The findings provide “clarity and confidence around the substantive role” of forests in fighting the climate crisis, says Wayne Walker, chief scientific officer of the Woodwell Climate Research Center, who was not involved in the research. Although there’s still “much uncertainty” as to the exact amount of additional carbon that forests could take up, Walker says the evidence is strong enough to justify acting to plant, restore, and protect forests.Humans have cut down a significant fraction—perhaps as much as half—of the forests that once existed. And every year, deforestation contributes 15% of all the greenhouse gas emissions caused by humans. So, scientists have been interested in finding out how much carbon trees could take out of the atmosphere if forests are allowed to regrow.
In 2019, a group led by ecologist Thomas Crowther of ETH Zürich published a paper in Science estimating that 205 gigatons of carbon could be taken up if forests were restored across 0.9 billion hectares of land, an area that’s roughly 10 times the size of China. Other scientists criticized the study, saying the figure was much too high because it assumed that each hectare of forest would capture and store 227 tons of carbon. The 227 tons was “an absurd figure,” says Simon Lewis, a global change scientist at University College London and the University of Leeds who wrote a critique of the 2019 paper. Some also worried the study would encourage climate mitigation efforts such as the planting of large tree plantations, which aren’t as beneficial for biodiversity and carbon capture as natural and diverse forests.
Crowther’s group came up with the figure by using satellite data to estimate how much carbon is currently stored by forests and by modeling how much more they could sock away if reforestation efforts took place. In the new study, Crowther’s group used satellite data as well, but it also created a separate estimate of forest carbon from ground-based data, which were extrapolated to a global scale using machine-learning models. The two estimates of global forest carbon differed by about 12%.
The convergence of findings based on the two methods “helps to increase confidence” in the estimate, says Susan Cook-Patton, a forest restoration scientist at the Nature Conservancy who was not involved.
Crowther and colleagues also calculated how much carbon would be stored in the forests if they were allowed to mature into old-growth forests. The team’s estimate included not just the carbon stored in trees, but also in dead wood, leaves on the ground, roots, and soil. They found that if existing forests were protected from logging, they could eventually absorb 138 gigatons of carbon. “That is a really exciting opportunity to achieve massive scale carbon capture, simply by protecting the ecosystems that we have,” Crowther said in a press conference. They also found that restoring tree cover where it once existed would take up another 88 gigatons of carbon—a considerable figure, although far less than the team’s 2019 estimate of 205 gigatons.
The authors acknowledge that demand for timber and other wood products will lower the potential for carbon uptake by forests. “We’re not talking about getting rid of timber,” Crowther said. “No one is advocating for turning everything into old-growth forests. We’re just saying what the full potential is.”
They and others also argue that efforts to minimize deforestation and promote reforestation will only go so far. For forests to take up and store carbon in large amounts, they must be protected from the heat and drought caused by climate change, which can fuel devastating wildfires. “The reality is that we need to slash fossil fuel emissions, end deforestation, and restore ecosystems to stabilize the climate,” Lewis says.
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