When recreational marijuana use becomes legal in Colorado on January 1, medical retailers expect demand to increase by 400 per cent. Increasing supply to meet that demand will carry with it dramatic economic and environmental costs.

The monthly energy bill for River Rock, a medical marijuana retailer with two dispensaries and a warehouse in the Denver area, exceeds $21,000. According to John Kocer, one of River Rock’s owners, that’s nothing: one of his competitors pays a monthly bill of $100,000 for its warehouse operation. In Colorado and elsewhere in the U.S., the pot industry generates more than enough revenue to pay its bills. The environmental cost of doing business, though, is a different matter.

In 2011, a study by researcher Evan Mills at Lawrence Berkeley National Laboratory showed indoor marijuana production accounting for 1 per cent of national electricity production, using $6 billion worth of energy per year, and creating greenhouse gas pollution equivalent to that of 3 million cars. Faced with the rapid growth of those numbers, the City of Boulder put into place the kind of environmental regulation ordinarily associated with corporate polluters. Boulder requires marijuana growers to purchase wind or solar energy, or to buy carbon offsets. That increases marijuana growing costs by an estimated 20 per cent.

River Rock’s new 18,000 square foot greenhouse will primarily use solar power, rather than artificial lights.

“Our primary goal,” Kocer told CBS Denver, “is to lower our costs as much as possible and pass it on to our patients.”

Outdoor cultivation is more energy—and cost—efficient, of course, but as long as marijuana is illegal on the federal level, we can expect indoor growers to increase their market share. A 2009 Justice Department assessment of domestic cannabis cultivation makes it plain: the primary advantage of indoor cultivation is to avoid detection, arrest, and eradication by authorities. But chasing the industry indoors as a matter of procedure and policy has its own, increasing environmental impact.

In Boulder, City Councilman Macon Cowles is proud of his city’s efforts to lessen the industry’s emissions.

“The marijuana is greener in Boulder,” he says. How green the greenhouses prove may be an important part of Colorado’s legalization process.