Growing an Ounce of Pot Indoors Can Emit as Much Carbon as Burning a Full Tank of Gas

In some parts of the United States, the growing cannabis industry is responsible for significant greenhouse gas emissions

Indoor cannabis growing in California.
Indoor cannabis growing in California. Rusty Blazenhoff via Flickr under CC BY-NC 2.0

As more and more states legalize cannabis for medical and recreational use, demand for the intoxicating plant is increasing and around half of that commercial demand is being satisfied via fully indoor grow operations. A new study suggests that in certain parts of the country, these indoor grow houses are responsible for significant emissions of planet-warming greenhouse gases, raising questions about the environmental toll of the expanding legal cannabis industry, reports Krista Charles for the New Scientist.

The researchers behind the study, published this week in the journal Nature Sustainability, estimated that the emissions associated with growing 1 ounce of cannabis indoors is about the same as burning 7 to 16 gallons of gasoline, depending on where in the U.S. it’s grown.

These emissions come from the large amounts of electricity and heating required to keep the plants happy. Lighting is the most obvious energy sucking aspect of indoor pot production, but heating, cooling or, in some places, dehumidifying the air also requires huge quantities of electricity. Per the paper, many producers even pipe in carbon dioxide, which plants use for photosynthesis, as a way of accelerating growth.

“Policymakers and consumers aren’t paying much attention to environmental impacts of the cannabis industry,” Jason Quinn, an engineer at Colorado State University and senior author of the study, tells Dharna Noor of Gizmodo. “There is little to no regulation on emissions for growing cannabis indoors. Consumers aren’t considering the environmental effect either. This industry is developing and expanding very quickly without consideration for the environment.”

Growing an Ounce of Pot Indoors Can Emit as Much Carbon as Burning a Full Tank of Gas
A map showing the life cycle greenhouse gas emissions from indoor cannabis cultivation across the U.S. Jason Quinn

In a commentary about their research in the Conversation, the Colorado State University researchers write that in Colorado, for example, the weed industry’s greenhouse gas emissions (2.6 megatons of carbon dioxide) exceed those of the state’s coal mining industry (1.8 megatons of carbon dioxide).

According to the study, pot grown indoors in Southern California has the lowest emissions, with an ounce of dried cannabis resulting in the equivalent of 143 pounds of carbon dioxide being released into the atmosphere. The low emissions are thanks to the state’s power grid, which uses of renewable energy sources and benefits from the region’s mild climate.

The highest greenhouse gas emissions occur in the Mountain West, Midwest, Alaska and Hawaii, where keeping cultivation facilities at optimal temperature and humidity requires significant usage of electricity and natural gas. The study found the highest carbon emissions were in eastern O’ahu, Hawaii, with the equivalent of 324 pounds of carbon dioxide emitted per ounce of dried weed produced, per the Conversation.

To figure all this out, the study authors made a model that calculated the carbon emissions incurred by typical indoor pot production and tweaked its parameters for different regions using weather data from more than 1,000 locations across the U.S., according to Gizmodo. In other words, the model could estimate, based on a region’s climate, how much energy it would take to keep a grow house at the optimal temperature and humidity for cannabis. The model turned that energy usage into equivalent emissions of carbon dioxide using government data on local power grids to account for the varying emissions associated with different modes of electricity production. Finally, the model also factored in so-called “upstream” emissions from materials such as fertilizer, water and fungicide used by indoor cannabis growing as well as “downstream” emissions from waste products.

On average, the model found that producing a kilogram (2.2 pounds) of dried cannabis released the equivalent of between 2 to 5 tons of carbon dioxide into the atmosphere.

Evan Mills, a retired energy efficiency researcher formerly at the Lawrence Berkeley National Laboratory in California, tells New Scientist the paper’s figures may be an underestimate, because they don’t account for emissions from storage or processing.

The study authors say that moving operations outdoor or to glass-walled greenhouses that don’t require grow-lights could dramatically reduce the cannabis industry’s emissions.

John Timmer of Ars Technica reports that the researchers estimate a wholesale switch to outdoor production in Colorado would reduce greenhouse gas emissions to the tune of 96 percent, cutting the state’s total emissions by 1.3 percent. Switching to greenhouses would decrease emissions by 42 percent.

However, moving everything outside doesn’t make pot’s environmental impact disappear.

“Beyond greenhouse gas emissions, there are also many additional environmental factors that need to be considered, including illegal water diversion, pesticide use, and land-use change,” Hailey Summers, a sustainability researcher at Colorado State University and the study’s lead author, tells Inverse’s Tara Yarlagadda. “The most ideal solution will probably be a combination of all three major growth systems: indoor, greenhouse and outdoor.”