Nuances in Parsing Cannabis Cultivation Energy Use: Comment on "Cannabis and the Environment: What Science Tells Us and What We Still Need to Know"

Abstract

The scientific literature has begun to shed much-needed light on the complex environmental impacts of cannabis cultivation. Among the pathways—land cover change, water use, pesticide use, energy use, air pollution, and water pollution—energy use may prove to be the most significant, particularly where climate change is of concern.

However, some estimates of energy use and greenhouse-gas emissions suffer from critical analytical errors that bias estimates in a downward direction. Among these are narrowed boundary conditions that exclude non-electric energy consumption, off-grid electricity, transportation energy, and less-efficient small-scale or black-market operations. Indoor cultivation presents one of the more complex energy analysis challenges, involving a combination of many end uses and bio-chemical factors at the core of any meaningful definition of "energy efficiency".

Comprehensive analyses must also include a diversity of pre- and post-cultivation energy uses. These include energy embodied in inputs such as single-use growing media, fertilizer, water supply and treatment; post-cultivation drying, processing, extractions, and preparations; refrigerated storage; retail operations; and transportation. Other accounting considerations include energy embodied in failed and interdicted crops or those destroyed due to unacceptable lab-test results.

Estimates based on field data--particularly using samples of convenience--can be biased in either direction if they do not reflect actual market conditions driven by local climate, strain choice, or production processes and their associated efficiencies.

Greenhouse-gas emissions can be further underestimated if geographically varying electricity carbon emissions factors are not properly weighted, manufactured CO2 used to enhance plant growth is not counted, and particularly carbon-intensive black-market operations are excluded.

While it has been stipulated that energy efficiencies have likely improved over time, major transformations in the industry (and increasing cannabis production) could be driving energy use either up or down, but most likely upward. It is thus critical to track efficiencies as well as aggregate demand metrics that can trend in opposite directions.

