Reefer sadness: How is cannabis growth impacting climate?

Cannabis cultivation as an industry is taking off from Lesotho to Latin America, and that’s raised questions about the sector’s impact on land deforestation, pesticide use and related concerns in an era of climate crisis.

As soothing (or lucrative) as the thought of legal marijuana growth may be for some, a new study from scientists in Canada and the United States looks at how cultivation impacts water resources. They focused on a specific area of northern California to fill the gaps in what researchers know about cultivation benefits and harms, with the work published in the journal Environmental Research Communications.

“Cannabis is an emerging agricultural frontier, but thanks to its long illegal and quasi-legal history, we know very little about the impacts of cannabis cultivation on water resources,” said lead author Sam Zipper of the University of Kansas. The use of cannabis is legal in some form in half of all U.S. states but remains illegal under federal law; Canada legalized recreational and medical marijuana in 2018. Germany, Mexico and the United Kingdom are expected to become top producers too by 2024.

“What we do know is that there has been a big increase in cannabis cultivation in in recent years. Researchers have found that the area under cultivation in (California’s) Mendocino and Humboldt counties nearly doubled between 2012 and 2016,” adds Zipper.

So he and his team looked at the region’s Navarro River Watershed and determined that between the pot growers and the much larger residential use, the potential exists for “significant streamflow depletion” because of water diversion for irrigation.

That’s expected to be at its worst in late summer, precisely when the local stream ecosystem, and the fish and marine life depending on it, need groundwater inflows most. While residential use places far more pressure on the system, cannabis fields add to that pressure – and it’s not really on the radar.

“It has often been assumed most cannabis cultivators irrigate using surface water. But recent evidence from Northern California shows that groundwater is the primary irrigation water supply in this region,” said Zipper in a press release. “That means it is essential to understand how groundwater pumping for cannabis cultivation affects water resources, particularly given that regulations governing cannabis legalization and management are currently being debated and designed in many U.S. states.”

This new work is focused on a specific area, but it adds to what the wider international community may know about the tradeoffs involved with cultivating cannabis as a cash crop. Legalization tends to protect the environment on the one hand because it reduces the damage illegal grow operations cause. But, as the industry’s DOPE Magazine pointed out earlier this year, the environment remains an afterthought as the legal cannabis industry takes off. Few governments and utility companies have taken action to promote water and energy conservation to growers, or ensure compliance with environmental targets.

“Issues such as whether illegal cannabis cultivations are diverting water from the state’s economically important mainstream agriculture, or the amount of pesticide contamination leaching into their waterways from illegal grows, or the tracking of individual grow sites,” the DOPE report said, counting the questions but noting that data is not readily available. Hopefully, the new report from Zipper and his team on water will stimulate the search for additional data on the industry.

Image: Streamflow depletion (within the Navarro River Watershed only) caused by groundwater pumping for cannabis cultivation and residential use after 1, 10, and 50 years of pumping, expressed (a) volumetrically and (b) as a percentage of mean monthly baseflow (1999–2018 water years).