Housing the world’s rapidly-growing population will require massive urban expansion and lots of concrete and steel, but these materials have a huge carbon footprint. A shift to building cities out of wood could avoid more than 100 billion tons of CO2 emissions, according to a new study.
Replacing reinforced concrete with timber might sound unwise, but innovations in engineered wood mean it’s now feasible to construct multi-story buildings without traditional materials. So-called “mass timber” is increasingly being used for structural and load-bearing elements in mid-rise developments, which refers to buildings between 4 and 12 stories high.
One of the main selling points of mass timber is that it’s much less carbon-intensive than steel and cement. In theory it’s actually carbon negative, because trees absorb CO2 in the process of producing wood. But question marks have remained over exactly how much more climate-friendly wood-based construction is, and what impact demand for timber could have on the environment.
Now, researchers at the Potsdam Institute for Climate Impact Research in Germany have shown that if at least 90 percent of the world’s new urban population is housed in buildings made from wood instead of concrete and steel, we could avoid more than 100 billion tons of CO2 emissions through 2100. What’s more, they say this can be achieved while protecting biodiversity and without demand for timber competing with agricultural land.
“Our study underlines that urban homes made out of wood could play a vital role in climate change mitigation due to their long-term carbon storage potential,” Abhijeet Mishra, who led the research, said in a press release. But, he added, “Strong governance and careful planning are required to limit negative impacts on biodiversity and to ensure a sustainable transition to timber cities.”
To assess the potential impacts of a wholesale shift to mass timber, the researchers calculated how much carbon would be stored in wooden building materials, the CO2 emitted in the production of different construction materials, and how land use would change in response to increased demand for timber.
In a paper in Nature Communications, they investigate four different scenarios in which 10 percent, 50 percent, and 90 percent of new housing is made from wood, as well as what would happen if there was no change in building practices. In the most optimistic scenario, they calculated that the world would need 140 million hectares of new timber plantations and significant harvesting of natural forest to meet demand.
But their simulations showed that this can be achieved by growing trees on areas of harvested woodland—avoiding any competition with land used to grow food—and avoiding harvesting natural wood from pristine forests or biodiversity conservation areas.
Nonetheless, the researchers admit that there is still likely to be some impact on biodiversity due to the replacement of virgin woodland with timber monocultures. “The explicit safeguarding of these protected areas is key, but still, the establishment of timber plantations at the cost of other non-protected natural areas could thereby further increase a future loss of biodiversity,” said Alexander Popp, a co-author of the paper.
Others are more explicit about the potential risks. “Natural, biodiverse forests are more resilient to drought, fires, and disease, so are a much safer carbon store than the tree plantations we’ve seen go up in smoke this summer from Portugal to California,” Sini Eräjää, Greenpeace’s European food and forests campaign lead, told The Guardian. “Wood can play a bigger role in construction but to double the world’s tree plantations at the expense of priceless nature is just bonkers, when modest reductions in meat and dairy farming would free up the land needed.”
The study’s estimates of the carbon impact of such a transition also gloss over the details of what happens to these buildings at the end of their lives, says building design expert Ljubomir Jankovic from the University of Hertfordshire in the UK. “If the material from a deconstructed building is sent to landfill and left to rot without the gases being captured, or if it is incinerated, the carbon stored in the engineered timber will return to the atmosphere, and there will be no net carbon storage,” he says.
And there are also big question marks over the safety implications of shifting to predominantly wood-based construction. While the paper states that engineered wood is “associated with fire resistance,” the jury is still out on whether it is truly as safe as traditional building materials.
Nonetheless, the study does show the significant impact that increasing use of wood in construction could make on our efforts to tackle climate change. While a mass timber revolution might not be around the corner, engineered wood is likely to be a major fixture of the cities of the future.