Maybe Wooden Skyscrapers Aren’t As Climate-Friendly As We Think
New research casts doubt on a popular climate solution.
A lengthy report from the nonprofit World Resources Institute released Thursday warns of a “growing land squeeze” where increasing demand for food, housing, and wood is threatening the world’s prospects for tackling climate change. Adding to the competition, the authors argue, is something that’s been broadly advertised as a climate solution — the use of mass timber.
Architects and sustainable building advocates have been spreading the gospel about mass timber for at least a decade. The idea is that replacing carbon-intensive materials like concrete and steel with wood can reduce the climate impact of building stuff. Forests suck up carbon from the atmosphere, and using that timber in the built environment is one way to lock it away more permanently.
Countless articles and photo essays and magazine stories featuring sanctuary-like skyscrapers made of wood have painted it as a no-brainer for sustainability. The concept has also been backed up by academic research published in peer-reviewed journals.
But according to Timothy Searchinger, a senior research scholar at Princeton University and the lead author of the land squeeze report, they’ve been looking at the carbon footprint of timber the wrong way. “What they’re really doing is treating land and plant growth as free,” Searchinger told me.
Mass timber advocates often emphasize that the wood must be “carbon neutral” and come from sustainably managed forests. The idea is that as long as the amount of wood removed from a forest for construction matches the forest’s growth that year, there’s no net impact on the climate. “What that misses,” said Searchinger, “is that if you didn’t harvest it, the forest would grow and absorb carbon. You’re keeping that added growth from happening.”
This is often called the “opportunity cost,” i.e. “the loss of potential gain from other alternatives when one alternative is chosen,” as the Oxford dictionary puts it. Not all researchers agree that it’s always appropriate to account for this kind of what-if scenario. Some told me that you can't assume forests have the ability to perpetually accumulate more carbon — mature forests reach a sort of stasis.
But Searchinger and his co-authors highlight another frequent accounting error with mass timber. Only a small portion of the wood harvested makes it into the final product. Some of it is lost to roots and bark and other debris left behind in the forest or burned, and some of it goes into shorter-lived products like wood chips and paper that decompose and release carbon in a matter of years. “So only a small amount actually gets into the building. All that other carbon is emitted. That is what they’re ignoring,” said Searchinger.
The authors analyzed a number of different scenarios with different types of wood sourced from different types of forests, with greater and greater amounts diverted to construction, searching for any conditions that would make mass timber pencil out as a net benefit for the climate compared with concrete and steel. Few did.
There were more or less two conditions that had to be met to see significant carbon savings. At least 70% of the wood harvested had to make it into the construction product, and the wood needed to be sourced from a fast-growing tree farm. The problem with that, Searchinger told me, is that all of our existing tree plantations are meeting existing demand for other wood products. “So there’s no free lunch out there.”
The calculus could shift if we’re able to reduce demand for other wood products, he said, but by then we may have figured out how to affordably cut emissions from the production of steel and concrete.
I sent the paper to several outside experts who were critical of its findings. One issue they raised was that some forests, when they are not managed, become more susceptible to severe wildfires, disease, and other disturbances, and can thus turn into net sources of carbon emissions as trees burn or rot. Austin Himes, an ecologist at Mississippi State University, told me that in the western U.S., for example, there's good evidence that removing timber and excess fuel can make the remaining forest more resilient and enable it to suck up more carbon.
Himes also stressed that this kind of analysis is complex, and the results are sensitive to tons of assumptions about location, transportation, manufacturing, and what happens to any material that doesn’t make it into the final product. But most of the literature he’s seen strongly suggests that using wood in construction to meet growing demand in our cities is going to have long term benefits.
“There’s uncertainty around that conclusion and this report highlights some of that, and so there’s obviously need for continually assessing a lot of those assumptions,” he said, “but this is one report based on one model and one set of assumptions.”
I also spoke with Beverly Law, a forest ecologist at the Oregon State University, whose research is cited extensively in the report and who praised its findings. She echoed Himes' statement that there is a lot of uncertainty about how to accurately account for the emissions benefits of substituting wood for concrete or steel, but she agrees with the new report that those benefits have been widely overestimated. “Substitution gets really hard,” Law said. “It’s a number that people can fiddle with.”
She pointed me to a 2019 paper by ecologist Mark Harmon which questioned common assumptions made when calculating the emissions benefits of substituting wood for concrete or steel, including not accounting for the fact that the energy used to produce concrete and steel is getting cleaner as coal is replaced with natural gas and renewables on the grid. Innovations in concrete also have the potential to turn the material into a carbon sink.
The bigger picture painted by the land squeeze report should give any mass timber advocate pause, even putting the carbon analysis aside. Demand for wood is expected to rise dramatically between now and 2050, without a growing mass timber industry. The authors estimate that an area roughly the size of the continental United States could be harvested for wood by then, releasing 3.5 to 4.2 billion tons of carbon dioxide per year, or more than 10% of recent annual global emissions.
Searchinger’s team does offer recommendations to shrink those numbers, including expanded recycling of wood products, reduced use of packing materials, the adoption of more efficient wood-burning stoves, and aid to developing countries to move away from wood-based heating systems. There's also potential to increase yields from existing tree farms.
Beyond wood products, the report also raises big, difficult questions about how we might use land more efficiently to feed and house a growing population on a finite planet, especially as tackling climate change requires preserving and restoring natural habitats to store more carbon.
As Searchinger and his co-authors wrote in a blog post about the report, “Given this squeeze, it is dangerous to adopt policies that encourage yet more human demands for land and its outputs.”