Cannabis is a new agricultural crop, so it is important to begin to understand its impact on greenhouse gas emissions considering growing threats from climate change. Cultivating cannabis in a commercial setting can require high energy and water use for irrigation, lighting, and climate control. A new study  evaluates the energy and materials used to grow indoor cannabis and quantifies the corresponding greenhouse gas emissions.
Since recreational cannabis legalization in Colorado and Washington in 2012, cannabis in the US has grown into an industry with $13.6 billion in annual sales. Colorado alone sells more than 530 tons of legally grown cannabis flower per year. As states continue to embrace legalization, that number will continue to grow. However, increases in greenhouse gas from production can be expected with 41% of North American cannabis growers reporting use of indoor cultivation. One previous estimate of greenhouse gas emissions from indoor cannabis production reported 4,600 kg of carbon dioxide equivalent (CO2e) per kg of cannabis produced.  That report was conducted prior to legalization and was based on a case study of a small-scale grow. A new model recently published (2021) in Nature Sustainability accounts for all factors from seed to distribution and considers geographic differences. 
Greenhouse gas emissions of cannabis cultivation vary widely from 2,283 to 5,184 kg CO2-e per kg of dried flower based on location. The main contributing factors include electricity use for lighting and cooling, natural gas use for heating, and use of CO2 enrichment (emissions are due to compression and storage). Modifying outside air with heating, ventilation, and air conditioning (HVAC) is the largest contributor to greenhouse gas emissions and varies significantly with geography.
Highest greenhouse gas emissions from cannabis production can be observed in Kaneohe Bay, Hawaii, with the lowest values in Long Beach, California. The high usage in Hawaii is associated with dehumidifying “consistently hot and humid outside air.” Additionally, Kaneohe Bay has a power grid relying more on oil compared to Long Beach with a power grid relying more on gas and solar power.
The median value of emissions was determined to be 3,658 kg CO2e kg–1. The Mountain West and Midwestern United States are especially intensive for growing cannabis indoors, while California is among the lowest emitters. Within Colorado, mountainous areas were associated with significantly higher emissions than plains areas, and the researchers suggest that transporting product from optimal locations would curb overall emissions.
Cultivating cannabis indoors anywhere leads to significant greenhouse gas emissions. Regulators will need to pay attention to the results of this study so they can implement strategies and guidance to prevent cannabis production from becoming problematic for the environment. Finally, allowing for federal legalization and interstate cannabis trade can encourage businesses to grow cannabis in areas where it has the least environmental impact instead of cultivating in states with more energy- and resource-intensive requirements. 
1- Summers HM, Sproul E, Quinn JC. The greenhouse gas emissions of indoor cannabis production in the United States. Nature Sustainability. doi:10.1038/s41893-021-00691-w. https://www.nature.com/articles/s41893-021-00691. [Impact Factor: 9.647; Times Cited: 2 (Semantic Scholar)]
2- Mills E. The carbon footprint of indoor cannabis production. Energy Policy. 2012;46:58–67. [Impact Factor: 5.042; Times Cited: 59 (Semantic Scholar)]
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