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Comment on “Cannabis and the Environment: What Science Tells Us
and What We Still Need to Know”
Cite This: https://doi.org/10.1021/acs.estlett.1c00062
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Arecent Global Perspective by Wartenberg et al. in
Environmental Science &Technology Letters sheds much-
needed light on the complex environmental impacts of
cannabis cultivation.
1
Among the pathways identified by their
ambitious reviewland cover change, water use, pesticide use,
energy use, air pollution, and water pollutionenergy use may
prove to be the most significant. Cannabis factory farming
involves energy-intensive lighting, space conditioning, dehu-
midification, artificial CO2fertilization, water reclamation, and
other processes to boost yields while maintaining year-round
tropical indoor conditions irrespective of the outdoor climate.
The authors do a service by bringing further attention to the
role of energy in indoor cultivation, as the literature focuses
largely on outdoor practices.
2,3
The Global Perspective exemplifies the accounting and
methodological complexities in cannabis energy analysis. The
authors suggest that my estimate (Mills
4
) of 20 TWh-y (1012
Wh-y) electricity consumption for legal and illicit indoor
cannabis cultivation in the U.S. may not be plausible, equating
it to the 1.7 quads (1015 BTU) site energy use for all U.S.
agriculture.
5
However, the energy equivalent of 20 TWh is only
0.068 quads (4% of U.S. agriculture).
The authors inadvertently cite a grey literature estimate of
national energy use for indoor cannabis cultivation (New
Frontier Data
6
) as 4.2 MWh-y (106Wh-y), lower than Mills
4
by more than six orders of magnitude. However, the source
document reports 4.2 million MWh-y (1012 Wh-y), admittedly
a peculiar phrasing of units. The authors also restate that
source’s estimated greenhouse gas emissions only for legal
cultivation, one-quarter of the total presented therein. The
remaining factor-of-three difference between these two
estimates is non-trivial and merits examination.
Mills
4
models energy intensity ((kWh/gram of finished
flower produced)), while the New Frontier Data study
6
utilizes
limited field measurements. Both studies extrapolate intensities
to estimate national energy demand and emissions. Impor-
tantly, the studies’boundary conditions and findings are not
directly comparable, notably that off-grid and transportation
energy are not counted in the New Frontier Data study,
6
which
also excluded direct fossil-fuel use within the facilities and did
not explicitly include small-scale operations which may be
more energy intensive than larger professionally operated
facilities.
Greenhouse gas emissions differ even more, as the New
Frontier Data study
6
excluded manufactured CO2used to
enhance plant growth, transportation energy, and off-grid
electricity typically produced by inefficient diesel generators.
The New Frontier Data study
6
also utilized a proprietary,
nonrandomized, self-reported sample of cultivation sites
heavily representative of milder climates with cleaner grids
(California and Oregon). While various data are presented for
81 benchmark sites, only 24 (from seven states) yielded the
intensity metric underlying their national estimate. The sample
also likely represented atypical adoption of efficiency features
(e.g., 20% of the sites had LED lighting, which was rarely
deemed cost effective at the time
7
). Lastly, the manner and
extent to which strain choice (a significant factor),
8,9
crop
drying, cold storage, and particularly carbon-intensive black-
market operations were addressed is not documented.
Wartenberg et al.
1
caution about the unavailability of
accurate energy-use data from individual cultivators. Fortu-
nately, such information is increasingly found in the
literature,
8−12
which supports model validation, as are
market-level data,
2,10,12−15
which support estimates of
aggregate energy demand. While they note an absence of
data on cultivation area and planting densities, these are not
inputs to either study’s methodology.
While others arrive at similar or higher measured energy
intensities
9,15
and shares of total regional electricity de-
mand
15,16
as in Mills
4
, the authors note that energy efficiencies
likely improved during the six-year interval between the two
studies. That said, during this period, there were major
transformations in the industry (and increasing cannabis
consumption
17
) that could have driven energy use either up
(e.g., larger facility volumes) or down (e.g., more greenhouses,
which average 25% lower energy intensity
6
). One data set
8
indicates a trend toward larger facilities, which could elevate
energy intensities. It is thus critical to track efficiencies as well
as aggregate demand metrics that can trend in opposite
directions. Meanwhile, a recent and exhaustive peer-reviewed,
model-based assessment
18
identifies a carbon-intensity range
bracketing the value in Mills
4
, and demonstrates the strong
influence of climate and electric generation mix on carbon
emissions. The New Frontier Data study
6
is a significant
outlier (Figure 1) among the other studies in terms of its very
high yield density (grams/m2-y, twice the average of a large
Received: January 26, 2021
Accepted: May 4, 2021
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meta-analysis
8
) and correspondingly low energy intensity (one
quarter to one seventh that of other studies
4,9,18
).
Many pre- and post-cultivation energy uses must also be
quantified. These include energy embodied in inputs such as
growing media, fertilizer, water supply and treatment;
postcultivation processing, extractions, and preparations;
refrigerated storage; retail operations; and transportation.
Other energy-accounting considerations include energy
embodied in failed and interdicted (black-market) crops or
those destroyed due to unacceptable lab-test results.
Energy use in this industry is a largely unattended issue
among policymakers.
13
Yet, at energy intensities of ∼6 kWh/
gram,
4
indoor cannabis cultivation is 500 times more energy
intensive than aluminum smelting,
19
and energy per unit floor
area is 50−100 times that of homes and offices.
13
Wartenberg
et al.
1
outline traditional policy recommendations, including
performance standards, incentives for energy efficiency, and
data disclosure. While such measures yield large savings in
other contexts, a confounding factor exists in the case of indoor
cannabis. Consider one planned 55-acre [22 ha] windowless
industrial park in the California desert which includes a
dedicated natural-gas power plant that could power 90,000
average
20
all-electric homes.
13
To instead employ solar
electricity would require photovoltaic arrays spanning 25
times the roof area, or roughly 1400 acres [567 ha], far more
than outdoor cultivation would require for the same yield. This
draws into question the potential for sustainabile indoor
cannabis cultivation, where “best practices”promise only to
optimize the suboptimal. Meanwhile, the alternativevirtually
zero-energy outdoor cultivation, which would require only
0.01% of U.S. agricultural lands
13
to meet current demand
receives no financial or policy inducements and indeed is
banned in many jurisdictions, rendering it at a competitive
disadvantage to indoor practices eligible for market-distorting
subsidies (utility rebates)
21
to finance costly measures like
LED lighting.
Notably, some indirect nonenergy impacts identified by
Wartenberg et al.
1
may prove as or more severe for indoor than
outdoor cultivation. These include land use (energy extraction,
infrastructure), water use (evaporation during electricity
generation), water pollution (hydroponic systems shunting
pollutants into wastewater treatment systems), indoor/outdoor
air pollution (VOC emissions in urban air basins), and solid
waste production (soil and artificial growing media landfilled
after each cropping cycle at intervals as short as 40 days,
14
while frequently replaced high-pressure sodium and metal-
halide grow lights contain mercury).
13
In sum, indoor cannabis production is a particularly complex
and fast-growing driver of energy demand and greenhouse gas
emissions and yet is far less studied than other comparable uses
of energy. In particular, despite the passage of a quarter century
since cannabis was first legalized for medical use, U.S. energy
and environmental agencies have conducted no known
technical or policy research on associated energy impacts.
Even leading states have only recently begun considering it.
22
There remains a pressing need for more analysis, particularly
given that legalization of indoor cannabis cultivation by the
incoming Biden Administration would directly undermine its
ambitious climate goals.
Evan Mills
■AUTHOR INFORMATION
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.estlett.1c00062
Notes
The author declares no competing financial interest.
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