How much can trees fight climate change? Massively, but not alone, according to Study.

Restoring global forests where they naturally occur could potentially sequester an additional 226 gigatons of planet-warming carbon, equivalent to about a third of the amount humans have released since the dawn of the industrial age, according to a new study published Monday in the journal Nature.

The study, with input from more than 200 authors, used vast amounts of data collected by satellites and on the ground, and was partly an attempt to address the controversy surrounding an earlier paper. That 2019 study helped boost the Trillion Trees movement, but it also caused a scientific stir.

The new conclusions were similar to those in a separate research published last year. The additional storage capacity would mainly come from existing forests being able to recover until they reach maturity.

But major caveats remain: If we protect all current forests, where will people get wood, rubber and palm oil? Could forests store carbon quickly enough? And how much forest carbon would be lost to fire, drought and pests as climate change increases?

The 226 gigatons of storage cannot be achieved without reducing greenhouse gas emissions, said Thomas Crowther, senior author of the study and professor of ecology at ETH Zurich, a university in Switzerland. “If we continue to emit carbon as we have done so far, droughts, fires and other extreme events will continue to threaten the size of the global forest system, further limiting its potential.”

Forests are essential for tackling both the climate and biodiversity crises. They provide food, shelter and shade to humans and countless other species. They purify our air and water. And they remove climate-warming carbon from the atmosphere. As the climate crisis intensifies, that ability has made it controversial: How much can we rely on trees to get us out of this mess?

Dr. Crowther was the lead author of a polarizing study on forest carbon in 2019, which caused scientific backlash but also inspired a World Economic Forum effort to grow and preserve one trillion trees.

In 2019, he acknowledged, careless language led to trees being wrongly painted as a panacea for climate change. Now his biggest fear is that countries and companies will continue to treat forests that way and use them for carbon offsets to enable the continued use of fossil fuels.

“We are all terrified that this potential of nature is being abused,” said Dr. Crowther. “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 damage to our planet.”

The World Economic Forum’s boom program, 1t.org, was launched with funding from Salesforce CEO Marc Benioff and endorsed by figures from then-President Donald Trump to Jane Goodall. Dr. Crowther himself, a charismatic and media-savvy scientistis an advisor to the group.

The number of 226 gigatons of carbon in his new study approaches his previous one of 205, but it comes out very differently. Both articles exclude urban areas, croplands and pastures, but include rangelands, where animals can graze at lower densities. According to the new research, 61 percent of the additional carbon storage would come from protecting existing forests and the remaining 39 percent from growing trees in deforested areas with a low human footprint.

In the 2019 study, all the carbon came from growing trees where they could occur naturally outside of existing forests. More than fifty scientists published seven critiques in Science that year, contesting both the analysis and its implications. One accusation was that the study endorsed the inappropriate planting of trees on grasslands and other non-forested ecosystems, which destroyed native biodiversity. Another was that carbon storage estimates were far too high for the amount of land in question.

Simon Lewis, professor of global change science at University College London, made such a criticism in 2019. But the new study was, he said, “reasonable.”

Still, he emphasized that carbon absorption from forests must be kept in perspective. “There is still only a limited amount of land that can be devoted to forests,” he said, “so only a small portion of the potential carbon capture has a chance of being realized.”

Another 2019 critic, Joseph Veldman, a professor of ecology and conservation biology at Texas A&M University, praised the vast amount of data the study revealed, but said the findings still relied on inappropriate densities of trees in landscapes where they naturally grow. occur, but should remain scarce, such as savannas and deserts.

Despite global commitments, leaders have struggled to rein in deforestation. The world lost 10 percent more primary tropical rainforest last year than in 2021, although Brazil’s current government has made recent progress.

Recovery efforts have also proven problematic. In the name of fighting climate change, countries and companies have often invested in failed mass tree plantations or monocultures of commercial, non-native species that harm biodiversity. Although the latter can grow quickly, they store only half as much carbon over time, said Dr. Crowther.

He emphasized that recovery must be driven by local communities who choose to work with nature to help themselves. A nonprofit he founded, Restor, connects community projects, such as an agroforestry farm in Ethiopia, with potential supporters.

“Instead of cutting down the forests to grow coffee, they preserve the forests,” said Dr. Crowther. “And because the forest captures water and nutrients, those trees grow very well without the need for fertilizer or irrigation, and as a result, nature makes their farm more productive.”

It is unclear to what extent such efforts can be scaled 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 didn’t take people and fire into account.

“The fact that it aligns with other rough estimates of global carbon emissions has more to do with the unfortunate reality that they share methods and data sources than with the truth,” he said.

He and other scientists also raised concerns about the warming effects that trees may have in colder and drier climates because they absorb heat that would otherwise be 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 much more to reduce fossil fuels and end the deforestation of old-growth forests.