Smoke from Canadian wildfires hung in the air as McCarthy spoke. Asked about his plans to prevent further fires and other disasters fueled by climate change, the speaker suggested a strategy popular among Republicans: Plant a trillion trees.

The plan has some prominent backers. Then-President Donald Trump announced in 2020 that the United States would join a global initiative to plant a trillion trees, despite his antagonism toward climate science. The chairman of the House Natural Resources Committee has introduced legislation to plant a trillion trees as “a comprehensive, practical solution to the climate issues we’re facing today.”

But in recent years, climate scientists have grown more skeptical about relying on tree-planting programs. They have warned that heat waves, famines and infectious diseases could claim millions of additional lives by the end of this century unless humanity swiftly phases out the burning of oil, gas and coal.

Now, new research finds that planting a trillion trees would have a minimal effect on halting global warming, partly because of the long lag time for trees to reach maturity and absorb large amounts of carbon. The analysis by John Sterman, a professor at the MIT Sloan School of Management, and Andrew P. Jones, executive director of the nonprofit Climate Interactive, found that planting a trillion trees would only prevent 0.15 degrees Celsius (0.27 Fahrenheit) of warming by 2100.

“Trees are great. I personally love to be out in the forests as much as I possibly can,” Sterman said. “But the reality is very simple: You can plant a trillion trees, and even if they all survived, which wouldn’t happen, it just wouldn’t make that much difference to the climate.”

The analysis relied on a global climate simulator called En-ROADS, developed by Climate Interactive and the MIT Sloan Sustainability Initiative. It also found that planting a trillion trees would only sequester 6 percent of the carbon dioxide that the world needs to avoid emitting by 2050 to meet the goal of the Paris climate accord: limiting global warming to 1.5 degrees Celsius (2.7 Fahrenheit) above preindustrial levels.

“Planting a trillion trees is not a serious solution to the climate crisis,” Jones said. “It is too little, too late.”

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Trees do store vast amounts of carbon dioxide in their trunks, branches and roots. But old-growth forests sequester much more carbon than younger forests, and it usually takes 20 to 30 years for trees to reach full maturity. That means a tree planted today would do little to reduce emissions over the next crucial decades.

Trees are also especially vulnerable to drought, wildfires and pests, all of which are becoming more common as the world warms. In Montana, where the average temperature has increased by nearly 2.5 degrees Fahrenheit since 1950, a mountain pine beetle infestation has damaged or killed hundreds of thousands of acres of forests.

The researchers also highlighted that planting a trillion trees would require an enormous amount of land — 900 million hectares, or nearly three times the size of India. It would be nearly impossible to acquire that much land without disturbing grasslands or farmland, which already store carbon. Although producing renewable energy is also land-intensive, avoiding the same amount of carbon emissions by building more wind and solar farms would require only 15 million hectares by 2050, the authors found.

The research has not yet been peer-reviewed or published in a scientific journal. But Zeke Hausfather, a climate scientist at Berkeley Earth and the payments company Stripe, said the findings are “generally consistent” with his own analyses.

“I don’t think there’s too much uncertainty here that that amount of trees would reduce global temperatures by a bit more than a tenth of a degree,” he said. “So it’s by no means a silver bullet.”

At the same time, Hausfather emphasized that “every tenth of a degree matters. So if you can reduce temperatures by 0.15 C, that still opens up more room for us to be able to achieve our most ambitious climate targets. It just means that it’s not a replacement for reducing emissions as quickly as possible.”

Both Hausfather and the authors of the analysis also clarified that they support preserving existing forests, including rainforests. Although the Amazon has historically acted as a massive carbon sink, deforestation is pushing the critical rainforest toward a tipping point, scientists say.

How deforestation is pushing the Amazon toward a tipping point

Rep. Bruce Westerman (R-Ark.), who chairs the House Natural Resources Committee and earned a master of forestry degree from Yale University, defended tree planting as a climate solution.

