It is conventional wisdom that building with wood has lower carbon emissions than building with steel or concrete. In this post, Dr. Stephanie Taylor, MD, wrote, “If buildings are constructed of wood, not only are the emissions from manufacturing steel and other building materials reduced, the innate carbon-storing property of wood is utilized to benefit decarbonization of the outdoor atmosphere.”

There has long been debate about how much carbon is stored or sequestered in wood construction; much depends on how much of a tree actually makes it from the forest to the mill to the building. But the consensus, according to the Mass Timber Report, is that “When wood is chosen over steel or concrete building materials, the net effect is a reduction in fossil fuel use.” The rule of thumb is that a cubic meter of wood sequesters a ton of carbon dioxide (CO2).

Another take

However, a new study and report challenges the conventional wisdom. The study,  The carbon costs of global wood harvests, claims that we should not consider the carbon to be sequestered in the building, because “the forest growth and regrowth used to offset the effects of new harvests would happen anyway.” In other words, the tree in the forest is already storing the carbon, chopping it down stops the process, and turning it into mass timber doesn’t magically store new carbon.

The authors of the study, led by Tim Searchinger of Princeton University, have also written a report, The Global Land Squeeze: Managing Growing Competition for Land for the World Resources Institute (WRI), as well as a summary with the controversial headline, Wood is Not the Climate-Friendly Building Material Some Claim it to Be. I shared their work on Linkedin; the resulting comments range from “It’s utter bollocks” to “I’m with most of your commentators who don’t want to drive traffic to an article so superficial and skewed as to not even merit your boost.” My initial reaction on reading the article was similar to Dorothy Parker’s review of Atlas Shrugged: “This is not a novel to be tossed aside lightly. It should be thrown with great force.”

But Tim Searchinger is a respectable scientist and the WRI is not a bunch of industry flacks and climate deniers. And when you get into all the reports and studies, you find that they are not so superficial; and while the headline is provocative, it is not entirely untrue.

Impact on forests

We also have to recognize their agenda, which is clear in the title of the report: they are worried about the global land squeeze and the pressure on forests if we keep expanding the use of wood. As for the controversial title of the post, some people have made pretty outrageous claims about wood construction. I was at a Wood Solutions Fair a few years ago where the director of a major Swedish homebuilder justified the use of cross-laminated timber instead of wood framing on a low-rise building by saying, “If we use more wood, we are then growing more trees and absorbing more CO2.”

In the WRI post, Searchinger’s first point is that “most wood (and its stored carbon) is lost during production,” noting that a third or more is left as roots or branches. Bark and sawdust is often burned. Other parts of the tree are turned into paper or furniture, which store carbon longer, but rarely for decades; and are also burned or decompose when thrown out. “These processes all emit carbon that would otherwise remain stored if forests were kept standing.”

None of this is news, nor is it controversial. Dave Atkins, author of the 2019 mass timber report, told me at the time that the consensus is that 50% of the carbon in the form of wood makes it into mass timber. But Atkins also noted that if the trees were left in the forest, 100% of the carbon would eventually be released into the air, so 50% is pretty good. He said, “If you don’t grow it, you mine it.”

Carbon sequestration truths

Embodied carbon pioneer Chris Magwood of the Endeavor Centre has also expressed reservations about how much carbon is actually sequestered in wood construction, writing in his EMBARC study: “There remain important and unresolved concerns with current accounting methods related to virgin forest products like lumber. Some of these concerns include uncertainty about the amount of carbon released from soils during logging operations; the amount of carbon returning to the atmosphere from roots, slash and mill waste; the amount of carbon storage capacity lost when a growing tree is harvested; and the lag time for newly planted trees to begin absorbing significant amounts of atmospheric carbon dioxide.”

(When you look at the information in the Nature study referenced earlier, 49% of the logs make it into wood and plywood—not far off the consensus.)

It is important to note that Searchinger doesn’t distinguish between “fast”carbon that comes from burning vegetation and “slow” carbon from burning fossil fuels. The word “biogenic” does not appear anywhere. He is not alone here; lots of people, including me, who complain about burning wood pellets, have said “Carbon is carbon, the atmosphere doesn’t know the difference.” Those who promote the use of wood note that by replanting trees in a sustainable manner, that carbon will be re-absorbed. Searchinger discounts this claim, literally; he applies a discount rate to future carbon storage compared to emissions today.

