Tree-planting is hard, urgent work. The season is November to April because you’re supposed to plant when the saplings are dormant. The location tends to be high in the north because that’s where the land is. And there’s no shelter from the wind, because you haven’t planted the trees yet.
So Rob White and Scott McCaskill are dressed as if for the Arctic. If it were a shade warmer they’d have a team of 15 up here on Keltie Farm in Perthshire, but there are patches of snow and the ground is hard. They’re here in the freezing wind for a demonstration, to show what they can do.
Keltie Farm is being turned over to Scots pine trees and Sitka spruce after 300 years of grazing sheep. Its owner is getting old – he wanted to sell up – but there’s another reason these ancient hillsides have been ploughed up like corduroy to be turned from light green to dark; from open space to coniferous forest at 2,700 trees per hectare. There’s a big push on in Scotland to plant more trees to suck more carbon out of the air, because of climate change.
It’s not uncontroversial. This is a commercial project, for a pension fund. It’s the kind of thing that splits climate change people into those who want whatever works right now, and those who want nature to take the lead; between trees as part of a holistic approach to saving Planet Earth, and trees as geo-engineering.
The former has the advantage of promoting biodiversity along the way. The latter, arguably, has the advantage of speed.
I ask how many trees Rob could plant in a day in perfect conditions, and he says a couple of thousand; three at a push. After which “you do ache”, he says. “Your wrist gets it. Your back gets it.” We see a kestrel hovering for a mouse over a ravine not far away, and then the sun comes out. “Come on,” he says. “Let’s see if I can get into a rhythm.”
Rob walks to a nearby furrow and stands with one boot in the low part and one on a raised berm of peaty soil. He jabs his spade in and quickly jabs it in again, cutting two sides of a square hole. He levers the soil towards him a couple of inches to create what he calls a channel for the seedling in his other hand. In it goes, followed by 10 grammes of slow-release potassium-potash fertiliser from a belt pouch. He compacts the soil round the seedling with two well-aimed stamps, giving it a tug as he does.
Later he will explain the tug. The point is to make sure the seedling is vertical because otherwise it’ll grow up with “butt sweep” – a curved base that creates problems in the sawmill. But that’s later. At the moment he’s not talking. He’s moving down the furrow, planting his next tree and his next, reaching for the seedlings from an orange shoulder bag with his left hand as he levers the channel open with his right.
The seedlings are tiny; not much longer than a pipe-cleaner and much more fragile. The soil has to be compacted round each one at exactly the right height, where the root stops and the tree starts. “If you put it in too deep the bark will rot and the tree will die,” Rob says. “If it’s not right you’d have to come back and replant it, but that doesn’t happen on my sites.”
He takes two steps between each seedling. They add up to 1.9 metres, give or take a centimetre or two. It’s a distance he’s been eyeballing for 26 years straight.
The sun has softened the soil just enough. The rhythm seems effortless. He plants 11 trees in a minute and 11 seconds, all in one neat row. Each hectare of these trees will sequester about 100 tonnes of CO2 from the air over the next 35 years. So, long term, we’re looking at the groundwork for nearly half a tonne of carbon sequestration in 71 seconds.
“I’ve done my duties to the carbon footprint over the years,” Rob says. “I wouldn’t put a number on it, but I’ve planted millions.”
Trees are the lungs of the earth. They absorb carbon dioxide and release oxygen through photosynthesis. Everyone with a GCSE in general science knows this, but no one knew with any confidence until the age of satellite photography how many trees there are, or were, or could be.
Then, five years ago, a paper in the journal Nature by Thomas Crowther, a young British ecologist then at Yale, caused a stir well beyond specialist tree circles. He used a combination of ground-level counting and space-based extrapolation to estimate the world’s total tree population at about three trillion. That was half as many as before humans started to cut them down, he said, but more than most tree experts had thought.
Many more. One estimate widely cited until Crowther came along held that there were fewer than half a trillion trees left on the planet. That was of a piece with the narrative of breakneck destruction of the Amazon rainforest, which appears to be accelerating under Brazil’s President Jair Bolsonaro. It was less easy to reconcile with a newer narrative of natural reforestation in much of the northern hemisphere, from France and Canada to Kazakhstan.
This was the context for another paper with the Crowther name attached, this time in Science, that sent much bigger ripples of excitement through the febrile world of climate change journalism. It was titled “The global tree restoration potential”. It made the front page of The Guardian, which reprinted the story on the cover of its weekly edition under a picture of a tree and the headline, in shocking pink: ‘This Machine Kills CO2’.
