Can today’s hottest sustainable building method actually slow climate change?
Washington Governor Jay Inslee stepped to a lectern in a sprawling 270,000-square-foot factory outside Spokane and declared it the “best day so far” in his six years in office. Earlier that day, he had marched downtown as part of the youth-driven climate strike that united 4 million people worldwide. Now he was in nearby Spokane Valley, heralding a new factory with an innovative product that, he said, answered the “kids'” calls for climate action.
The plant — one of the largest of its kind in North America — produces today’s hottest sustainable building material, called cross-laminated timber, or CLT. Rollers and lifts shuffle and stack lumber, feeding a giant press that glues the assembled boards to form immense 12-foot by 60-foot panels. With up to nine stacked layers, each laid perpendicular to its neighbors, the panels are stronger — pound for pound — than concrete. The multi-ton panels are then precision-cut to fashion them into floors and walls for office and apartment towers — pre-fabricated panels that will snap together at the construction site like Lego blocks.
What fired up Inslee was that those panels are designed to replace steel and concrete — traditional construction materials whose production accounts for about 13 percent of global carbon dioxide emissions. Cross-laminated timber panels, in contrast, contain the carbon-cutting dividends of photosynthesis: While growing as trees, the wood in the panels pulled CO2 from the Earth’s atmosphere. And as long as a CLT building stands, that carbon should remain locked up in its walls and floors.
What’s more, advocates of the technology say, the snap-together fabrication of CLT panels may be both cleaner than and as affordable as building with concrete and steel. Here, gushed Inslee, was an example of a “somewhat older generation who’s using their technological prowess and entrepreneurial zeal” to create a “solution to climate change” — not to mention jobs in economically depressed eastern Washington.
The governor was in top rhetorical form, fresh off his short-lived, climate-focused presidential bid. But CLT’s Pacific Northwest juggernaut is lacking in one crucial element: Proof that it will really help slow climate change. Some forest scientists, climate modelers, and materials experts are raising tough questions about the wisdom of boosting the region’s wood harvests. They argue that forestry’s carbon footprint is far more complex than the “wood is good” message pushed by CLT’s supporters. Forests are only able to regenerate if the lumber industry makes sustainable choices, they say, and plenty of carbon gets dumped into the atmosphere when logs are transformed into snap-tight CLT buildings.
“The carbon story is actually rather complicated for wood compared to other building products,” says Frances Yang, a San Francisco-based sustainable materials specialist with the global engineering giant ARUP. “There’s carbon dioxide constantly going in and out of the system.”
Such complexity is obscured in most carbon tallies for wood projects, including those that Katerra, the Silicon Valley-based building firm that poured more than $150 million into the Spokane Valley plant, is preparing to market its CLT. Those analyses simply assume an equal balance of CO2 is emitted and absorbed by forests — a “gaping hole” in the methodology according to David Diaz, director of forestry analytics and technology at Ecotrust, a Portland-based nonprofit.
It’s possible that Katerra’s megafactory and other CLT plants might not be the climate-change weapon that Inslee and the industry claim they are — especially in one of the world’s most productive forested regions, which is already suffering from warmer temperatures and larger and more frequent wildfires linked to climate change. Increased wood harvesting to feed CLT demand could also harm ecosystems and degrade water quality.
“If we rev up demand for this material tomorrow in a big way without paying attention to the consequences,” says Mark Wishnie, until recently the director for global forestry and wood products at The Nature Conservancy, a Virginia-based environmental advocacy group, “we are very likely to get all kinds of outcomes that we don’t want.”
First developed in Europe in the 1990s, CLT has been gaining traction in North America over the past decade. Growing demand for it in the Pacific Northwest, projected just a few years ago to reach 6.6 million cubic feet by 2035, may meet — and exceed — that volume as early as 2021, as new plants such as Katerra’s get up to speed. Capacity is similarly rising across the border in British Columbia.
Meanwhile, there is growing pressure to cut carbon emissions as quickly as possible. The influential sustainable design alliance Architecture2030 has exhorted the building sector to slash its embodied carbon — the sum of greenhouse gas emissions associated with a building’s materials and assembly — by 45 percent by 2025. “We need to be identifying pathways for radical reductions of emissions from building materials,” says Kate Simonen, a professor at the University of Washington who specializes in assessing the environmental impacts of building materials.
CLT is one possible pathway, but research published last year by Simonen and some University of Washington colleagues reveals several fault lines in the CLT carbon debate.