So all that carbon is standing in the growing trees. If we chop them down they stop absorbing more. If we 1. deny any sequestration benefits, 2. ignore that trees die and release their carbon or that forests burn, 3. discount future growth “to express our preference for mitigating climate change by reducing emissions right away,” and 4. treat fast carbon the same as slow carbon, then wood doesn’t look so good. If we total up only the negatives, then we can claim that building with wood contributes to increased emissions.

With steel and concrete, we total up the positives, noting that “possible techniques for reducing emissions from steel or concrete include capturing the carbon emitted from their production and putting it underground.” This isn’t denying the benefits of substituting wood for steel or concrete, but it does downplay it. That’s how we get to statements like this:

“Our analysis also shows that ‘sustainable forest management,’ as conventionally understood, does not mean that wood use is carbon neutral or that using wood in construction in place of concrete and steel necessarily provides a net climate benefit. Harvesting wood comes with a time-discounted cost in lost carbon in the forest. The climate benefits of harvesting wood include the storage of some of that forest carbon elsewhere and avoided emissions from other carbon-intensive products such as concrete and steel. But the climate costs are reduced storage of carbon in the forest.”

Land is the bottom line

In the end, it’s clear that Searchinger and his team are most concerned about land use, and the increasing pressure that mass timber construction will put on the forests. In their report they note: “Producing 50 percent of new urban construction with wood would likely require more than a 50 percent increase in uses of industrial roundwood. These levels of competition, along with the vast competition already inherent just from rising incomes and population, pose enormous challenges for both the climate and biodiversity.”

Once again, this is not controversial. Andy Simmonds of the Association for Environment Conscious Building (AECB) and journalist Lenny Antonelli wrote in Passive House+ magazine last year that we need to be more careful, saying:

“In Sweden, natural forests are being systematically clear-cut and replaced with even-aged plantations, according to five environmental NGOs. Recent logging of protected forests has also been reported in Estonia, Lithuania, and Romania. And with extractive demand on land increasing, nature is more at risk of being squeezed out. So, while supporting a move from concrete and steel to timber and other natural fibres, our primary goal should be to dramatically reduce the quantity of raw materials needed in the first place. When specifying timber or other natural materials, how efficiently they are used can minimise the pressure on landscapes. As well as prioritising reclaimed or recycled materials where possible, smart choice of build system matters too.”

This is not something we can ignore. And fortunately, architects are not ignoring it. While I was touring his new Black and White Building, Andrew Waugh explained that his new buildings use 40% less fiber per square meter than they once did. The LVL lumber is much stronger, resulting in smaller beams and columns; and the process uses 90% of the log.

Many other architects are using stick framing instead of mass timber on low-rise buildings, recognizing that, as Life Cycle Assessment expert Jane Anderson says, we need “more buildings in timber, not more timber in buildings.”

In the end, I think it is fair to say that Searchinger and his team make some good points. Counting sequestered carbon as a benefit is a mug’s game. As engineer Will Hawkins has noted, “When sequestration is reported as a negative emission, it can create the counterintuitive impression that using timber excessively can have environmental benefits.”

We should never forget about the forests; this is the main point that Searchinger and his team are trying to convey. As Martin Holladay recently wrote, “We all depend on forests. Forests provide clean water and support thousands of species that are key players in complicated ecosystems. When we lose our forests, we lose a precious part of our planet’s complicated web of life. In short, without forests we will die.”

Multi-pronged approach is best

No material is perfect, including wood. We should be harvesting it more carefully, using it more efficiently. We should also recycle and reuse as much of it as possible, and stop burning wood for fuel.

But wood grows back. Concrete and steel do not. The carbon footprint of concrete and steel are known; if the numbers for wood are still iffy, they are getting better all the time.

Searchinger concludes: “Because the world already faces a global land squeeze as it tries to balance growing human demands for food, wood and urban development with nature and climate goals, it is dangerous to adopt policies that encourage yet more human demands for land and its outputs. The world has a fixed quantity of land; people should be working hard and creatively not to expand, but rather to reduce their footprint on it.”

But it is more dangerous to continue burning fossil fuels to make concrete and steel and to continue losing our forests to fire and disease exacerbated by climate change. The authors have their thumb on the scale, making wood construction look worse and steel and concrete look better. The appropriate answer is to use less material, whatever it is.

________________________________________________________________________

Lloyd Alter is a former architect and developer. His journalism career includes 15-plus years as design editor at Treehugger.com. Today he teaches sustainable design at Toronto Metropolitan University. His work can be found at Carbon Upfront.