Action on climate change has been focused on the electrical grid and our transportation systems, but there are other hurdles between here and a zero-greenhouse-gas-emissions world. Agriculture and deforestation are major hurdles, but so is cement.

Producing this humble, gray, ubiquitous building material actually accounts for about eight percent of global CO₂ emissions. Firing the kilns required for this process is very energy intensive, but the process itself also releases CO₂ from its starting materials. The primary ingredient in concrete is calcium oxide, which is taken from limestone (CaCO₃)—and that leaves CO₂ as a byproduct.