Legal cannabis production in Colorado emits more greenhouse gases than the state’s coal mining industry, researchers analysing the sector’s energy use have found. The production and use of cannabis for medical or recreational reasons is now legal in several US states, which has led to a booming industry. Hailey Summers and her colleagues at Colorado State University have quantified and analysed the greenhouse gas emissions produced by cannabis growers. They found that emissions varied widely by state, from 2.3 to 5.2 tonnes of carbon dioxide equivalent (CO2e) per kilogram of dried flower produced. “The emissions that come from growing 1 ounce, depending on where it’s grown in the US, is about the same as burning 7 to 16 gallons of gasoline,” says Summers.