The team compared options for an eight-story office tower in Seattle. One employed a typical structure of steel-reinforced concrete. Two others used different volumes of structural wood delivered from a hypothetical local CLT plant on Washington’s Olympic Peninsula. On average, the CLT structures used 1,495 cubic meters, or about 52,795 cubic feet, of mass timber panels.
To compare the designs’ carbon footprints, the researchers used a life-cycle assessment (LCA) — a scientific method for tallying the overall environmental impact of a process or product. They estimated the amount of CO2 released during the creation and shipping of each design’s materials, then added CO2 generated on-site by construction equipment. The result: On average, the CLT office towers required 26.5 percent less carbon than using steel-reinforced concrete, avoiding about as much CO2 as 250 cars pump out in a year.
And that was without taking stock of the wood buildings’ stored carbon. Advocates such as the industry group WoodWorks routinely cite far better performance by factoring in the CO2 locked up in a CLT building’s floors and walls. Unlike the fossil carbon spewing from coal-fired steelworks or the diesel air compressors at worksites, they note, the carbon in a CLT panel is biogenic — it was once atmospheric carbon that was captured and photosynthesized by trees. Subtract the tons of biogenic CO2 that the academic team estimated would be sequestered within that Seattle office tower, and the CLT designs’ carbon footprint shrinks considerably, from 26.5 percent smaller than business-as-usual construction to as much as 80 percent less.
But Simonen, who founded the university’s Carbon Leadership Forum to help designers and builders reduce carbon emissions, warns against laying claim to that extra benefit. In fact, she highlights several limits to her team’s LCA. She noted, for example, that the study used a “simplistic” model of a reinforced-concrete building that might require more concrete and steel than necessary compared with the more optimized CLT designs.
The analysis might also have overstated concrete’s embodied carbon, because they plugged in standard carbon figures that do not account for recent research showing that concrete actually absorbs a lot of CO2 over its lifetime. Such “carbonation” may sequester more than 40 percent of the CO2 released by the chemistry of cement production, according to a 2016 study in the journal Nature Geophysics. That is the equivalent of trimming the embodied carbon in concrete by about 20 percent.*
As for CLT’s capacity to store carbon? Simonen says that while wood buildings store biogenic CO2, the process of harvesting and turning trees into building materials also spurs the release of biogenic CO2. Carbon escapes, she says, when logs are fashioned into panels. Sawmills and CLT plants typically burn bark, sawdust, and other wood wastes for heat. Combusting such castoffs would release 1.7-times more CO2 than would be stored in the Seattle tower’s CLT panels, according to a tally in the University of Washington report.
And more biogenic CO2 escapes in the forest. Slash piles left behind after a harvest — gathered branches, treetops, and smaller trees — are often burned or left to decompose. Soil and decomposing stumps and roots release further CO2 for years after the loggers have left.
The University of Washington study used the standard “cradle-to-gate” methodology for life cycle analysis, which assumes a neutral value for carbon emissions from the forest — wood’s ultimate cradle. Simonen’s co-lead on the Seattle tower comparison, University of Washington environmental science associate professor Indroneil Ganguly, says leaving out biogenic carbon in LCA calculations remains a valid approach. That’s because forests absorb and release carbon in a continuous cycle. And as long as the forest regrows, Ganguly says, there is no net release of biogenic carbon to the atmosphere.
The study’s hypothetical CLT towers pass that test, he adds, since they are built from locally harvested trees. Thanks to tighter habitat protections mandated in the 1990s, Western Washington’s coastal forests consistently grow more wood each year than timber firms truck to market.
Zoom out to a regional scale or fast-forward a decade or two, however, and the picture darkens, other experts say. Consider British Columbia, whose vast forests cover an area larger than Utah. For more than a decade those lands have given up more carbon every year through harvesting than photosynthesis has recaptured, according to the province’s official greenhouse gas inventory. (Forest fires and beetle infestations have also plagued forests in British Columbia during that time.)
In 2017, the worst year on record, loggers harvested 2.5 times more carbon than growing trees added back, according to the inventory. Add in CO2 emissions from wildfires, decomposition, and other practices, and forestry lands gave up over 200 megatons of CO2 — more than triple all of the province’s other carbon sources combined.
Research suggests climate change and fires, among other factors, make it difficult to predict when the forests will recover to become a net “sink” for carbon rather than a net source, according to Caren Dymond, a senior scientist and forest carbon expert with British Columbia’s Ministry of Forests, Lands, Natural Resource Operations, and Rural Development. What’s clear is that recovering is decades off. “Our research shows the most optimistic scenario is that it will be a net sink by 2070,” she says. “But if climate change causes more fires and more emissions from decay in the forest floor, then it might not be a net sink, even by 2070.”
