Mass timber’s biggest advantage may be environmental. Buildings are by some estimates responsible for a third of global greenhouse-gas emissions. Much of a building’s carbon footprint results from its lifetime energy use, but another big part derives from its construction. The manufacture of concrete and steel accounts for an estimated 10 percent of greenhouse-gas emissions. Trees, however, are “carbon sinks”—they absorb and hold carbon until they decompose or are burned. According to a study in the Journal of Sustainable Forestry, substituting wood for other materials used in buildings and bridges could prevent 14 to 31 percent of global carbon emissions. (This assumes that forests regrow, and that wood is reused or disposed of responsibly at the end of a structure’s life.)