The research, which comes with important caveats, was partly an effort to address the scientific uproar surrounding an earlier paper.
Restoring global forests where they occur naturally could potentially capture an additional 226 gigatons of planet-warming carbon, equivalent to about a third of the amount that humans have released since the beginning of the Industrial Era, according to a new study published on Monday in the journal Nature.
The research, with input from more than 200 authors, leveraged vast troves of data collected by satellites and on the ground and was partly an effort to address the controversy surrounding an earlier paper. That study, in 2019, helped to spur the Trillion Trees movement but also caused a scientific uproar.
The new conclusions were similar to those in a separate study published last year. Mainly, the extra storage capacity would come from allowing existing forests to recover to maturity.
But major caveats remain: If we protect all current forests, where will people get timber, rubber and palm oil? Would forests be able to store carbon quickly enough? And how much forest carbon would be lost to fire, drought and pests as climate change intensifies?
The 226 gigatons of storage cannot be achieved without cutting greenhouse gas emissions, said Thomas Crowther, the study’s senior author and a professor of ecology at ETH Zurich, a university in Switzerland. “If we continue emitting carbon, as we’ve done to date, then droughts and fires and other extreme events will continue to threaten the scale of the global forest system, further limiting its potential to contribute.”
Forests are essential to tackling both the climate and biodiversity crises. They offer food, shelter and shade to humans and countless other species. They clean our air and water. And they pull climate-warming carbon out of the atmosphere. As the climate crisis intensifies, that ability has made them controversial: How much can we rely on trees to get us out of this mess?
Dr. Crowther was the senior author of a polarizing study on forest carbon in 2019 that drew scientific backlash but also inspired an effort by the World Economic Forum to grow and conserve one trillion trees.
In 2019, he acknowledged, careless language led to trees being wrongly painted as a silver bullet for climate change. Now, his biggest fear is that countries and companies will keep treating forests that way, using them for carbon offsets to enable the continued use of fossil fuels.
“We are all terrified that this potential of nature gets misused,” Dr. Crowther said. “Nature has such spectacular potential to help us tackle global threats, but it will be devastating if major organizations use nature as an excuse to do more harm to our planet.”
The World Economic Forum’s tree program, 1t.org, was started with funding from Marc Benioff, the chief executive of Salesforce, and endorsed by figures from then-President Donald Trump to Jane Goodall. Dr. Crowther himself, a charismatic and media-savvy scientist, is an adviser to the group.
His new study’s number of 226 gigatons of carbon approximates his previous one of 205, but it gets there very differently. Both papers exclude urban areas, croplands and pastures but include rangelands, where animals may graze at lower densities. In the new research, 61 percent of the additional carbon storage would come from protecting existing forests and the other 39 percent from growing trees in deforested areas with low human footprints.
In the 2019 study, all the carbon came from growing trees where they could occur naturally outside of existing forests. More than 50 scientists published seven critiques in Science that year, disputing both the analysis and its implications. One accusation was that the study endorsed inappropriate tree planting on grasslands and other nonforested ecosystems, destroying native biodiversity. Another was that the estimates of carbon storage were far too high for the amount of land concerned.
Simon Lewis, a professor of global change science at University College London, submitted one such critique in 2019. But the new study, he said, was “reasonable.”
Still, he emphasized that carbon drawdown from forests should be kept in perspective. “There is still only a finite amount of land to dedicate to forests,” he said, “so only a small fraction of the potential carbon uptake has a chance of being realized.”
Another critic from 2019, Joseph Veldman, a professor of ecology and conservation biology at Texas A&M University, praised the enormous amount of data the study brought to bear but said its findings still relied on inappropriate densities of trees in landscapes where they exist naturally but should remain sparse, like savannas and deserts.
Despite global pledges, leaders have struggled to rein in deforestation. Last year, the world lost 10 percent more primary tropical rainforest than in 2021, though Brazil’s current government has made recent progress.
Restoration efforts have also proven problematic. In the name of fighting climate change, countries and companies have often invested in failed mass tree plantings or monocultures of commercial, nonnative species that hurt biodiversity. While the latter might grow quickly, they sequester only half as much carbon over time, Dr. Crowther said.
He emphasized that restoration should be driven by local communities that choose to work in concert with nature to help themselves. A nonprofit he founded, Restor, connects community projects, like an agroforestry farm in Ethiopia, with potential supporters.
“Instead of removing the forests to grow coffee, they instead keep the forests standing,” Dr. Crowther said. “And because the forest captures water and nutrients, those trees grow really well without the need for fertilizers or irrigation, and as a result, nature makes their farm more productive.”
It’s unclear how much such efforts can scale up. Matthew Fagan, a professor of geography and environmental systems at the University of Maryland, Baltimore County, who works on global forest monitoring, said he believed the new estimate was too high because it did not account for people and fire.
“The fact that it aligns with other rough estimates of global carbon owes more to the unfortunate reality that they share methods and data sources in common than to the truth,” he said.
He and other scientists also raised concerns about the warming effects that trees can have in colder and drier climates as they absorb heat that would otherwise have been reflected by snow or grass.
But there is one thing they all agree on: To tackle both climate change and biodiversity loss, the world must do far more to cut fossil fuels and end deforestation of old-growth forests.