Traveling north from San Francisco, the rolling vineyards of wine country soon give way to stands of redwoods and the famed cannabis lands known as the Emerald Triangle, where the local microclimate gives rise to some of the world’s most storied weed. But while you can pick up a bottle of Sonoma County zinfandel or Napa Valley chardonnay at any wine shop in the country, you won’t find famed pot varietals like Humbolt Kush or Mendocino Purps in dispensaries outside the Golden State. In fact, federal law classifies interstate commerce in cannabis – even among pot-legal jurisdictions – as drug trafficking, a serious felony. 

America’s state-led march to legalization – recreational weed is now legal in 17 states and the District of Columbia – has created a cannabis archipelago, where consumers can only partake of marijuana produced locally. For much of the country, that means pot must be grown indoors, in warehouses, with artificial grow lights and supercharged HVAC systems. As groundbreaking new research published in the journal Nature Sustainability demonstrates, this indoor cultivation comes at an alarming climate cost, turning what could be a green enterprise into yet another dirty business – with a carbon output that rivals major extractive industries. 

“The crux of the problem is that federal prohibition means that you cannot transport any cannabis across state lines,” says Baylen Linnekin, an agriculture lawyer and senior fellow at the Reason Foundation. “That means that everything that’s going to be sold in any state has to be grown in that state,” regardless of whether it makes any sense agriculturally. “And that introduces a big problem.” Market research indicates that 60 percent of the nation’s pot growers now rely on warehouses, with 44 percent growing strictly indoors. “It’s a much more resource intensive, and hence climate impacting, way to grow,” Linnekin says, “than growing in the field.”

Culturally, Colorado was a natural state to pioneer weed legalization in 2012, and the state now claims one of the nation’s biggest markets, selling more than $2 billion worth of weed in 2020 alone. But weatherwise, most of Colorado is a ludicrous place to grow pot, and as a result its industry has one of the dirtiest cannabis footprints in the country. According to the science published in March, the state’s pot cultivators now account for 1.7 percent of Colorado’s total CO2 emissions – greater than the state’s active coal mining industry. 

“There’s two big reasons behind that,” explains the paper’s lead author, Hailey M. Summers, a Ph.D. candidate at Colorado State University’s Energy Institute. “One is the weather, and one is the electric-grid mix.” Outdoor temperature is a driving factor, because indoor grows require constant circulation of fresh air. “If it’s winter in Colorado, you can’t bring in that cold air and shock the plants. So you have to use heating, ventilation, and air conditioning systems to modify that air,” Summers says, “and that air is very, very energy intensive.” 

Heating is often achieved by burning natural gas on site. But the local electricity mix is also critical to the industry’s climate damage. Warehouse cultivation requires grow lights, fans and other electrical equipment, and in Colorado, coal is still a dominant fuel burned for electricity, “leading to further, higher greenhouse gas output,” Summers says. Colorado’s climate impacts are compounded, Linnekin notes, by local regulations that have required cannabis be grown close to the point of sale. “If you have to grow it where you’re selling it,” he says. “You’re kind of stuck growing in a warehouse.”

While growing weed in frigid Alaska or Minnesota poses similar problems, indoor grows in hot weather states are not necessarily cleaner. In fact, the worst state to grow indoors, the climate researchers found, is tropical Hawaii, where HVAC systems must work overtime to lower humidity levels, and local electricity is often sourced from oil combustion. The Nature Sustainability researchers found that cultivating one kilogram of dried cannabis flower indoors in western Oahu produces 5 metric tons of CO2 emissions. To put that in more relatable proportions: It takes the carbon equivalent of a 16-gallon tank of gasoline to produce a single ounce of that Hawaiian indoor weed.

To keep marijuana plants happy, warehouse growers will also bathe them directly with CO2 gas, much of which escapes to the environment. While the researchers give the industry a pass on some of these emissions, reckoning they would have been released to the atmosphere regardless, the energy required to capture, compress, bottle and transport this CO2 for indoor cultivation creates a third major climate cost – one that limits the potential to clean up the industry simply by using more renewable electricity. “If your CO2 use is accounting for 25 percent of your total facility’s greenhouse gas emissions,” Summers says, citing a high-end estimate, “no matter how much you clean up that grid, you’re still going to have a significant contribution.”

