forest agenda, because there are signs that this vital part of the planet’s ecosystem is failing. 

Observations in rainforests and research in labs can help project what’s ahead for warming forests. In the University of Arizona’s Biosphere 2 habitat, scientists simulate rainforest conditions, then turn the sprinkler off. Such experiments, together with the onsite measurements, suggest that parts of the Amazon are already emitting more carbon dioxide than they capture.

Increasing drought could push even more of the rainforest over the tipping point, a recent study concluded. In the deforested eastern part of the Amazon, carbon dioxide emissions are already nine times as high as in the relatively undisturbed western part, said Brazilian forest scientist Luciana Vanni Gatti, lead author of the study. 

“We think that we know everything about nature, but we don’t,” she said. “I imagine nature like a domino. We understand the first, second, third, and maybe, some illuminated minds, the fourth. But when we make changes to nature, the dominoes fall, and that can have consequences on a very large scale.”

Some scientists say there are areas where forest restoration could help in the long-term effort to curb greenhouse gas heating, including the Atlantic Forest zone of Brazil, which has been stripped of trees over the last 200 to 300 years.

But in the May 8 issue of the journal Science, University of California, Santa Cruz restoration ecologist Karen Holl wrote that simplistic tree-planting formulations can overshadow other actions that could do more to slow global warming, like quickly reducing deforestation and cutting greenhouse gas emissions.

“The forest carbon bill proposed in Congress is not going to fix global warming, because we can’t plant our way out of climate change,” she said.

Tree-planting plans have to make sure that projects don’t displace food production and living space, and they have to include a long-term commitment to land protection, management and funding, she added.

“People love trees, and there are good reasons they love trees,” Holl said. “We’re not saying tree-planting is bad. But it’s more complicated than it seems.”