Science

Why Is Growing Pot So Energy-Intensive?

Federal regulations, budtender preferences, and a weird trick with carbon dioxide.

Rows and rows of weed plants growing in a greenhouse
Richard T on Unsplash

In his 2022 budget proposal, President Joe Biden included a rider first introduced by Republican Maryland Rep. Andy Harris that would block D.C.’s ability to legalize and tax recreational marijuana, even after the city’s residents voted to legalize possession back in 2014. While the budget provision is plainly at odds with Biden’s support of D.C. statehood and autonomy, it’s also the latest example of Biden’s conservative views on marijuana—views that could have major consequences for the climate crisis.

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Currently, because marijuana is not legal at the federal level, cannabis growers are not allowed to ship their products over state lines, as Emily Atkin explains in a recent edition of her newsletter, Heated, aptly titled “Biden’s anti-weed budget is anti-climate.” Rolling Stone dubs weed a “climate villain,” as cannabis sold in any state where it’s legal must be grown in-state. Because not every state boasts the year-round warm climate that cannabis thrives in, the vast majority of cannabis is grown indoors in large facilities. And there are other reasons indoor growing is thriving, as Evan Mills pointed out in a recent Slate piece, from misconceptions about the potency of cannabis grown indoors to budtenders who “tend to toss outdoor-grown product onto the bottom shelf.” Mills argues that the only way to make cannabis truly “green” is to grow it outside.

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A study by researchers at Colorado State University published in March has been cited again and again in the discussion over weed, regulations, and the environment. In the study, researchers calculated the energy costs of growing weed indoors in a thousand locations across the U.S. and found not only that indoor weed-growing is a major contributor to greenhouse gas emissions, but also that the environmental impacts vary by state (in Colorado, cannabis emissions outpaced coal). Slate spoke to Hailey Summers, the first author on the study and a Ph.D. candidate at Colorado State University, to learn more about marijuana’s climate impacts and the federal government’s role in the problem. This conversation has been condensed and edited for clarity.

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Sofia Andrade: Is there a reason why growing cannabis is so energy-intensive? Are all plants like that?

Hailey Summers: Right now, we actually have an extension project that’s looking at other crops and how they might compare, but if I had to speculate, I think that most indoor agriculture will have high emissions, mostly from the lights and the environment controls needed to maintain certain indoor climates to make the plants happy.

I think that there’s also some things about cannabis that might make it have a little bit higher emissions, like the added carbon dioxide that cannabis growers are putting into the room. They are putting in bottled CO2, or they’re burning natural gas on-site to basically get CO2 in the room.

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Why add carbon dioxide?

That increases your rate of photosynthesis and makes your plants grow bigger and faster, so you can get more flower faster. Some other indoor cropping systems are doing that, but most crops can’t afford to because the commodity just isn’t as valuable economically, so they can’t pay for all those added operational expenses.

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What’s the impact of adding CO2, in terms of total emissions?

Just the CO2 accounts for between 11 and 25 percent of the total emissions for all the locations across the U.S. that we ran. It can be up to a quarter of your total emissions in any one location. And that’s a conservative estimate. We only accounted for the emissions associated with actually bottling the CO2, and to transport and use that CO2 in the building. We didn’t account for, say, if growers are putting in a molecule of CO2, and it’s not taken up by the plants, but then it’s released through the ventilation systems to the environment.

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In your study, you found that it’s more carbon-intensive to grow cannabis in some places over others, even when comparing indoor facilities. Why is that?

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There’s two primary considerations that we figured out when it comes to geographic location. The first is grid mix and the second is weather. With grid mix, essentially, if you’re pulling electricity in California versus New York versus Texas, the emissions per kilowatt-hour of electricity that’s coming out of the wall will be different in each location based on what the utility providers are using to generate electricity in the region. So California has a lot of solar. Solar doesn’t emit that many emissions compared to coal when you’re actually generating electricity, so California has a pretty clean grid. If you were locating your facility in Texas or Colorado, we have a lot of coal and natural gas, and so those energy sources lead to a lot higher emissions when you’re generating electricity.

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Weather comes into play because the plants need access to fresh air from the outside. The heating, ventilation, and air conditioning system uses a constant supply of fresh outside air, and then it just changes it to meet what the plants want to be comfortable at. When that heating, ventilation, and air conditioning equipment is modifying outside air, it needs more energy to modify it if there’s extreme weather events. If you’re always growing in Michigan and it’s really cold half the year, but the plants want to be at 75 degrees inside, it’s going to take more energy in Michigan to constantly meet and change that outside air to meet the comfortable inside air.

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Is it more environmentally friendly to grow cannabis outdoors?

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We did back-of-the-envelope, precursory estimates of what emissions outdoors would be. We [looked at the indoor environment and] said, if we take away all of the artificial environment applications—so all of the artificial lights, the climate control from your heating, ventilation, and air conditioning—what’s left? If you strip away everything that’s purely associated with indoor growing, your emissions drop by 80 percent. But it hasn’t been studied to the level that I studied indoor growing. So the next step of research is really to understand the emissions from greenhouse and outdoor with quantitative numbers.

How would the effect on the climate change if marijuana were legal on the federal level?

I think one thing that could happen is having the industry adopt how our food is grown in the U.S. We have central locations for a lot of crops, and then it’s just transported across the U.S. Right now, you can’t do that with cannabis. You can’t grow in one state and cross state lines and sell in another. Even if you’re growing indoors, if you could just locate those indoor facilities in places that are better suited, whether that’s with clean grid mixes or weather that doesn’t need to be as modified, and then ship it to states that don’t have the suitable climate or clean emissions from their grid, the trucking emissions are very, very small relative to the differences [in emissions that come from] just relocating the facility.

I think federal legalization can help that become a reality. I’m also hopeful that more funding will become available for people that want to research this. Right now, there’s no federal money allocated for cannabis research because it’s illicit.

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