Dymond says research suggests it is best practice for LCA studies in British Columbia not to assume that all biogenic carbon released from forests will be recaptured in the near-term: “They shouldn’t be considering it carbon neutral.”
Materials experts say ignoring biogenic carbon makes more sense in Europe, where historical forest decimation led to the creation of strict forestry codes and a centuries-long record of sustainable forestry. But even there, scientists are raising concerns. The European Academies’ Science Advisory Council in 2018 raised a red flag about power companies fueling their generators with wood instead of coal to meet carbon mandates. The scientists warned that this fuel switch was “drastically” reducing forests’ ability to capture carbon, and thus could be “perversely” accelerating climate change.
According to experts such as Yang, Simonen, and Diaz, more nuanced approaches are needed to assess CLT’s impact on the global carbon cycle — methods that link the new wood buildings to forestry methods that preserve more carbon in forests. As Diaz puts it: “What’s lost in the generic ‘wood is good’ branding is the recognition that forest practices can be improved.”
Materials experts say more comprehensive LCAs are needed to more carefully account for biogenic carbon flows in forests — methods that can tally the carbon debt incurred when a forest is harvested, and track repayment on that debt as forests regrow.
Dymond recently coauthored a peer-reviewed conference paper presenting one such study: an LCA evaluating expanded forestry in the United Kingdom that tracked all relevant carbon flowing in and out of the atmosphere. The paper determined that planting trees on cow pastures to supply materials for timber-framed homes would avoid 2.4 gigatons of CO2 releases over the following 100 years compared to business-as-usual. That’s about five years’ worth of all U.K. greenhouse gas emissions.
While that study examined lumber and plywood, she says the same methods could be applied to CLT. Ganguly’s group plans to do just that, according to Francesca Pierobon, a scientist at the University of Washington. She says their team is also working on next-generation LCA methods that incorporate biogenic carbon. But such methods have been in development for some time and are still not widely accepted or practiced. In 2012, the U.S. Environmental Protection Agency’s science advisory board declared that treating wood as carbon neutral was “not scientifically valid” but it has yet to agree on a better alternative.
Pierobon says their integrated LCA methods will not be ready for “several years.” But builders and designers, of course, want guidance now. They already have well-documented options for selecting the most sustainable steel and concrete. Concrete producers are adding ash to reduce CO2 emissions during curing, for example, while dedicated solar panels will soon be decarbonizing a Colorado steel mill’s arc furnace. LCAs conducted by consultants for each supplier document the carbon reductions they have achieved, enabling builders to select the greenest products.
Materials experts say forestry research is just beginning to identify similar opportunities for CLT producers. Two appear in recent research by Diaz, who modeled the carbon impacts of different forest practices at 64 sites in western Oregon and western Washington. He found that extending harvests from the roughly 40-year rotations favored by forestry firms out to 75 years allowed trees to more fully develop, boosting the amount of carbon stored by the forests and the amount of embodied carbon in the wood products they supplied.
In addition, environmentally responsible harvesting methods established by the Bonn, Germany-based Forest Stewardship Council, or FSC, such as leaving more trees standing beside streams and limiting the scope of clear-cuts, also show carbon-storage benefits, according to Diaz’s modeling. Those forestry methods, which are widely adopted by producers of paper products, were designed to protect ecosystems and water quality, but Diaz’s modeling found that they also increased carbon storage as much as 40 percent compared to Washington state forestry codes. “If you leave more and bigger trees, you store more carbon,” says Diaz. “FSC is essentially giving you a guarantee that the forest is storing more carbon on the landscape over time.”
Another harvesting approach is underway at a second newly completed CLT plant 70 miles north of Spokane. Vaagen Timbers’ plant resembles Katerra’s, with its cacophony of rollers and vacuum-cranes, giant presses, and robotic saws and drills. But his plant’s production is quite different, according to Russ Vaagen, the company’s founder. Whereas most of Katerra’s lumber travels more than 250 miles from British Columbia, Vaagen’s factory uses lumber thinned out of the nearby Colville National Forest with support from regional conservation groups.
