The idea that cannabis cultivation can help mitigate the effects of climate change is not new. However, there is a great deal of disinformation, pseudoscience and outright propaganda from both sides of the legalisation argument, and it can be very difficult for the public to ascertain the reality behind the claims.A prevalent argument holds that planting cannabis alone could replace terpenes released by trees lost to deforestation, which have an important role to play in regulating atmospheric conditions. Specifically, monoterpenes (which all share the chemical formula C10H16) are of particular importance in this mechanism, although other types may also be of significance. As monoterpenes are released, they travel up to the stratosphere, carried by convection currents, and undergo oxidation reactions with ozone, OH, and NO3 in the atmosphere to produce a range of by-products.
The idea that cannabis cultivation can help mitigate the effects of climate change is not new. However, there is a great deal of disinformation, pseudoscience and outright propaganda from both sides of the legalisation argument, and it can be very difficult for the public to ascertain the reality behind the claims.
The monoterpene argument
A prevalent argument holds that planting cannabis alone could replace terpenes released by trees lost to deforestation, which have an important role to play in regulating atmospheric conditions. Specifically, monoterpenes (which all share the chemical formula C10H16) are of particular importance in this mechanism, although other types may also be of significance. As monoterpenes are released, they travel up to the stratosphere, carried by convection currents, and undergo oxidation reactions with ozone, OH, and NO3 in the atmosphere to produce a range of by-products.
Our understanding of the precise mechanisms by which monoterpenes can assist in shielding the planet from the sun’s harmful UVB rays is not complete; however, it is known that the products of their oxidation in the atmosphere assist in the formation of clouds, which reflect solar radiation and produce enhanced precipitation. Monoterpenes are released in higher levels in warmer weather, allowing for a localised cooling effect as clouds seed above the forest, thereby regulating its temperature.
What’s wrong with this argument?
While it is certainly true that cannabis releases monoterpenes, the quantities and types released have not been definitely established (and vary between strains), and their specific mode of action in atmospheric regulation has not been assessed. There is apparently no basis for suggesting that cannabis releases more monoterpenes than all other agricultural crops.
On the basis that cannabis does release most of the monoterpenes considered most crucial to oxidation and cloud formation (a-pinene, ß-pinene, D-limonene, myrcene, camphene, among others), there may very well be a specific role for cannabis to play, but this is far from established. Furthermore, it may well be that there is another, or perhaps many other, plant species that can do an equivalent or better job of releasing atmospheric monoterpenes. Cannabis does grow particularly fast, but the rate at which it releases terpenes is not understood.
The sweet chestnut (Castanea sativa) is known to be among the highest emitters of monoterpenes, and achieves its full height of around 35 metres (and girth of around 2m) in around eighty years, achieving as much as 20m in the first twelve years. Eucalypts too are known to produce very high levels (the blue haze that gives the Australian Blue Mountains their name is caused by light scattering off the cloud of monoterpenes released by them), and have many applications in medicine and industry. They are fast-growing, with up to four metres in new growth per year. Black bamboo (Phyllostachys nigra) grows at a rate of up to three metres per year, and is another high producer of monoterpenes, as well as having various applications in timber, fibre, food and medicine.
As such, unsourced claims such as “the cannabis plant is exceptional in producing copious amounts of 58 monoterpenes in less time, in more soil and climate conditions, with greater ecological and economic benefit than any other agricultural resource on Earth” are unscientific, and not particularly helpful to the ongoing cause for legitimising cannabis.
The importance of CO2
Forested regions are well-known to be crucial carbon sinks, sequestering tonnes of carbon per
hectare annually and thereby reducing the overall level of atmospheric CO2, while also emitting oxygen. CO2 is a known greenhouse gas, and its increasing atmospheric concentration is firmly linked to anthropogenic climate change.
As deforestation continues to impact the world’s forested regions, the percentage of overall tree cover has drastically fallen. It is thought that deforestation alone has accounted for some overall increase in global temperatures in recent decades, as it has caused an increase in CO2 of between 12% and 20% independent of that caused by industry and other sources of atmospheric pollution.
Another oft-repeated argument holds that cannabis is a better candidate for carbon sequestration than any other plant. An article from the ubiquitous sensationalist website NaturalNews states that “the soil-restorative benefits of cannabis are virtually unmatched in nature, as this miracle plant naturally pulls in far more carbon dioxide than virtually any other tree, shrub or plant known to man”; the article goes on to quote from an obscure blog, Carbon Planet, asserting: “A field of medicinal weed will sequester around 22 tonnes of carbon dioxide per hectare”—a claim which is not backed up by any official source in either post.
Hemp is indeed considered a valuable carbon sequestering crop—along with many other commercial crops, such as kenaf, rice, wheat, and sugarcane. However, net carbon sequestration by industrial hemp is elsewhere estimated at 0.67 tons (0.61 tonnes) per hectare per year, far lower than the previous claim, and comparable to other common crops. While annual crops have undoubted potential as carbon sinks, particularly if they are not ultimately burned or used as biofuel (processes which emit carbon back into the atmosphere), perennial trees are generally seen as more important in the effort to control atmospheric CO2.
Cannabis and atmospheric CO2
Interest has been shown in the ability of cannabis and many other plant species to increase their growth rate and nutrient utilisation in CO2-enriched environments. A study published in 2011 demonstrated that atmospheric concentrations of CO2 of 700ppm significantly increased net photosynthesis and water use efficiency in four high-yielding cannabis varieties compared to ambient concentrations of 390ppm; on the other hand, concentrations of 545ppm had negligible effects.
However, atmospheric concentrations of CO2 have not yet reached the magic figure of 700ppm that will allow cannabis to grow more efficiently. According to estimates, this point will be reached in around 2100 at present rates of emission. Thus, we still have over eighty years to attempt various other strategies that will potentially maintain lower concentrations.
These strategies include replanting of a large range of different plant species that are currently threatened or subject to fragmented habitats. If implemented now, reforestation schemes involving even relatively slow-growing species would still have a discernable impact in eighty years’ time. If successful, these strategies will negate the need for abundant planting of cannabis as a carbon sink.
As our understanding of the complex dynamics of carbon sequestration by terrestrial biomass grows, it is increasingly clear that large, old trees are by far the best carbon accumulators over time. A recent study in Nature found that for the majority of tree species, the largest, oldest individuals are in fact the fastest-growing in terms of mass, and increase their carbon-sequestering capacity each year as they grow. In extreme cases, one large tree may add equivalent carbon to the forest in one year as a mid-sized tree may accumulate in its entire life to date.
So what role can cannabis play?
With properly-managed techniques, there is no reason that cannabis and hemp should not be
planted as part of a sustainable strategy for carbon sequestration. However, the question of whether or not any one plant is a better carbon sink or monoterpene emitter is not the correct one to ask when it comes to the issue of resolving anthropogenic climate change. Loss of biodiversity is one of the biggest factors affecting the long-term survival of our own species and other species we depend on, so strategies which focus on single crops are generally impractical in real terms.
Rather than viewing cannabis as the one plant that is going to protect against further
anthropogenic global warming, the correct approach would be to seek to protect all remaining ecosystems, preserve as much biodiversity as possible, and intensify already-ongoing planting strategies that comprise a range of species appropriate to habitat and potential utilisation. Cultivating more cannabis and hemp where appropriate would undoubtedly bring multiple benefits, but its importance should not be overstated for the purpose of winning support for legalisation. There are already more than enough good reasons for cannabis to be legalised without diluting the argument with wild claims and unsupported facts, and thus giving the opposition fuel to discredit the movement.