Earth Day and Weed Day Are an Odd Couple

Evan Mills, Ph.D.
5 min readApr 20, 2021


Long before there was an Earth Day — and very long before there was a Weed Day, observed on 4/20 by those in the know — cannabis culture was aligned with environmental values. That went out the window when cultivation moved indoors, particularly conspicuously in California.

While this pair of April celebrations are separated by only a day, the expanding carbon footprint of indoor factory farming creates a chasm running wide and deep.

The local environmental effects of irresponsible outdoor cultivation are relatively well known, but scant attention has been paid to the arguably more serious global impacts of indoor methods, which, mind-bogglingly, require around 400-times as much energy pound-for-pound as needed to smelt aluminum.

In the rush to legalize, state governments from California to New York turned a blind eye to this elephant in the room as indoor cultivation quietly became the dominant industry practice. Federal agencies have also been disengaged, and prohibitions on interstate transport mean that cultivation has to happen even in the most illogical and severe climates.

Policymakers should heed a new peer-reviewed study from Colorado State University that pegs the greenhouse-gas emissions of indoor-grown cannabis at 2,000 to 5,000-times its retail weight (varying climate and carbon-intensity of the grid). This jives with estimates I developed a decade ago, equating the emissions to those of 3 million cars nationally. This suggests that the push towards higher-tech windowless farming may not have achieved the hoped-for efficiencies. More tangibly, producing an ounce of the stuff emits as much as a tank of gas. Colorado’s emissions exceed those of the state’s entire production of coal. And emissions for individual Coloradans — the nation’s top consumers of cannabis — are more than half their entire household’s carbon footprint.

Going solar won’t cut it. These facilities use energy so intensively (think data centers) that around twenty-times their roof area is required to generate enough clean power. For one planned mega-grow in Blythe, California this translates to 1400 acres of solar panels — a far greater use of land than “less productive” outdoor farming would entail (see figure). Not surprisingly, evidence of such net-zero grow facilities is lacking. Centralized renewables feeding the grid have impacts, too, and what power we manage to generate that way needs to be prioritized for energy users who have no alternative (think steel factories).

There is a far more elegant solution: outdoor cultivation, or, as the industry likes to say, “Sungrown”. Indeed, national demand could be met with 0.01% percent of all farmland, an area only surpassed birdseed and only one-seventh that used or tobacco.

Urban myths about the necessity of industrialized indoor cultivation are readily debunked. These include superior quality, medicinal value, and water efficiency. In any case, indoor cultivation is an entirely unaffordable luxury in a warming world. It also yields far more solid waste, stresses the grid, and is now a suspected contributor to urban air pollution.

In yet the latest instance of science being ignored in setting public policy, some states favor or forbid outdoor cultivation altogether. Among them is Illinois, which happens to have the highest cannabis-production carbon footprint in the nation, yet energy hungry Costco-sized grows are ironically being plopped onto beautiful farmland. In “climate-friendly” California, state legislation is neutral, but many local jurisdictions prohibit outdoor cultivation and other policies favor indoor growers.

Ethics and politics aside, it is arguable that indoor cultivation will become the next boom-and-bust industry, creating new headaches for policymakers. This reckoning would stem from the collision of rising energy costs and declining product prices, facility construction costs reaching into the tens of millions of dollars, plus more millions spent over time on energy. Such bankruptcies could result in lost tax revenues, abandoned buildings, and unemployment. Cannabis companies may soon find themselves excluded from green investment funds. Layered atop this are a host of political, regulatory, and reputational risks as consumers become aware of just how un-green indoor-grown can be.

Consumers may eventually vote with their wallets, but that can’t be expected in the current information void together with “budtenders” generally pushing indoor- over outdoor-grown products and indoor growers slickly green-washing their image.

Meanwhile, as Congress intensifies its pursuit of national legalization, a window of opportunity for nipping this burgeoning source of carbon emissions in the bud is closing. The Biden administration will have to reckon with the conspicuous conflict between drug policy and environmental policy. Suggestions for Team Biden include:

  • Lift the interstate transportation barriers so that cannabis can be grown were it makes sense and emits least.
  • Double-down on the black market. Excess production is suppressing prices, particularly of outdoor-grown, making indoor-grown artificially more profitable.
  • Revisit a carbon tax (non-starter, I know, but one can ask).
  • Work with states, who ultimately will continue to set key policies, which could range from mandatory net-zero building codes for cannabis facilities to bans on indoor cultivation.
  • Push for emissions disclosure and product labeling — consumers can’t vote with their wallets unless they know there is a problem — and work to make bud-tenders conversant on product carbon footprint.
  • Deploy climate-stimulus incentives to outdoor growers, as they will save more carbon than indoor producers, without exception.

Policymakers don’t need to choose between climate and cannabis, they just need to ensure that cultivation occurs sustainably outdoors. This has done the trick for thousands of years. That prospect is something to celebrate this Earth Day.

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Evan Mills is a California-based energy and climate analyst who participated in the work of the Nobel-Prize-winning Intergovernmental Panel on Climate Change. He was previously a Senior Scientist at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, where he is now an Affiliate. His new study of this issue, coauthored with Scott Zeramby, is forthcoming in a collection entitled The Routledge Handbook of Post-prohibition Cannabis Research (Routledge & CRC Press).



Evan Mills, Ph.D.

Energy & environment scientist, with 40 years experience developing and advancing climate change solutions.