There are many groups within NZ including the Green Party that are calling for the legalisation of marijuana for personal/medicinal use and my question for them is: - How can they reconcile that stand with the negative environmental effects from cannabis cultivation?

No matter where you sit on its legalization, growing marijuana affects our environment and that can be in a negative way.

Growing marijuana indoors requires copious electricity through the use of high-intensity lamps, air conditioners, dehumidifiers and much more. In order to grow it outside, streams become sponges, being sucked dry as seen in the outdoor grow-ops in California.

What Happens When Marijuana is legalised:

Illegal marijuana growers have typically kept their crops indoors to safeguard themselves against prosecution. Now that marijuana has been decriminalized in some US states, many will start to cultivate cannabis outdoors meaning that it will require more water consumption. A study by the California Department of Fish and Wildlife in 2015 found that weed cultivation was excessively diverting water from creeks, which are home to endangered salmon species.

An estimated 22 litres of water is needed a day to water marijuana plants, compared to a wine grape plant, which uses 12 litres.

With any expansion of an agricultural crop comes the need to clear-cut more forest and construct road. This increases the risk of erosion, river diversion, and habitat destruction. Wild animals are left fending for themselves and possibly endangering themselves and others.

There is also the case of pollution with the use of chemicals, which are often used to kill rodents which may damage the crop. These chemicals make their way into the sewage system and into our water supplies. They also make their way into the food chain, and can pose significant health risks to predators.

Using the State of Colorado in America as an example, the legalization of cannabis (recreational weed was approved in a state wide ballot in 2012) has reinvigorated previously dilapidated industrial areas of Denver and generated more than $1bn a year in taxable sales. But the voracious energy consumption of growers is rubbing up against the city’s ambitions of cutting greenhouse gases.

And with around half of all US states now allowing cannabis for various uses, hothouse cultivation is increasingly a concern for governors and mayors promising to fill a hole in emissions reductions.

Evan Mills, a senior scientist at the University of California, was one of the first researchers to quantify how energy hungry the cannabis industry is, estimating in 2011 that indoor cannabis cultivation represents 1% of total electricity use across the US, a figure backed up by a New Frontier study last year.

Lighting can comprise up to half of a cannabis grower’s energy use, with the desire to create a round-the-clock version of natural growing conditions requiring hugely powerful high pressure sodium (HPS) lights.

As a result, producing just a couple of pounds of weed can have the same environmental toll as driving across America seven times.

“The legalization of recreational or medicinal marijuana in eight states including California, Florida and Massachusetts, means some of the nation’s hard-earned progress towards climate change solutions is on the chopping block as regulators continue to ignore this industry’s mushrooming carbon footprint.”

Denver’s electricity use has been edging up at a rate of more than 1% a year, with nearly half of that increase due to marijuana-growing facilities, the city has said. While just a small percentage of Denver’s electricity is used by cannabis operations, they are far more energy intensive on a per-square-foot basis than most other types of businesses. This demand, in turn, drives fossil fuel use, because Colorado gets the majority of its energy from coal-fired power plants.

“It’s definitely an area of concern,” said Emily Backus, sustainability advisor for the city. Denver has a goal to shrink its greenhouse gas emissions 80% by 2050, largely through boosting renewable energy, improving the efficiency of buildings and promoting public transit and electric cars.

Denver also signed on to the Paris climate agreement’s goals with Mayor Michael Hancock decrying the “serious threat to our economy and way of life” posed by global warming.

In order to calibrate conditions to reap multiple harvests a year, growers have to bake the plants in light while cranking the air conditioning to ensure rooms stay at a finely balanced temperature. A dehumidifier is used to prevent mould, carbon dioxide is pumped in to bolster growth and fans mimic the presence of a breeze. Irrigation systems are often hoses plugged into the plants, leading to tubs of water. The goal is healthy, weighty buds.

All of this uses a lot of energy and LEDs, despite saving a lot of power, have been deemed by many growers to be less effective and more expensive than HPS lights.

The cannabis industry is often pigeonholed as being very heavy users of resources. LEDs aren’t in themselves a perfect answer. Because marijuana generally takes longer to mature under LEDs, it can result in not much energy being saved.

Marijuana might look and smell natural, but its ecological footprint is anything but green. Marijuana is power hungry.

The $3.5bn USA cannabis industry is one of the nation’s most energy intensive; often demanding 24-hour indoor lighting rigs, heating, ventilation and air-conditioning systems at multiplying grow sites.

As many as 10 states could legalize recreational marijuana this year, which means the resultant electricity consumption could cause problems for public utilities and city officials.

A study by scientist Evan Mills, with the Lawrence Berkeley National Laboratory, revealed that legalized indoor marijuana-growing operations account for 1% of total electricity use in the US, at a cost of $6bn per year. Annually, such consumption produces 15m tons of greenhouse gas emissions (CO2), equal to that of three million average cars.

In 2012, Colorado became the first state to legalize recreational marijuana. Two years later, Denver’s 362 marijuana grow facilities consumed more than 2% of the city’s electricity usage. State wide facilities are behind roughly half of Colorado’s new power demands. Electricity represents roughly 20% of the total cost of a cannabis operation.

In Boulder County during the second quarter of 2015, a 5,000 square foot indoor cannabis facility was eating about 29,000 kilowatt hours (kWh) of electricity monthly. A local household in the county was consuming about 630kWh.

In other states where the recreation market has taken off, cannabis production is having a similar effect.

According to a report by the Northwest Power and Conservation Council in Oregon – where recreational marijuana has been legal since 2014 – an indoor grow system for only four plants sucks up as much energy as 29 refrigerators.

The report also estimated that the emerging market could warrant the electricity demands of a small city in the next 20 years.

Skyrocketing energy levels confirm that the marijuana business is growing.

Grow operations consume $6 billion a year or enough energy to power 1.7 million American homes. The industry is not as green as you'd think. Growing consumes electricity for powerful lamps, CO2 generators, fans and air conditioning. On a personal scale, it takes as much electricity to produce one joint-worth of cannabis as lighting a lightbulb for 25 hours.

In some cannabis-loving states like California marijuana production accounted for 3 percent of the state's entire energy output.

Evan Mills is a member of the UN Intergovernmental Panel on Climate Change. "Most key parties are not yet engaged. California and elsewhere has begun to address the destructive impacts of unregulated outdoor cultivation, but have yet to recognize what may be even greater environmental consequences from the prodigious amounts of energy used by indoor operations," said Mills.

Outdoor production also has environmental consequences with deforestation and high levels of water and pesticide use but outdoor producers will have to abide by pre-existing environmental laws, just like everyone else. In effect, that makes indoor production the chief climate change and energy concern.

As the industry grows, so will its negative impacts, therefore legalizing marijuana use should also require the growing industry to power itself cleanly.

Annual consumption of electricity in the USA for the purpose of growing marijuana produces 15m tons of greenhouse gas emissions (CO2), equal to that of three million average cars.

So we have these groups calling for legalisation of marijuana on one hand and also calling for steep reductions in greenhouse gas emissions on the other. They are quite vocal about blaming agriculture and the burning of fossil fuels for the country’s GHG emissions but it seems, taking the USA consumption as an example of the likely effects from legalisation in NZ, that logic does not come into their thinking it is just about self-satisfaction (the nimby style of thinking strikes again).

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