DAILY SCIENCE

First comprehensive study of indoor pot farm emissions uncovers a giant climate hot spot
Cannabis growing regulations often focus on drug policy, producing unintended environmental consequences
March 9, 2021

Since individual U.S. states began legalizing recreational cannabis about a decade ago, the industry has become a lucrative but greenhouse gas intensive slice of agriculture: Producing a kilogram of dried cannabis bud can result in carbon emissions equivalent to driving a car more than 9,000 miles, a new study of indoor growing operations reveals.

“More and more states are legalizing and decisions around how [cannabis] will be cultivated need to include environmental considerations,” says study team member Jason Quinn, director of the Sustainability Research Laboratory at Colorado State University in Fort Collins. The new study is the first comprehensive analysis of greenhouse gas emissions from large indoor growing operations typical of commercial producers for the legal market.

Quinn and his team gathered information on the impacts of lighting, indoor temperature and humidity control, ventilation, water, fertilizers, fungicides, and waste disposal associated with indoor cannabis production. They calculated the greenhouse gas emissions per kilogram of dried bud for each of 1,011 different locations across the country.

The climate impact per kilogram of dried cannabis bud ranges from the equivalent of 2,283 kilograms CO2 in Long Beach, California to 5,184 kilograms CO2 in Kaneohe Bay, Hawaii, the researchers report in Nature Sustainability. The median value is 3,658 kilograms CO2, the equivalent of driving 9,077 miles in an average passenger car.

The total varies from place to place depending on the regional mix of energy for the power grid, and how much local weather differs from optimal cannabis growing conditions (that is, how much outdoor air has to be modified when it’s brought in to the warehouse-like growing rooms). Indoor cannabis cultivation is particularly carbon intensive in the Mountain West and Midwestern United States.

Regardless of location, the top two factors contributing to carbon emissions from indoor cannabis growing are climate control (cannabis plants need stable temperature and humidity but regular turnover of fresh air) and high-intensity grow lights. “While it is well known that lights are energy intensive, maintaining a comfortable environment for the plants is just as intensive if not more,” says Quinn.

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Another surprising source of emissions: “Bottled carbon dioxide is added to indoor grow spaces for increased plant growth and accounts for 11-25% of cannabis emissions, depending on location,” says study team member Hailey Summers, a graduate student at Colorado State University. The emissions don’t come from the CO2 itself (which is a byproduct of other industrial processes and would otherwise be released into the atmosphere) but from compressing it into liquid form and bottling it.

The results illustrate how cannabis regulations that have been made with drug policy in mind can have unintended environmental consequences, such as Colorado’s requirement that cannabis growing and retail operations must be co-located, which has led producers to move operations indoors.

The researchers calculated that more than 80% of the greenhouse gas emissions associated with indoor cannabis production are from practices specific to indoor growing. “The significant energy required to create an artificial climate for plants indoors causes cannabis to be extremely GHG emissions intensive,” the researchers write. Moving all cannabis cultivation outdoors could shave 1.3% off the state’s total greenhouse gas emissions.

Of course, there are downsides to outdoor growing, too – water use, exposure to theft, and perhaps greater land use. “There are tradeoffs between greenhouse gas emissions and land use, we just don’t know the magnitude yet because it hasn’t been researched,” Summers says. “This is on our ‘to-do’ list for what’s next.”

Source: Summers H.M. et al.The greenhouse gas emissions of indoor cannabis production in the United States.” Nature Sustainability 2021.

Image: David Gabrić via Unsplash.

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