Some research has shown that cross-laminated timber—which can store carbon—could be a big improvement over steel and concrete, but the benefits depend on where the wood comes from.
Construction materials account for 9 percent of global greenhouse gas emissions from energy use (1)—more than the energy emissions of the whole European Union (2). And demand for these materials is only growing (3), which means these climate-warming emissions will also grow if we don’t find new ways of manufacturing them or constructing our buildings.
Today, most emissions from construction come from just two products: steel and concrete. Housing and commercial construction account for more than half of global steel demand (4) and, in the U.S., over 40 percent of concrete use (5). Both of those materials cause significant greenhouse gas emissions, because their production typically involves burning fossil fuels to create high industrial heats. Manufacturing concrete also involves a chemical reaction that releases the greenhouse gas carbon dioxide (CO2).
So one way to lower emissions from the building sector is to replace steel and concrete with other, less emitting materials strong enough to support high-rises—like cross-laminated timber.
Cross-laminated timber, or CLT, is made by criss-crossing sections of wood on top of one another, sandwiched with glue, to form a lightweight material with a strength-to-weight ratio that rivals concrete (6). The product can be used in many different types of construction, and in the last couple of decades, builders have started using it in large-scale buildings.
“Substituting steel and concrete with timber and engineered woods is something that's gaining momentum,” says John Fernández, professor of building technology, director of the MIT Environmental Solutions Initiative, and an architect who has designed buildings around the world.
The theory behind CLT is that it not only replaces emissions-heavy steel and concrete, but actually helps slow climate change by keeping CO2 out of the atmosphere. Here’s how it works: as trees grow, they suck up CO2 from the air and store it. Normally when trees are cut down and destroyed, that carbon goes back into the atmosphere. But if most of their wood remains intact and is used in a building, that carbon will instead be locked up for decades, if not centuries. And even after a building comes down, there are examples of that wood being used in new products like furniture, says Fernández. A review of 27 studies on the lifetime emissions associated with CLT found that the material could lower the carbon emissions of large buildings by roughly 40 percent over traditional materials (7).
But using CLT comes with caveats, says Fernández. Logging releases carbon and threatens biodiversity, so forests supporting CLT production need to be carefully managed. If the trees used for CLT are harvested unsustainably, wiping out forests, or aren’t replaced and allowed to grow at a sustainable rate, we don’t get the steady drawdown of CO2 from the atmosphere that CLT promises—and we can cause serious harm to natural ecosystems.
“If it's not managed well, a forest can end up releasing more carbon than it captures,” says Fernández. “Also, growing trees takes many decades, and their full potential to capture carbon is too slow to meet the need for rapid decarbonization today. There needs to be a lot more work done on that side before anyone can claim that wood buildings are going to be overall a positive solution for sustainability generally, including for climate change.”
For these reasons, Fernández says the building sector needs an all-of-the-above strategy that also improves on the ways we make concrete and steel. And this is possible: companies today are looking to make steel with electricity or concentrated solar power instead of fossil fuels, or embed CO2 in raw cement to store carbon. Whatever materials builders choose, the details of how they were produced are crucial to understanding whether they’re a problem or a benefit for the climate.
Thank you to a reader from Lynwood, California, for the question. You can submit your own question to Ask MIT Climate here.
Published June 29, 2023.
FOOTNOTES
1 United Nations Environment Programme: "2022 Global Status Report for Buildings and Construction." November 2022.
2 International Energy Agency: "Global Energy Review: CO2 Emissions in 2021." March 2022.
3 United Nations Environment Programme: "CO2 emissions from buildings and construction hit new high, leaving sector off track to decarbonize by 2050." November 9, 2022.
4 World Steel Association: Steel in buildings and infrastructure.
5 Portland Cement Association: Introduction to Concrete.
6 North Carolina State University College of Natural Resources News: "5 Benefits of Building with Cross-Laminated Timber." Andrew Moore. August 1, 2022.
7 Younis, Adel and Ambrose Dodoo. "Cross-laminated timber for building construction: A life-cycle-assessment overview," Journal of Building Engineering, Vol. 52 (2022), doi:10.1016/j.jobe.2022.104482.