The reality of the modern cannabis business is such that, even in California, many growers prefer to cultivate indoors. Warehouses can operate year round, close to consumers, and precisely calibrate cultivation conditions to tease out desired flavor profiles and precise THC levels. In the highly regulated world of legal weed, warehouses can also be secured under lock and key, alleviating concerns about theft and drug diversion. But it turns out that California warehouse grows are far less carbon intensive than other states’ – because the ambient outdoor air requires little temperature adjustment, and because the electrical grid is sourced with a high mix of renewables. For a warehouse grow in Long Beach, for example, researchers measured a climate impact of just over 2 metric tons of CO2 emissions per kilogram of weed- less than half the climate toll of growing inside in Hawaii.

There’s a clear opportunity for federal drug reform to minimize the carbon impact of the burgeoning cannabis industry. The Biden administration claims to be serious about combating the climate crisis “with bold, progressive action” combining “the full capacity of the Federal Government” and “every sector of our economy.” Ending the preposterous federal regime of cannabis prohibition, which classifies marijuana as a dangerous drug on par with heroin, would open the free trade in cannabis among states where it’s legal. Sun-grown, greenhouse, or even warehouse-grown pot from the West Coast could then supply cannabis consumers nationwide, in much the way California farmers provision 90 percent of the country’s strawberries and tomatoes. 

Fred Gunnerson is a manager at SoFresh Farms, a cannabis grower in the fertile Willamette Valley, south of Portland, Oregon, which specializes in “regenerative farming” using “minimal energy inputs.” A Navy vet who got involved in the medical marijuana movement more than 30 years ago, Gunnerson blames federal prohibition for giving rise to indoor cultivation in the first place. “There was a time when we risked going to jail for 25 years just for growing our own medicine,” he says, “so we had to keep everything secret.”

In Oregon, where voters approved legalization in 2014, the SoFresh operation features an outdoor farm, but also cultivates cannabis in greenhouses that combine natural light and grow lights. Growing outdoors is cheaper and requires less pest control, but mold is a constant worry, and the crop is seasonal and less lucrative: “You can never get the same prices for outdoor,” Gunnerson says. “Outdoor weed is always a little leafier and the quality is just not the same.” 

With greenhouse grows, he says “you get the best quality product at the best price,” as well as up to five harvests a year. While Oregon offers excellent growing conditions, the state’s small population and the inability to export has also created an absurd weed surplus. “Being able to market to other states would change our world,” Gunnerson says. “The Oregon market is saturated. The large farms with big investors can afford to lose money, but the small farms are starting to go under. If they could sell their products nationwide, they wouldn’t just save their farm, they’d prosper.”

Normalizing marijuana commerce is, in fact, consistent with the platform Joe Biden ran on in 2020, which vowed to “decriminalize marijuana use and reschedule it through executive action on the federal level.” But the Biden White House has not yet displayed any boldness or progressive action on cannabis – instead making headlines for ousting qualified administration staffers from the West Wing over past marijuana use. “President Biden is a longtime drug warrior, and he seems to be disinclined to cast off that legacy,” says Linnekin. “So despite the administration’s rhetoric on climate change, this is one area where they’re just not putting their money where their mouth is.”

Extracting marijuana from the tentacles of the federal drug war will more than likely require an act of Congress. And change is brewing. Senate Majority Leader Chuck Schumer, of newly cannabis-friendly New York, has found religion on maijuana reform and is publicly challenging the president to evolve, vowing that Congress will take the lead if the White House won’t: “We’re going to move forward, period,” he recently told Politico. 

For now, however, it’s up to newly legalizing states – like New York, New Jersey, and Virginia – to learn from the mistakes of the nation’s early adopters, says Summers, the Colorado State scientist. State regulators of these new industries could adopt climate-centered rules to maximise the efficiency of indoor grows, she argues, by mandating LED lights, insulation, and high efficiency HVAC systems. Better yet, she says, they could push pot cultivation out of the warehouse and into greenhouses or the open air. “For these states, developing policy to encourage greenhouse and outdoor cultivation can drastically reduce GHG emissions,” the Nature Sustainability authors write, “by avoiding the practice of indoor cannabis cultivation altogether.”