Harvesting locally slashes direct CO2 emissions from transportation, and thinning protects mature trees that excel at storing carbon. Vaagen is thinning up to 10,000 or so smaller trees per acre from forests that naturally contain just 60 to 80 giant, centuries-old trees per acre. A fire sparked in that dense growth can feed a megafire that burns hot enough to penetrate even the thick, fire-resistant bark that would otherwise protect the older trees.
“There’s nothing more discouraging than seeing a Ponderosa pine or a Western larch with three or four different lightning scars on it, that may be 200 to 400 years old, getting wiped out because we aren’t managing the forest appropriately,” says Vaagen.
Washington Public Lands Commissioner Hilary Franz praised thinning at Katerra’s September plant opening, urging Katerra to consider buying local too: “We have a forest health crisis in Washington State, with 2.7 million acres of forest just in eastern Washington alone that are in the process of dying and in need of urgent care, urgent treatment.”
Clearly, more research is needed, including the use of statistical tools to clarify how often wildfires will strike a given forest and how much thinning will enhance its survival. But the more immediate challenge is economic. Thinning forests and extending harvest cycles tend to make lumber more expensive than fast-rotation, clearcut-intensive industrial forestry. Diaz’s simulations suggest that lumber milled from timber harvested in Oregon in accordance with the Forest Stewardship Council’s guidelines — a rarity in the market today — needs roughly a 5 percent to 15 percent price premium over non-certified lumber to break even.
Vaagen says his family accepts a lower rate of return in exchange for a more reliable future. But the big corporations that dominate forestry generally prioritize near-term over future profits. “That’s the barrier,” says Jason Grant, a sustainable forestry consultant and Sierra Club activist. “It makes financial sense for large companies to clearcut and replant on 40-year cycles rather than doing more ecosystem-based forestry. They have a fiduciary obligation to maximize profit within the law.”
Even so, some large developers are paying to build CLT projects with more expensive FSC-certified lumber, according to Hardy Wentzel, the chief executive officer of Structurlam, a mass timber producer based in Penticton, British Columbia. Wentzel says his firm has supplied FSC-certified panels for Silicon Valley tech firms such as Microsoft and Google.
Engaging the rest of the building market will require helpful incentives. “We need good public policy to change the economics,” says Grant.
There are a growing number of policy proposals on the table.
Oregon environmental groups petitioned Portland’s mayor to require FSC-certified lumber for the city’s growing number of CLT building projects. The Oregon-based Center for Sustainable Economy has proposed a market mechanism — a “carbon tax and reward” scheme — that would tax conventional forestry operations and use the funds to incentivize more sustainable ones. In addition, Gov. Inslee’s presidential campaign promised budget boosts to improve forest management and a “reward” for landowners who enhance their forests’ carbon absorption.
Such proposals are getting a mixed reception. The Portland petition for FSC lumber hasn’t gained traction, but Inslee recently signed legislation that begins to integrate forestry into Washington’s carbon-reduction policies.
Federal policy, meanwhile, threatens to erode forestry practices. President Donald Trump’s secretary of agriculture, for example, is trying to accelerate old-growth logging in Alaska’s Tongass National Forest, which sequesters more carbon than any other U.S. forest.
What’s more, even if CLT construction flourishes in the future, rising lumber demand may spur greater harvesting from less responsible suppliers. A CLT boom could entice imports from Russia’s Far East, for instance, where clearcutting can melt permafrost, accelerating the release of methane from soil.
Add in undocumented flows of biogenic carbon and the overall decline in forest health, and it is easy to see how CLT could be a mixed blessing at best.
CLT boosters are not backing down. Katerra recently released a life-cycle assessment — produced by the University of Washington experts — for its first CLT structure, a five-story office building in Spokane. The final report notes that more sophisticated analysis is required “to rigorously incorporate the benefit of carbon storage into LCA results.” But sponsored content from Katerra published by an architectural magazine this month discards the nuance, flatly stating that “the carbon sequestered in the building’s mass timber structure nearly offsets all of its upfront embodied carbon, making the building carbon neutral.”
Wishnie, previously of the Nature Conservancy, says there is no alternative but to move ahead with eyes wide open. Building is inevitable, he says, but all means of minimizing the associated negatives — and maximizing potential positives — must be explored. “Under the wrong circumstances CLT can increase carbon and reduce biodiversity and impact all kinds of things that we care about,” says Wishnie. “But when we’re thinking about climate change there are no easy options. The only reason we’re going down this road is that we feel that we need to.”
*Correction: This article has been updated to clarify that the absorption of CO2 by concrete may reduce its carbon footprint by about 20 percent. The article originally stated this reduction could be over 40 percent.