In an effort to reduce the 40 trillion kg of carbon dioxide humans produce each year, three companies have been working to build machines that can capture the gas directly from the air. One such machine in Iceland has begun operation. Quartz reports: Climeworks just proved the cynics wrong. On Oct. 11, at a geothermal power plant in Iceland, the startup inaugurated the first system that does direct air capture and verifiably achieves negative carbon emissions. Although it's still at pilot scale -- capturing only 50 metric tons CO2 from the air each year, about the same emitted by a single U.S. household -- it's the first system to take CO2 in the air and convert the emissions into stone, thus ensuring they don't escape back into the atmosphere for the next millions of years. Climeworks and Global Thermostat have piloted systems in which they coat plastics and ceramics, respectively, with an amine, a type of chemical that can absorb CO2. Carbon Engineering uses a liquid system, with calcium oxide and water. The companies say it's too early in the development of these technologies to predict what costs will be at scale.