“It’s proven. It’s low-cost. It’s large-scale,” Westerman said in an interview last week. “I don’t know of any other solution anybody’s proposing that you can get two of those benefits from.”

Asked about phasing out U.S. production of fossil fuels, the Republican scoffed, noting that China last year approved the largest expansion of coal-fired power plants since 2015.

“That’s a pipe dream,” Westerman said. “If we phase them all out here, they’re still going to be increasingly used around the world. ... But if you want to kill people and starve people to death and go backwards in technology and innovation, then yeah, phasing out fossil fuels would certainly do that and reduce greenhouse gas emissions.”

The Trillion Trees Act, which Westerman introduced in 2021, garnered 105 co-sponsors, including three Democrats and three members of the current House GOP leadership. Westerman said he hopes to reintroduce the measure after this month’s congressional recess.

Chris Barnard, vice president of external affairs at the American Conservation Coalition, a group that seeks to engage younger conservatives on climate change, said he thinks Republicans’ remarks about planting trees have been misconstrued. He said GOP lawmakers are not suggesting that planting a trillion trees is the only way the world should address climate change. The party, for instance, has also proposed expanding nuclear power, accelerating the permitting process for energy projects, and bolstering America’s supply chains for the critical minerals used in electric vehicles and other green technologies.

“These are all things Republicans have championed,” Barnard said. “But the party has not done a good job of messaging that to people, especially to young people who care about climate issues.”

Polling has found that while Republicans and Democrats are deeply divided over climate change, younger voters in both parties prioritize the issue, and tree planting remains popular across ages and ideologies. A 2020 survey by the Pew Research Center found that 90 percent of Americans in both parties support planting a trillion trees to absorb carbon emissions, compared with 71 percent who support tougher fuel efficiency standards for cars and trucks.

Although many Democrats favor a rapid transition to renewable energy, they also support a targeted approach to planting trees. The bipartisan infrastructure law of 2021 authorizes the U.S. Forest Service to plant more than a billion trees in national forests over the next decade. President Biden’s landmark climate law, the Inflation Reduction Act, also provides more than $1 billion to increase access to trees in urban neighborhoods experiencing the “heat island effect,” in which heat reflects off surfaces such as concrete and asphalt.

Where the most U.S. residents bake because of concrete and lack of trees

The funding will help ensure that “families have a place to go to cool off,” Biden said last week while announcing new measures aimed at protecting communities from extreme heat this summer.

Public interest in planting a trillion trees exploded in 2019, when an eye-catching study claimed that such a move could remove two-thirds of all carbon emissions from human activities that remained in the atmosphere. The paper in the journal Science received intense media coverage, with a headline in the Guardian proclaiming that “tree planting ‘has mind-blowing potential’” to tackle the climate crisis.

But the backlash was swift. Several scientists criticized the authors’ assumptions and findings, saying they wildly overestimated trees’ potential to store carbon. The authors later issued an “erratum” acknowledging several errors and clarifying that they had never intended tree planting to be a replacement for emissions-cutting measures.

During his visit to Ohio in June, however, McCarthy cited the 2019 study’s original findings. “If we planted a trillion trees, we would take two-thirds of all the emissions created from the beginning of the Industrial Revolution till today,” he said.

McCarthy’s office and Jean-François Bastin, the lead author of the 2019 paper and a professor at the University of Liège in Belgium, did not respond to requests for comment.

The paper seems to have resonated with policymakers and the public, despite its “astronomically incorrect” numbers, said Carla Staver, an associate professor of ecology and evolutionary biology at Yale University. It is important to correct the record, she said, and spread awareness of the potential downsides of planting trees as a climate strategy. Her work has found that planting trees in African savannas can threaten plants and animals that have not adapted to forest ecosystems.

“There are real impacts on real ecosystems,” Staver said, “that probably shouldn’t have more trees in them.”