Wood: Nature’s carbon capture and storage technology
Wood is a pretty amazing material. It has superb engineering qualities and it literally grows on trees. It is one of the few renewable construction materials we have. Additionally, half the mass of wood is carbon captured from the atmosphere. Wood might be nature’s most prodigious carbon capture and storage technology. So with devastating global climate change impacts looming why aren’t we trying to build everything out of wood?
Some researcher’s are urging that we do just that. A recent publication by Churkina et al. in Nature Sustainability suggests that meeting the demand for new urban construction with wood over the next 30-years could store up to 20 billion tons of carbon in the world’s cities. To put that in perspective, it is almost one tenth of the carbon currently stored in all living trees on earth. And next generation wood products, like cross laminated timber (CLT) make it possible. These engineered wood products are being used to construct the mid- and high-rise apartment and office buildings, the same type of buildings that will make up the bulk of new urban construction in coming decades. Building with wood also means using less concrete and steel which emit lots of greenhouse gasses when they are manufactured. Churkina et al.’s study found substituting wood for concrete or steel in urban buildings could reduce the emissions of construction materials by about 40%.
Too good to be true?
Of course, things are more complicated than that. The numbers above are from the most extreme scenario Churkina et al. simulated where 90% of new cities buildings are made out of wood. The amount of timber needed each year for that many new buildings exceeds the global saw-log harvest of 2015. Churkina et al. analyzed more realistic assumptions, where 10% or 50% of new urban construction used wood and the potential benefits dropped by an order of magnitude. However, even the more conservative figures are compelling and it seems realistic that the more modest increase in wood demand could be met through intensified management of existing forest plantations.
Maybe we should leave more trees in the forest
Despite the potential of wood buildings to be a major carbon sink, some research suggests that carbon is better kept in the forest. A 2018 article in Proceedings of National Academy of Sciences by Law et al. found that reducing harvests levels and increasing rotation lengths in forests of the Pacific Northwest, USA could reduce atmospheric carbon more than storing that carbon in wood products. In addition, reducing harvest by increasing preserves and lengthening rotation age may have additional co-benefits, like improved water quality and wildlife habitat.
Where do we go from here?
One possible reason for the discrepancy between the results is that simulating carbon pools and fluxes requires a lot of assumptions that can effect the results of a study. In the face of all this uncertainty, studies that incorporate dynamic forest harvest models into traditional life-cycle assessment of building products might be useful. These track carbon sequestered by forest over time using growth and yield models coupled with harvest simulations. They also consider emissions from harvesting, transporting, manufacturing and construction with wood, the lifespan of buildings and what happens to the wood after demolition.
In the face of imperfect information and imminent climate challenges, policy makers, developers and consumers need to decide where wood belongs. Should we intensify forest management to increase stores of carbon in cities buildings while re-growing trees rapidly or should we leave trees to grow and store carbon in the forest where they are likely to provide other ecosystem services?
Austin Himes, Ph.D., is an assistant professor in the Department of Forestry at Mississippi State University where he teaches and conducts research. His research is focused on silviculture, forest ecology, ecosystem services, and environmental values. He is interested in understanding interactions between forests and people through an interdisciplinary lens and finding ways silviculture can be used to improve the many ways forests benefit people. He and Adam Polinko, Ph.D., run the Silviculture at MS State lab, which you can follow on Twitter @silviculture.
Feature image photo from Moelven. It is Mjøstårnet, the world tallest timber building located in Norway.