Forests, highly vulnerable to the ravages of climate change, are a risky bet for offsetting greenhouse gas emissions, according to research published Thursday in the journal Science.
That’s because any cap-and-trade program or tree-planting effort that relies on forests to slow the snowballing effects of climate change must consider how wildfire, drought, shifting climate zones and other factors will affect trees’ ability to store carbon, the study says.
The only way for forests to work as part of a climate strategy, it says, is if they’re able to grow old and store carbon dioxide in their trunks, roots, and soil for centuries.
“Not fully accounting for the range of climate- and human-driven risks to forests can result in an overestimation of the carbon storage potential of forest-based mitigation projects,” study co-author Deborah Huntzinger, a climate scientist at Northern Arizona University, said in a statement.
If the forests burn in a wildfire, die because of higher temperatures, or are killed by insect infestations or drought, all that stored carbon goes right back into the atmosphere, wiping out decades of carbon storage.
The research is being published as environmental groups and lawmakers on Capitol Hill support the “Trillion Trees” initiative, which calls for planting 1 trillion trees globally to help store carbon in forests.
In the U.S., The Trillion Trees Act (H.R. 5859), sponsored by Rep. Bruce Westerman (R-Ark.), is the Republicans’ answer to using public lands to tackle climate change. Thirty-three Republicans, including House Minority Leader Kevin McCarthy (California), and three Democrats are co-sponsors.
The bill aims to support a global tree-planting effort while bolstering the timber industry.
Westerman said at a House hearing on the issue in February that tree planting is key to any climate strategy.
The Utah research lends “a lot of credibility” to the Trillion Trees Act because the study says existing forests should be taken care of and deforestation should be stopped, Westerman said Thursday.
“We need to act now to make sure the forests we’ve got are healthy, and we’ve got to stop deforestation and make sure we’re regenerating healthy forests,” he said. “Trees are still the best carbon capturing mechanism that we have.”
The new research, which doesn’t mention the legislation specifically, says that any mass tree planting must consider how long the trees will live.
The climate benefits of planting trees would take decades or even centuries to materialize, said study lead author William Anderegg, an assistant professor of biology at the University of Utah.
Underestimating climate risks to forests and overestimating their carbon storage potential calls into question the legitimacy of massive global tree-planting efforts to address climate change, he said.
For example, California’s cap-and-trade program, which includes forest offsets based on their ability to store carbon, factors in various risks posed by climate change over the next century, but not beyond that. The study says that California’s and similar programs must consider what climate change will mean for those trees long-term because future climate conditions could mean all those carbon-storage efforts could go up in smoke.
Anderegg said tree planting should be a “third-tier” option after halting deforestation and cutting fossil fuel use.
The research was conducted by a team of climate scientists and biologists from universities and nonprofit groups around the country, including Princeton University, Stanford University, and Resources for the Future.
‘Overly Optimistic’ Estimates
Planting trees for carbon storage is also among many strategies the United Nations Intergovernmental Panel on Climate Change has suggested as a way to pull carbon from the atmosphere to address climate change, something known as “negative carbon emissions.”
UN scientists have concluded that pulling carbon dioxide from the air is an essential part of halting climate change.
The UN also promotes forest health and combating deforestation in developing countries as part of the its climate program and the negotiations that produced the Paris Climate Agreement.
Scientists unaffiliated with the study said it fills a gap in research that has been ignored by overly optimistic evaluations of forests’ potential to store carbon as a climate mitigation strategy.
“The central argument is that, for trees to store carbon, they need to stick around. So, anything that kills trees, be it fire or drought or a hurricane, decreases the potential of forests to store carbon,” said Carla Staver, an associate professor of ecology at Yale University.
“The paper is spot on in identifying that gap, and calls for permanence to be a core component of planning for carbon storage in forests,” Staver said.
One of the failings of the Trillion Trees initiative in the U.S. is that it calls for logging, followed by planting new trees, as a way to store carbon, she said.
“This flies in the face of the notion that we need forests to stick around in order for them to store carbon — basically, that deforestation is the worst thing you can do if you want to sequester carbon,” Staver said.
Westerman wants to plant trees in part to support the wood products industry. Trees grown for wood products naturally store the carbon they pull from the atmosphere through photosynthesis, and the carbon remains stored in the wood building materials until they rot or burn.
The study shows that climate change poses very large risks to forests in the future, and their role in mitigating climate change is highly uncertain, Nate McDowell, an Earth scientist at Pacific Northwest National Laboratory, said.
Tree-planting efforts need to anticipate how the climate will shift over the coming century or more in a particular place in order for the trees to live long enough to store enough carbon to make a climate difference, he said.