The paper’s lead author was Jean-Francois Bastin, a Belgian expert on forests and climate change, but it was Crowther who became seriously well known as a result of it. By then he was 33 and had secured €17 million in Dutch grants to set up a tree lab in his own name at the Swiss Federal Institute of Technology in Zurich.
By the lab’s estimates the Guardian story was followed up in 700 other newspapers. The study on which it was based used the same remote sensing of the world’s surface as Crowther’s earlier one – it was essentially a forensic analysis of nearly 80,000 images made available by Google. But it went further. Its central contention was that tree-planting is the single most promising mechanism available for climate change mitigation because:
- there are nearly a billion hectares available for forest restoration excluding existing “agricultural and urban areas”;
- that land should be able to sustain a trillion trees, absorbing over their lifetimes about 205 gigatonnes of carbon; and
- that in turn is equivalent to a third of the carbon dioxide added by humans to the atmosphere (as opposed to the oceans or soil) since the dawn of the industrial age.
To some it felt like a eureka moment. A fresh pair of eyes had seen a solution to climate armageddon right in front of our noses – or at least out there in wide open spaces that would support trees if left alone or visited by planters.
Christiana Figueres, architect of the 2015 Paris climate agreement, called the paper “a hugely important blueprint for governments and the private sector”. Joseph Poore, a remote sensing specialist at Oxford, called it “excellent research”. Crowther himself called it “mind-blowing”.
What blew his mind was a sense of possibility. Most climate scientists agree we will have to find ways to take carbon out of the atmosphere even after cutting emissions to net zero. If not, excess CO2 left in the air will go on warming it for about 100 years.
This is why energy and climate people talk about the need for “negative emissions” – usually a shorthand for prodigiously expensive technologies for taking CO2 from industrial waste streams and pumping it underground, or removing it from ambient air with machines that look like jet engines. The former is called CCS, for carbon capture and storage; the latter DAC, for direct air capture. Both will do the job for upwards of $600 per tonne.
Sitka spruce seedlings go for about 27p, wholesale. So what about trees as a negative emissions strategy? Their role as a natural carbon sink has long been understood, but the sheer scale of the numbers in the Crowther Lab paper suggested a new role for trees as the spear tip in an existential fight. Go all-in on reforestation and maybe we could actually beat global warming.
The land area identified as suitable for extra trees in the Science article was equivalent to that of China. Half of it is in six countries – the US, Russia, China, Canada, Australia and Brazil – but the rest is spread across many more.
Crowther calls this land “degraded forest”, although in parts of France, Russia, China and western Canada it rubs up against forest cover that has been naturally expanding for several decades. Elsewhere the damage is the story, but also the opportunity. A separate 2019 study identified Brazil, Indonesia, India, Madagascar and Colombia as the countries with the largest areas of cost-effective forest restoration potential.
Across most of these territories climate-inspired tree planting is still theoretical. In Scotland it is real and accelerating. In 2018 the devolved government set a target of planting a record 10,000 extra hectares of forest, and met it easily. For 2019 the target was 15,000. Tax-funded grants cover the cost of planting and fencing (to keep deer away) as long as the new trees aren’t simply replacing old ones felled for timber. The key, says Ian Robinson of Scottish Woodlands, is “additionality”.
The 400-hectare Keltie project will produce one of Scotland’s biggest new patches of forest of the 2019-20 planting season. It all meets UK forestry standards, which means among other things that open spaces will be left for kestrels and their prey. You might have thought, then, that Rob White and Scott McCaskill could relax at the end of a hard day’s planting as uncontested climate mitigation heroes.
If only it were that simple.
As soon as the Crowther lab paper was published, experts from other institutions set about taking it apart.
In the 17 October issue of Science, in something close to a coordinated academic mugging, six groups levelled three main criticisms. They said Bastin et al had over-estimated the carbon sequestration potential of the forest types in their study by a factor of five or more. They said it assumed large areas of grassland would revert to forest, or support it, when in fact savannah was its natural state. And they accused the authors of forgetting about the albedo effect (which increases the amount of heat that land reflects back into space when covered in snow, and increases the amount it absorbs when covered in dark vegetation such as trees). This means mass tree-planting in the far north could end up warming the planet rather than cooling it.
