Restoring global forests where they occur naturally could potentially capture an additional 226 gigatons of planet-warming carbon, about a third of the amount humans have absorbed since the beginning of the world, according to a new study published Monday in the The industrial age was released by the magazine Nature.
The research, which involved more than 200 authors, used vast datasets collected from satellites and on the ground and was partly an attempt to resolve controversy surrounding an earlier paper. This 2019 study helped propel the Trillion Trees movement, but also caused scientific uproar.
The new conclusions were similar to those of a separate study published last year. The additional storage capacity would come primarily from allowing existing forests to recover to maturity.
However, major reservations remain: If we protect all current forests, where will people get wood, rubber and palm oil? Could forests store carbon fast enough? And how much forest carbon would be lost to fires, drought and pests if climate change worsens?
The storage capacity of 226 gigatons cannot be achieved without reducing greenhouse gas emissions, said Thomas Crowther, the study’s lead author and a professor of ecology at ETH Zurich, a university in Switzerland. “If we continue to emit carbon as we currently do, droughts, fires and other extreme events will continue to threaten the extent of the global forest system and further limit its contribution potential.”
Forests are crucial to tackling the climate and biodiversity crises. They provide food, shelter and shade to humans and countless other species. They clean our air and our water. And they remove climate-warming carbon from the atmosphere. As the climate crisis worsens, this ability makes them 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 that sparked scientific backlash but also inspired a World Economic Forum initiative to grow and conserve a trillion trees.
In 2019, he acknowledged, sloppy language led to trees being falsely portrayed as a panacea for climate change. His biggest fear now is that countries and companies will continue to treat forests this way, using them for carbon offsets to enable continued use of fossil fuels.
“We are all afraid that this potential of nature will be abused,” said Dr. Crowther. “Nature has such spectacular potential to help us deal with global threats, but it would be devastating if large organizations used nature as an excuse to do more damage to our planet.”
The World Economic Forum’s tree program, 1t.org, was launched with funding from Salesforce CEO Marc Benioff and supported by figures from then-President Donald Trump to Jane Goodall. Dr. Crowther himself, a charismatic and media-savvy scientist, is an advisor to the group.
At 226 gigatons of carbon, his new study’s figure is roughly the same as his previous study’s 205, but it arrives at a very different goal. Both papers exclude urban areas, farmland and pastures, but include rangelands where animals may graze at lower densities. According to the new research, 61 percent of additional carbon storage would come from protecting existing forests and the other 39 percent would come from growing trees in deforested areas with a low human footprint.
In the 2019 study, all of the carbon came from growing trees, where it could occur naturally outside of existing forests. More than 50 scientists published seven reviews in Science this year disputing both the analysis and its implications. One accusation was that the study advocated inappropriate tree planting in grasslands and other non-forested ecosystems, destroying native biodiversity. Another reason was that carbon storage estimates were far too high for the land area affected.
Simon Lewis, professor of global change science at University College London, made such a criticism in 2019. However, the new study was “reasonable,” he said.
Nevertheless, he emphasized that CO2 removal from forests must be kept in mind. “There is still only a finite amount of land that can be set aside for forests,” he said, “so only a small fraction 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 enormous amount of data the study produced, but said its findings still relied on inadequate tree densities in landscapes where they should occur naturally, but do so and remain barren, such as savannahs and deserts.
Despite global commitments, leaders are struggling to curb deforestation. Last year, the world lost 10 percent more primary tropical rainforest than in 2021, although Brazil’s current government has made progress recently.
Restoration efforts have also proven problematic. In the name of fighting climate change, countries and companies have often invested in failed mass plantings of trees or monocultures of commercial non-native species that harm biodiversity. While the latter could grow quickly, they only sequester half as much carbon over time, said Dr. Crowther.
He stressed that recovery should be driven by local communities who choose to work in harmony with nature to help themselves. Restor, a nonprofit he founded, connects community projects like an agroforestry farm in Ethiopia with potential supporters.
“Instead of cutting down the forests to grow coffee, they leave the forests standing,” said Dr. Crowther. “And because the forest stores water and nutrients, these trees grow very well without the need for fertilizers or irrigation, making nature their farm more productive.”
It is unclear how far such efforts can be expanded. Matthew Fagan, a professor of geography and environmental systems at the University of Maryland, Baltimore County, who works on global forest monitoring, said he believes the new estimate is too high because it doesn’t take people and fires into account.
“The fact that it agrees with other rough estimates of global carbon is more due to the unfortunate fact that they use the same methods and data sources than to the truth,” he said.
He and other scientists also raised concerns about the warming effect that trees can have in colder, drier climates, as they absorb heat that would otherwise have been reflected by snow or grass.
But everyone agrees on one thing: to combat both climate change and biodiversity loss, the world needs to do much more to reduce fossil fuels and stop deforestation.