Hemp and climate change

Cannabis is legalised in more places than ever before. Some activists are pushing for legalisation with claims of climate change benefits, but how true is it that cannabis can help prevent or reverse climate change?

The idea that growing hemp can help mitigate the effects of climate change is not new. However, there is a lot of misinformation, pseudoscience and pure propaganda on both sides of the legalisation argument. This can make it very difficult for the public to verify the reality behind the claims.

The Monoterpene Argument
A common argument is that planting hemp alone could replace terpenes released from trees lost to deforestation. These terpenes play an important role in regulating atmospheric conditions. In particular, monoterpenes (all of which have the chemical formula C10H16) are of particular importance in this mechanism, although other types may also be important.

As the monoterpenes are released, they travel up to the stratosphere, are 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 exact mechanisms by which monoterpenes can help protect the planet from the sun’s harmful UVB rays is incomplete. However, their oxidation products in the atmosphere are known to help form clouds, which reflect solar radiation and produce enhanced precipitation. Monoterpenes are released at higher levels in warmer weather, allowing a local cooling effect as clouds seed over the forest, thus regulating its temperature.

What is wrong with this argument?
Although it is certainly true that cannabis releases monoterpenes, the amounts and types released have not been definitively determined (and vary between strains) and their specific mode of action on atmospheric regulation has not been evaluated. There is obviously no basis for suggesting that hemp releases more monoterpenes than all other agricultural crops.

Given the fact that cannabis releases most of the monoterpenes considered most important for oxidation and cloud formation (alpha-pinene, beta-pinene, D-limonene, myrcene, camphene, among others), there may well be a specific role for cannabis to play, but this is far from established.

Cannabis grows particularly fast, but the rate at which it releases terpenes is not understood. In addition, there may well be another, or perhaps many other, plant species that can do an equivalent or better job of releasing atmospheric monoterpenes:

Sweet chestnut
Sweet chestnut (Castanea sativa) is known to be among the highest emitters of monoterpenes and reaches its full height of about 35 metres (and a girth of about 2 m) in about 80 years. In the first 12 years it can reach 20 m.

Eucalyptus trees are also known to produce very high levels (the blue haze that gives their name to the Australian Blue Mountains is caused by light diffusing from the cloud of monoterpenes released from them) and have many applications in medicine and industry. They grow rapidly, with new growth of up to four metres per year.

Black bamboo
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.

Therefore, allegations without sources such as “the cannabis plant is excellent at producing abundant quantities 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 of legitimizing the potential role of cannabis in contributing to reversing climate change.

The importance of CO2
Forest areas are vital carbon sinks, sequestering tonnes of carbon per hectare per year and thus reducing the overall level of atmospheric CO2, while also emitting oxygen. CO2 is a known greenhouse gas and the increasing atmospheric concentration of greenhouse gases is closely linked to anthropogenic climate change.

As deforestation continues to affect the world’s forest areas, the percentage of total tree cover has drastically decreased. Deforestation alone is thought to be responsible for some overall global warming 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 air pollution.

Another oft-repeated argument claims that hemp is a better candidate for carbon sequestration than any other plant. An article from the impressive Natural News website states that “the benefits of hemp for soil remediation are virtually unparalleled in nature, as this miracle plant naturally pulls in far more carbon dioxide than any other tree, shrub or plant known to man.” the article goes on to quote from an obscure blog, Carbon Planet, which argues: “A field of medicinal weeds will sequester about 22 tonnes of carbon dioxide per hectare” – a claim not supported by any official source in either post.

Hemp is indeed considered a valuable carbon sequestration crop – along with many other commercial crops such as soybeans, rice, wheat and sugar cane. However, the net carbon sequestration from industrial hemp is estimated elsewhere at 0.67 tonnes per hectare per year. This is much lower than the previous claim and comparable to other common crops.

Hemp is an annual crop, which means that it is harvested every year. Therefore, it is different from trees. While annual crops have undoubted potential as carbon sinks, particularly if they are not ultimately burned or used as biofuels (processes that emit carbon back into the atmosphere), perennial trees are generally considered more important in the effort to control atmospheric CO2.

As our understanding of the complex dynamics of carbon sequestration from 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 older, older individuals are actually the fastest growing in terms of mass and increase their carbon sequestration capacity each year as they grow older. In extreme cases, a large tree can add carbon equivalent to the forest in one year, as a medium-sized tree can accumulate over its entire life to date.

So what role can hemp play?
With properly managed techniques, there is no reason why industrial hemp and medicinal hemp should not be planted as part of a sustainable carbon sequestration strategy. However, the question of whether a plant is a better emitter of carbon or monoterpene emissions is not the right one when it comes to the issue of solving anthropogenic climate change.

Biodiversity loss is one of the biggest factors affecting the long-term survival of our own species and other species on which we depend. Focusing on individual crops to combat climate change is not a long-term solution.

Instead of seeing hemp as the only plant that is going to protect against further man-made global warming, we need to look at the bigger picture. A better approach would be to seek to protect all remaining ecosystems, conserve as much biodiversity as possible, and intensify planting strategies already underway that include a range of habitat-appropriate species and potential uses.

Growing more cannabis, where appropriate, will undoubtedly bring multiple benefits, but its importance should not be overestimated for the purpose of gaining support for legalisation. There are already many good reasons for legalising cannabis without diluting the argument with wild claims and unsubstantiated facts. This may just end up giving the opposition fuel to discredit the movement.

Source: sensiseeds.com

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