“There is merit in restoring forest to remove carbon from the atmosphere. No one is doubting that,” says Simon Lewis, professor of global change science at University College, London. “The question is what kind of scale can that be done at, and Bastin shows the absolute maximum biophysical reality” in the absence of social, political and economic considerations.
That’s unrealistic, he adds, and the science behind the figure of 205 gigatonnes for global carbon storage potential is “shockingly bad”.
The Crowther team also got it in the neck from people falsely accusing it of presenting tree-planting as an alternative to cutting carbon emissions. Even Oxford’s Joseph Poore, who had been so enthusiastic about the original paper, said when I reached him a few months later that, on reflection, tree-planting had been “way overcooked as a climate change solution”.
Bastin and Crowther replied defiantly to their critics, but seemed bruised. Bastin moved away from Zurich to the University of Ghent and Crowther stopped talking to reporters for three months. During this period, another scientist at the lab spoke up for him.
Felix Finkbeiner was 10 when he first addressed the European Parliament on the subject of trees, and 13 when he first spoke to the UN General Assembly. At age nine he founded a non-profit called Plant for the Planet that encourages and records tree-planting efforts around the world. In 12 years it has counted 13.6 billion new trees. In September it launched an app that lets you know when you’re near a climate-mitigating tree planting operation, whether it’s real or bogus, how to donate if it’s real, what species you’ll be supporting and how much carbon your donation will sequester. In its first two months the app raised funds for nearly a million trees.
Finkbeiner is now one of Crowther’s PhD students, examining the difference between carbon sequestration rates in tropical and temperate forests. He’s 22.
“What happened is what needed to happen,” he says of the Global Tree Restoration paper and the reaction it provoked. “The greatest contribution of Jean-Francois’ paper is to kick off the debate.”
But what about the three-pronged attack on its most basic findings?
“The scientific consensus is very clear that restoring forest has a very significant climate impact,” Finkbeiner says. He suggests the critics are forgetting that the paper was basically an exercise in mapping – in showing where trees currently grow and where they could grow without encroaching on human habitation or agriculture.
We’re at the nub of it. The “where” question. And Finkbeiner is glossing over a big grey area where the Crowther Lab may have done as much to confuse things as to clarify them. The Bastin paper emphasised carbon sequestration potential in six big countries, five of which are mainly in temperate zones far from the Equator.
This was a big part of its appeal as a story. The implication was that the rich north might actually be able to clear up its own mess by planting trees in its own back yard. Another plea to save tropical rainforests from loggers and ranchers would have been over-familiar. Focusing instead on reforesting the northern hemisphere tugged at an intriguing thread from recent history: an amazing effect of the closure of collective farms in the former Soviet Union since the 1990s has been the return of forest that has locked up carbon equivalent to the annual emissions of Belgium and the Netherlands combined.
But Finkbeiner prefers to talk about the potential of reforestation in Africa and Latin America. There’s no getting round it, he has found: tropical broadleaf trees grow bigger and fix carbon far faster than anything in temperate zones. His own research suggests the sequestration potential per unit of tree growth in the tropics is four to five times that of temperate forests. He also acknowledges the force of the albedo argument, which means that even if mass tree-planting does save the planet “it’s not going to be [in] Siberia”.
This makes me sad. I admit, I had visions of huge new forests of pine growing quietly from the Urals to Vladivostok, scrubbing the air, purging the poisonous legacy of northern hemisphere industrialisation, atoning for us all. Something tells me Finkbeiner did once too. He grew up in the mountains south of Munich, which wear a heavy year-round cloak of spruce.
The detail is difficult, so we take a step back. “I’ve been talking about trees for 12 or 13 years now,” Finkbeiner says. “Initially tree-planting was seen as kind of cute, but there was no real research into its global potential – until now. There has been a huge, significant shift in the narrative, and there’s a clear consensus tree-planting has to be part of the solution.”
There is, it has to be said, a rival consensus that tree-planting often turns out to be bogus. Particularly when sold as a carbon offset to individuals and companies more worried about perception than reality, it is vulnerable to scammers.
Four years ago a charity set up by the country singer and tree-lover John Denver published a 13-page pamphlet warning would-be donors and investors to beware tree-planting schemes that take your money but fail to nurture or even plant trees in the first place.
Even good schemes are often funded with flashy investments by airlines and oil firms with little intention of shrinking their own carbon footprints. And nothing seems able to stop the destruction of old-growth forests for fast-growing monocultures of eucalyptus (especially in Brazil and China) or palm oil trees (across south-east Asia).
