As it becomes more and more popular across the United States, indoor production of cannabis is gradually becoming a big source of greenhouse gas emissions because of its electricity and natural gas consumption. The good news is that there’s a way around the nasty emissions: focus on outdoor production.

Greener green

The US legal cannabis industry has rapidly grown from a US$3.5 billion industry to US$13.6 billion in annual sales. Colorado pioneered recreational legalization in 2012 and soon other states followed. There’s substantial consumer demand in the country, with 48% of the adults have tried cannabis at some point in their life and 13% having consumed it last year.

It’s grown to be quite the industry. But despite its rapid growth and spread there’s been no real research into its environmental impact — particularly greenhouse gas emission. A new study by researchers from the Colorado State University has now provided the most detailed accounting to date of the industry’s carbon footprint — which, as it turns out, is quite big.

“Policymakers and consumers aren’t paying much attention to environmental impacts of the cannabis industry,” Jason Quinn, co-author of the study, told 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.”

For growers, indoor cultivation is usually the preferred choice as it offers a larger control over the plants’ habitat and you can do it anywhere on the globe. But the advantages come at a cost to the climate, as the plants are quite picky: they require ventilation, air conditioning, and heating to grow at the ideal level. Plus, the grow lights have to stay on around the clock, leading to large energy consumption.

The researchers created a model to assess the energy and materials used by indoor growing, which is the method chosen by 41% of US growers. The model was designed to mimic an actual growing warehouse and all its elements – from the grow lights and the pesticides to the water used as part of the drip irrigation system.

They gathered data from over 1,000 locations nationwide to complement their model and found that the top two factors contributing to emissions from indoor cannabis growing are climate control and high-intensity grow light for the plant.

The emissions were estimated between 2,283 and 5,184 kilograms of carbon dioxide per kilogram of dried flower. Put another way, the eighth of weed you buy legally in the US comes with a 19-kilogram carbon footprint.

Up to 80% of the emissions created by cannabis growing are caused by practices directly linked to indoor cultivation methods, according to the researchers. This means there’s a large potential to tackle the climate footprint by shifting the industry towards a more outdoor-based operation. In Colorado, moving all production outdoors would cut 1.3% of the state’s total emissions.

Of course, there are tradeoffs to outside growing, such as larger water use, exposure to theft, and larger land use. This is what researchers want to focus on next.

“There are tradeoffs between greenhouse gas emissions and land use, we just don’t know the magnitude yet because it hasn’t been researched. This is on our ‘to-do’ list for what’s next,” Hailey Summers, lead-author, told Anthropocene Magazine.

The study was published in the journal Nature Sustainability.