33 3 Do Not Forget CO 2 The construction sector is directly or indirectly responsible for approximately half of all global CO 2 emissions. Carbon dioxide is produced at every stage of a structure’s use cycle. Building material production is a significant source of CO 2 emissions. Cement production releases 5 to 7 percent of all anthropogenic CO 2 emissions. Steel for construction may represent another 5 percent of CO 2 emissions. These represent significant areas for improvement, though reducing CO 2 output needs to be balanced with improving resource efficiency—two aims that are sometimes in conflict with one another. Building operations are also a significant source of carbon dioxide. Heating buildings emits about 20 percent of all anthropogenic carbon dioxide, and most of these emissions are produced in the northern hemisphere. Though this is in large part a question of which sources are used to supply energy, it is also a material question (at least until a total shift is made to renewable, non-harmful energy sources!). Putting thermal mass to use, insulating, or even reflecting upon thermal- comforts standards (wearing a sweater is easier than renovating a facade) may all be pathways to reducing the CO 2 output from heating through buildings. Cooling buildings currently produces comparatively less carbon dioxide than heating but is expected to increase. In general, low-income regions use less energy because of the prohibitive cost of energy. In Brazil, for example, only 2 percent of CO 2 emissions are associated with building use—and this is mostly for cooking.