DENVER, CO – MARCH, 4: Lights hang above cannabis plants in a “flower room” inside a medical cannabis cultivation facility in Denver, Colorado, U.S., on Monday, March 4, 2013. (Photo by Matthew Staver/For The Washington Post)

Across the country, there’s a growing trend toward the legalization of marijuana. Four states — Oregon, Washington, Colorado, Alaska — have voted to allow people to possess limited amounts of marijuana for personal use and also to let producers apply for licenses to produce and sell it. D.C. also just voted to allow personal possession. All of this is on top of the 23 states that allow it for medical reasons.

In some states, where businesses are also now legally cultivating and producing marijuana, a mainstream industry is emerging. Marijuana sales totaled $700 million in Colorado last year, for instance. But there’s a surprising catch. It turns out that indoor marijuana growth in particular — a cultivation method often favored in the industry for many reasons — uses a surprising amount of energy.

Indeed, the level of power use appears to be so significant that one scholar is now suggesting that as the industry grows, states and localities should take advantage of marijuana licensing procedures to also regulate the industry’s energy use and greenhouse gas emissions.

“Given that this is a new ‘industry’ that is going to be pretty highly regulated, I felt like the state and local policymakers have a unique opportunity to incorporate energy usage and climate assessments into their state marijuana licensing fees,” says Gina Warren, a professor at the Texas A&M University School of Law whose paper, titled “Regulating Pot to Save the Polar Bear: Energy and Climate Impacts of the Marijuana Industry,” will soon appear in the Columbia Journal of Environmental Law.

The published statistics on energy use from indoor marijuana production will blow your mind (whether or not you use the stuff). In a 2012 study of the “carbon footprint of indoor cannabis production” published in the journal Energy Policy, researcher Evan Mills noted that “on occasion, previously unrecognized spheres of energy use come to light,” and marijuana is a textbook example.

The study estimated that indoor cannabis (both illegal and legal) uses $6 billion worth of electricity every year, amounting to 1 percent of overall U.S. electricity. And in some production-intensive states like California, it was much higher — 3 percent, Mills found.

“One average kilogram of final product is associated with 4,600 kg of carbon dioxide emissions to the atmosphere, or that of 3 million average U.S. cars when aggregated across all national production,” wrote Mills.

The reason is simply the technology required. “Specific energy uses include high-intensity lighting, dehumidification to remove water vapor and avoid mold formation, space heating or cooling during non-illuminated periods and drying, pre-heating of irrigation water, generation of carbon dioxide by burning fossil fuel, and ventilation and air-conditioning to remove waste heat,” writes Mills.

Outdoor production also has environmental consequences — it has been charged with deforestation and high levels of water and pesticide use. But as pot becomes more legal and mainstream, notes Warren, outdoor producers will have to abide by preexisting environmental laws, just like everyone else.

In effect, that makes indoor production the chief climate change and energy concern. According to Warren’s article, while underground indoor marijuana production already consumed plenty of energy, legalization will increase energy use even farther. “As the industry grows, so will its negative externalities,” she writes.

Which is why she’s proposing that states that legalize marijuana use should also require the growing industry to power itself cleanly. And it’s not without precedent: Starting in October of this year, Boulder County in Colorado will require many marijuana facilities to “directly offset 100% of electricity, propane, and natural [gas] consumption” through renewables or other means.

Warren says she’s not “picking on the marijuana industry” with her proposals — it’s just that, well, we don’t often have new industries appear that use a lot of energy and are likely to be highly regulated as they become legal.

“I think it could actually be a marketing tool for the industry,” says Warren, “because if you have people who are purchasing the product who are the type of individual who cares about the environment, then they would gravitate towards the green marijuana production.”