The latest round of United Nations climate negotiations, which took place in November in Germany, delivered a banner moment for wood. At the conference in Bonn, Reuters reported that 19 countries, including China, Brazil and Canada, pledged to use more trees to generate electricity—a form of power called wood bioenergy that some have hailed as a green alternative to fossil fuels.
But a new scientific report suggests that those nations might want to think twice. Using computer simulations, a group of researchers examined how wood bioenergy, often created by burning wood pellets, might stack up against coal, the poster-child for dirty energy. Echoing what some environmental organizations and scientists have argued, the team found that burning trees could cause a short-term bump in carbon dioxide in the atmosphere – creating a “carbon debt” that the world might never pay off.
The question of what’s cleaner, wood or coal, might seem like a no-brainer. Yes, trees emit carbon dioxide when burned – and actually generate more emissions per unit of power than coal. But proponents of wood bioenergy point out that trees, unlike coal, grow back. And when they do, they reabsorb all of that carbon dioxide from the atmosphere. It’s less like fossil fuel power and more like recycling. That logic inspired governments like the European Union (EU) and the United Kingdom, and now the 19 nations at the Bonn climate change conference, to encourage the use of wood bioenergy.
In the new paper, the researchers, led by John Sterman at MIT, set out to examine those assumptions. The team turned to computers to model what might happen if wood producers logged a range of forest types in the United States to generate one exajoule of electricity. (Globally, people consume more than 550 exajoules of electricity each year). As expected, the spate of tree burning would send an immediate surge of carbon dioxide to the atmosphere. Over time, the trees in those forests would return, offsetting that increase.
But the team discovered that, as with wracking up huge credit card bills, such a carbon debt can take a long time to pay off. Depending on the logging methods used, it might take as much as 104 years for a forest grow back enough to make wood bioenergy as clean as coal. South-Central oak-hickory forests would reach that point in 52 to 82 years, while Northeast maple-beech-birch forests would need 79 to 101 years to recharge.
Even more years would pass before wood bioenergy could match the efficiencies of solar or wind power. And during that time, the excess carbon dioxide would sit in the atmosphere, worsening climate change. That’s if—and it’s a big if—the forests grew back at all, the researchers say. Wildfires, diseases or shopping malls could all wipe out those young trees before they’ve had a chance to become tall.
In the end,the team cautions that turning to trees to supply the world’s power could worsen “global warming over the critical period through 2100 even if wood offsets coal, the most carbon-intensive fossil fuel.”