To get these figures, the researchers did a whole lot of scaling up. They used accepted figures for the emissions involved in making a ton of steel or concrete, and the emissions you’d sequester by producing a ton of timber. Then they scaled that up to how much material you’d need for a building, then scaled it up to how many materials you’d need per capita to build new structures. The team accounted for population growth, as well as the increasing demand for space as people around the world ascend into the middle and upper classes.
“This transition is feasible under two conditions: That the harvested forests are sustainably managed, and that the carbon that is transferred from forests into the cities and stored in the buildings is preserved in some form after demolition of buildings,” says Churkina. That is, we can’t just go chopping down forests willy-nilly, and we can’t just burn the buildings after we tear them down—that’d just put the carbon right back into the atmosphere. Instead, that wood needs to be recycled, for instance as floorboards in new homes.
Now, we’re not just talking about cobbling together a 20-story building out of mere two-by-fours. Modern timber high-rises make use of cross-laminated timber, essentially large-scale plywood, made by gluing two-by-fours together into a sheet, then flipping the sheet 90 degrees and gluing still more two-by-fours on top. “You end up with a basically a sheet of wood that is in its size, and the way you use it when you engineer or design with it, very similar to a slab of concrete,” says Michael Ramage, director of the Centre for Natural Material Innovation at the University of Cambridge, who wasn’t involved in this new work. “It just weighs one fifth the amount.” (Keep that weight factor in mind—it’ll be important later when we talk safety.)
As an analog for steel, architects also use glue-laminated timber. It’s the same principle, only the end product is beams instead of sheets. These can support a structure, and architects can even bend them to craft flourishes like domes.
The beauty of both cross-laminated and glue-laminated timber is that they leverage the pound-for-pound strength of wood while getting rid of some of its organic weaknesses. In the manufacturing process, each component piece of timber is scanned for imperfections that could weaken the material like knots, which are cut out before the pieces are glued together.
In the last couple of decades, the manufacture of these kinds of composites has, ironically, turned an ancient building material into what’s really the only new structural material in the last century, says Ramage. “You have to go back to basically reinforced concrete and emergence of structural steel, both at the end of the 19th century, for a new structural material at the scale of buildings.”
Still, these newfangled materials inherently limit the scale of wooden buildings. A 10-story steel building weighs two or three times as much as a wooden version. “Because of that, as you get higher, wooden buildings counter-intuitively have to be stiffer than steel or concrete,” says Ramage. This is your fault as a human, really: Buildings have to sway in the wind or earthquakes so they fail, but they can’t sway too quickly, or they’ll make their occupants seasick. Because wooden buildings are so much lighter than ones made of steel and concrete, they move much faster in the wind.
“It’s not even the amount of movement, it’s the speed of movement that we’re susceptible to,” Ramage adds. “It’s like being on the rocking deck of a ship.” Accordingly, architects have to design these lighter wooden buildings to be stiffer than traditional skyscrapers—but not too stiff, or they’ll fail in the wind. That limits how high they can go. But there’s an upside: Because timber buildings are made of lighter stuff, workers can assemble them faster, saving the client money. (The cost of the laminated materials remains high, but prices are coming down.) They don’t have to weld steel beams and pour concrete.