Carbon dioxide, one of the main greenhouse gases, is demonized for being a suspected contributor to climate change. But the odorless, invisible gas is a natural part of the Earth's ecosystem, said Lee Spangler, director of the Big Sky Carbon Sequestration Partnership in Bozeman. Your breath contains CO2. "If you eat food, that's a hydrocarbon," Spangler said. "In order to get the energy out, you oxidize it and exhale