The United Nations predicts the world’s urban population will increase by 2.3 billion by 2050. And that means a building boom is ahead, because all of those new urban dwellers will need places to live and work.
Today’s favored building materials – cement and steel – result in a lot of carbon emissions. If current trends continue, emissions from manufacturing building materials will total 4.4 gigatons of carbon dioxide by 2050. If living standards and therefore dwelling sizes increase, the total could be up to 19 gigatons.
But there’s an alternative, an international team of researchers suggests in a paper published yesterday in Nature Sustainability. “We propose to exploit this projected demand for urban buildings as a means to mitigate climate change,” they write.
The researchers say that constructing new buildings out of wood and other bio-based materials like bamboo and hemp could actually turn the urban skyline into a carbon sink. In the most optimistic scenario, buildings could store a total of 20 gigatons of carbon over the next three decades.
To reach this conclusion, they analyzed the carbon implications of substituting mass timber for concrete and steel in mid-rise residential and commercial buildings. Their analysis covers the carbon stored in building materials and the emissions associated with manufacturing those building materials – though not building operations, maintenance, or disposal.
Mass timber is an emerging class of wood products in which small boards are glued or mechanically laminated into larger beams and panels strong enough to support mid-rise and even high-rise buildings. The technology minimizes waste and makes wood suitable for a modern building industry driven by uniformity and predictability.
The researchers constructed four scenarios for mass timber building over the next 30 years: business as usual, in which 0.5% of new buildings are constructed out of mass timber; 10% timber; 50% timber; and 90% timber.
“Our analysis suggests that construction of timber buildings for new urban dwellers could store 0.01–0.68 [gigatons of carbon per year] depending on the scenario and the average floor area per capita,” the researchers report. “The total carbon stored over thirty years would sum up to 2–20 Gt [gigatons] in the 90% timber scenario, 1–11 Gt in the 50% timber scenario, and 0.25–2.3 Gt in the 10% timber scenario.”
Mid-rise timber buildings can store more carbon per square meter than the most carbon-dense natural forests. In the 90% scenario, the pool of carbon stored in mass timber after 30 years could be one-tenth the size of that stored in living forests worldwide.
Of course, it’s not as simple as just deciding to use mass timber. Some countries will have to adopt new regulations to permit construction of mid- and high-rise buildings out of wood. We’ll also need longer-lived buildings to keep carbon locked up in the city as long as possible; expanded markets for used wood so that building components can be reused and recycled; less energy intensive manufacturing and forest harvesting practices; and, above all, sustainable forest management.
But if we manage all that, the benefits could big – both environmentally and culturally. “The use of timber or bamboo could keep or revive cultural-specific methods of construction and thereby preserve cultural heritage in many regions of the world,” especially traditional building forms in Asia, Oceania, and Africa, the researchers point out. And in imagining a sustainable urban future, the idea that a lot of us will live in treehouses just feels right.
Source: Churkina G. et al. “Buildings as a global carbon sink.” Nature Sustainability 2020.
Image: Don Earhardt / UBC Bioenergy Research via Flickr.