This sort of tree planting destroys biodiversity and is at best a short-term store of carbon. The idea of tree-planting as a climate change solution has caught on even so.
Google “Mr Beast Team Trees”. Mr Beast, real name Jimmy Donaldson, is a 21 year-old vlogger who three months ago set about raising $20 million to plant 20 million trees to celebrate gaining 20 million followers. He’s since hit both targets and pledged to make it an annual challenge. The Arbor Day Foundation is his partner, finding suitable planting sites across North America. Elon Musk of Tesla and Susan Wojcicki of YouTube are among his donors.
Tree-planting is at a confluence of zeitgeist and geopolitics. Last July Ethiopia claimed a world record for most trees planted in 12 hours (353 million). In November, Turkey took most-trees-planted-in-an-hour bragging rights from Indonesia in mass planting of 303,150 saplings. The Bonn Challenge aims to restore 350 million hectares of degraded forest around the world by 2030, including 100 million in Africa. And of course tree-planting is a British political egg and spoon race. The Tories, Liberal Democrats and Labour all plucked multi-million sapling planting targets out of the political ether for last month’s election.
Election promises aren’t worth much, but still. As Ian Robinson of Scottish Woodlands told me: “The snowball’s starting to roll and for once the politicians seem to agree on something.” One way or another there is every chance of a lot more Keltie-style tree-planting in Britain over the next five years.
The prospect makes a certain sort of purist wince. These are the wilders. They cherish biodiversity above all, as an end in itself but also as a solution to climate change. Their view is that nature left to itself will eventually fix more carbon than nature rushed along by humans who treat trees as a crop. Their holy of holies, in the UK at least, is a former farm on the Knepp Estate in Sussex. It’s the subject of a bestselling book, Wilding, by Isabella Tree (her real name; she lives at Knepp with her husband, Charlie Burrell, its ancestral owner).
I went there on a bright December afternoon and glimpsed the past and a wished-for future rolled into one. Ancient longhorn cattle and semi-feral pigs had roughed up the fields. Birds had planted acorns which, fertilised by dung, had grown into sturdy young oaks protected from fallow deer by tailor-made bramble thickets. Fleabane stood knee-high everywhere, waiting for spring and holding up spiders’ webs like zip-wires in the meantime. There were even nesting storks.
A long-term goal of projects like these – another is at Glenfeshie in the Cairngorms – is to re-wild the soil. Tree writes in her book that organic matter in the world’s soils accounts for 82 per cent of all the carbon in the biosphere. Increase soil’s worldwide CO2 uptake by just 0.4 per cent, she continues, and you could halt climate change in its tracks.
I put this to Felix Finkbeiner. He concedes at once that “natural regeneration can be a lot more effective and cost-effective” than the artificial variety. “It’s certainly true that there is far more carbon below ground than in vegetation above ground. But what’s important to note is it’s the vegetation that brings it down there.” You’ve got to grow something, in other words, and it might as well be trees.
It’s also important to note that we’re in a race against time. The UN’s Intergovernmental Panel on Climate Change gives us about a decade to take decisive action, without which the current carbon dioxide build-up could be irreversible and feedback loops could kick-in (as a result of melting permafrost and Arctic sea ice) that make it uncontrollable as well.
Do we have the luxury of waiting for re-wilding? People have been making educated guesses for a while. A briefing document prepared by the Woodland Trust for the UK government 20 years ago stated confidently that “if the aim is to sequester carbon rapidly then it is best to grow fast-growing species”. The good news was that Sitka spruce could fix 3.6 tonnes of CO2 equivalent per hectare per year, and poplar twice as much. The bad news: at that rate a hectare of spruce would barely absorb the carbon footprint of a single British adult.
Fast-forward to the present and there is little doubt that natural forest regeneration is the single cheapest, simplest, most effective carbon sequestration method we have. That means leaving nature to choose its own species and tree densities. But given that we’ve left it all so late there’s the short-term to consider as well.
I think back to Rob White and Scott McCaskill, transforming Keltie Farm into a different sort of forest – thick, fast-growing, predictable and measurable. It’s not hard to make the case for their 2,000 new trees each, each day. Not in these desperate circumstances. Not after three centuries of stunted grass and farting sheep.
All photographs Getty Images, Alamy Images, Giles Whittell
Tomorrow: an economist’s take on how to turn dreams of large-scale reforestation into reality
Join us in Birmingham for a ThinkIn on Wednesday 5 February at 6.30pm when we’ll be asking: Can trees save the world?