Cannabis might make people feel high, but from an environmental perspective it could be a real downer. In the US state of California a massive expansion in cannabis farms has resulted in deforestation, overstretched water supplies, wildlife poisoning, and destabilization of steep hillsides. Without suitable policy intervention and careful planning, other parts of the world are likely to suffer too as demand for cannabis continues to rise.

In Humboldt County, northern California, the number of greenhouses has increased nearly twentyfold over the space of eight years. Located far from markets and main roads, and often teetering on steep slopes, these greenhouses were never designed for growing tomatoes or cucumbers. Instead, they’re almost certainly a response to the burgeoning cannabis trade.

Using high resolution satellite imagery for the years 2012 and 2016, Van Butsic from the University of California, Berkeley and his colleagues found a boom in cultivation of cannabis in Mendocino and Humboldt Counties. By zooming right in, the researchers could identify the distinctive shape of the cannabis plants, the regular pattern in which the crop is planted, and the greenhouses perched in unusual places.

The number of cultivation sites and area under cultivation nearly doubled in five years, the team found, while the number of plants produced nearly quadrupled. Much of this expansion took place in areas of high environmental sensitivity, with a 40% increase on steep slopes, a 44% increase in remote regions, and as much as a 116% increase near rivers inhabited by endangered fish.

“The chances of environmental damage are much greater in these regions because of the high potential for erosion, which threatens water quality, high potential for using water directly from headwaters, and the need to build roads to access these farms,” says Butsic.

California has fairly robust environmental laws, but they appear not to have been applied to these remote cannabis farms. Cannabis was legalised for medical production in California in 1996, but there was little regulation of the market until 2016, with no state-wide collection of information on cultivation locations. Couple this with a booming illegal export market and it is hard to know whether the observed expansion was driven by the need for increased medicinal supplies, or for profits to be made on the black market.

“The paucity of regulation and enforcement around cannabis has been one of the major drivers in the increase of cultivation in environmentally sensitive areas, we think,” says Butsic.

Ultimately the authors believe that the trend to legalise cannabis, for both medicinal and recreational use, may lessen the problem of cannabis cultivation in environmentally sensitive areas.

“Through liberalization one can imagine a path where cannabis production is normalized and farmers maximize profit by using land that is best suited for growing cannabis instead of areas chosen to avoid detection,” they write in Environmental Research Letters (ERL).