Not seeing the big emissions wood for the millions of baby trees

That nice chap Chris Martin from Coldplay was on the news the other day promoting their latest tour. Although their music seems to polarise opinion, they are an admirably well-meaning bunch and in order to offset the impact of flying all over the world singing songs, they are planning to plant a tree for every concert ticket sold. This intent chimes with the phrase used by Boris Johnson at COP26 when he summarised the global agenda in four pithy words: “coals, cars, cash and trees”. Both Chris and Boris must surely be right. Trees absorb carbon from the atmosphere as they grow, they support a whole host of insect and bird life, and a mature tree represents a big chunk of stored carbon either as a tree or as a harvested timber product such as a table. You’d think that unlike Boris’s other three pledges (coals, cars and cash), planting trees is that rare thing, a simple solution to climate change that everybody can agree with and do something about. You’d think it sounds obvious and easy, but you’d be wrong. The reality of the relationship between trees and offsetting carbon emissions is not quite as straightforward as advocates of offsetting would have us believe.

Let’s step through what sceptics regard as the three major  issues with planting trees as a climate change mitigant. Firstly, trees grow very slowly and don’t really absorb much carbon until they reach maturity. A weedy sapling planted tomorrow won’t start to make any difference for at least twenty years, not to mention that the infant mortality rate of baby trees is pretty grim, at least 25% of them will die within 5 years.  So, if we plant one million trees tomorrow at least 250,000 of them will die before maturity and those that do survive won’t make any inroads into current emissions for a long time to come. The ones that unfortunately die and decompose along the way will of course release their small amount of captured carbon back into the atmosphere by way of methane (CH₄), which is a much more powerful (80x in the first 20 years) greenhouse gas than CO₂. To keep the remaining 750,000 alive and thriving takes time and effort and forests need to be carefully managed with regular thinning of young trees to create space. All of this of course releases more carbon from the felled trees so the relationship between planting a million saplings and the amount of captured carbon in the smaller number of final trees that survive is far from linear or straightforward.

Secondly, there is the obvious question of which trees to plant and where. Original forests are a complex blend of different tree species whereas man-made ones tend to be made up of a single species of fast growing soft wood such as pine. In Scotland for example, 70% of the man-made forests are planted with a single species, Sitka spruce, which was imported from the pacific Northwest coast of the US in the 18^th century. Those of you who have been to the Highlands will recall these regimented, dark, and eerily silent mono-culture forests of non-native trees. They are silent because they do not support a fraction of the wildlife of the original mixed forests they replaced and being all the same species, they are highly susceptible to disease. They may absorb a lot of carbon eventually but at the expense of a lot of displaced wildlife along the way.

So, if planting huge acres of trees as a climate mitigant is not all it’s cracked up to be, why bother? Well, we know that the world has lost a third of its forest over the last 10,000 years and a big chunk of this was cut down in the last 200 years to build ships and power the industrial revolution. Globally we have lost an area equivalent to twice the size of the United States and reforesting this lost resource is undoubtedly a good thing and vital for preserving endangered wildlife.

However, it has to be done carefully with mixed native species not mono-cultures. On a positive note, the rate of global reforestation is increasing and European forest cover is now 42% of the total European land area. Between 1990 and 2015, this increased by an area roughly the size of Portugal.

Please be under no illusions – I think reforestation is a great thing. However, where I part company with the prevailing orthodoxy is when tree planting of either mono or mixed species is used by carbon emitters to ‘offset’ their carbon production so they can cheerfully carry on as normal . This brings us on to the third issue: the numbers don’t stack up. This section involves some basic maths but bear with me on this as it is crucial to my position. An acre of mature trees will capture at most 2.5 tons of CO₂ per annum depending on the type of tree planted but as we saw above, it will have taken over 20 years to get to that point. A long haul flight produces ¼ tons CO₂ per hour so a return flight from Dublin to San Francisco (20 hours) produces 5 tons of CO₂. This means you would need to plant 2 acres of trees tomorrow to offset the emissions of just a single return trip flight in 20 years’ time. UK aviation alone produces circa 40 million tons of CO₂ every year so following the same maths, you would need to plant 16 million acres of trees. England is roughly 32 million acres so you’d have to cover half the country in new trees to offset emissions from UK aviation and this wouldn’t act as a mitigant for 20 years. This is clearly not viable and if we extrapolate to the global scale, we are looking at an area five times the size of India to offset total global emissions from all sources.

So, if the answer to climate change isn’t planting trees, what is it? Other forms of nature based carbon capture such as seaweed or kelp farming seem to have more potential. Kelp grows much faster than trees and grows in the sea instead of the land, so advocates argue that capturing carbon in kelp is twenty times more efficient than a terrestrial forest and far less resource intensive to manage long term.  However, kelp farming is still very much in its infancy and – unlike trees – there are virtually no uses for the captured carbon which ensure it stays captured long term. IKEA don’t sell tables made from seaweed so the carbon will make its way back into the atmosphere eventually.  

Some scientists have gone much further and propose growing then sinking millions of tons of kelp in the deep ocean to keep the carbon buried forever but these type of radical interventions into the marine environment are completely unproven at scale and may have unexpected and disastrous collateral consequences. 

However much we may like trees and seaweed, it’s clear we can’t just blithely plant our way out of the climate crisis. Tree planting and its emerging marine cousin, kelp farming, are parts of a solution but only small parts at best. I remain a huge fan of reforesting schemes, but they need to be done in the right way (mixed native species) and for the right reasons (habitat preservation) and with complete transparency as to their limited effectiveness as a way of carbon capture. You can plant a lot of trees to support insects and birds but don’t kid yourself that it absorbs enough carbon to offset the emissions produced by your annual ski trip. Reducing carbon emissions and expanding renewable energy generation must remain the central planks of global policy and we need to avoid being distracted by seductively simple answers to complex questions.