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WORLDWATCH CARBON Planting the Seeds The lead author of a scientific study, which claimed that planting trees has mind-blowing potential to tackle the climate crisis, reflects on the mixed response to the report Not every scientific study gets the attention it deserves, but every now and then one captures the public imagination. That’s what happened when a group of scientists demonstrated that the Earth is home to 1.7bn hectares of treeless land (around the size of the US), which they say could support an additional 500 billion native trees without affecting cities or agriculture. If planted, the researchers, led by Dr Thomas Crowther of ETH Zurich, claim that once mature, these trees could remove around two-thirds of all the carbon emissions produced by humans in the industrial era. The estimates were produced by combining satellite images of Earth’s surface with environmental data about where trees can grow and how much carbon they can store. While the frenzy that accompanied the release of the report was largely positive (Crowther says that donations to restoration projects spiked in its wake), 8 • Geographical there were criticisms. Geographical caught up with Crowther to discuss the response the report generated... Some commenters to the report suggested that planting 500 billion trees isn’t economically viable. ‘Well, the study was never intended to include an economic analysis but that’s certainly the next step. I’m optimistic that it can be done. We really want to start looking into the economics of global-scale restoration. I hope this will inspire more government support, but I don’t think that’s what’s going to make the difference. I really think its going to be industry and the general public donating.’ Even if that many trees were planted, it’s likely to be a long time before the amount of carbon predicted could realistically be sequestered. ‘Certainly. As with all climate change solutions we have at the moment, it’s a very long-term solution. It’s only by the end of the century if not into the next century that we’d start to see that amount of carbon sequestered. The critical thing is that we need to get going yesterday.’

CLIMATEWATCH WORLDWATCH Boreal forests, such as this one in Finland, capture less carbon than tropical forests Some observers had doubts about the figures, pointing in particular to the boreal forests of the Northern hemisphere and their reduced potential to absorb carbon. ‘True, but this doesn’t affect the overall figures. Boreal forests grow much more slowly than tropical forests, we’re well aware of that. On top of that, forests up there could actually warm the planet because the surface created by boreal forest is much darker than the snow that would otherwise be there. A lot more of the sun’s energy gets absorbed. But simply in terms of carbon capture, none of that changes any of our numbers. It just highlights the need to really understand the ecology before you do any restoration.’ What would you say is the real message people should take from your report? ‘That this is only the start. I would reiterate that all we’ve done is identify this as an incredibly powerful carbon draw-down solution, but it only benefits local biodiversity and sustainability when it’s done ecologically right.’ l The Trilemma Trees have many uses when it comes to better planetary health, but Marco Magrini asks if planting saplings is the only way we can best utilise our available land Everyone knows that, to help mitigate the ongoing climate crisis, we need to plant new trees. A paper recently published in Science estimates that the Earth could support an additional 9,000,000 sq km of forest, potentially hosting 500 billion trees capable of capturing more than 200 billion tonnes of carbon dioxide at maturity (see left). It would be a serious help. The Earth’s landmass is 149,000,000 sq km. Take out glaciers (15 million) and deserts (28 million), and we are left with 104,000,000 sq km. Subtract cities (1.5 million), freshwater (1.5 million), forests (39 million) and shrubs (1 million) and we finally get 51,000,000 sq km of arable lands, badly needed to feed 7.5 billion human beings. Very few people are aware of the immense complexity of the whole system. ‘Widespread use at the scale of several millions of square kilometres globally of tree-planting and bioenergy crops – reads a leaked IPCC draft report – could have potentially irreversible consequences for food security and land degradation’. In other words, more extensive monocultures and more bioenergy crops, fuelled by more fertilisers, could erode soil and its capacity to soak up carbon. Bioenergy has now a 50 per cent share in the world’s renewables consumption, said Fatih Birol, the IEA’s Executive Director, ‘as much as hydro, wind, solar and all others combined’. It’s good news, but not entirely. If we add that an increase in desertification and in ocean levels (both propped up by greenhouse gases in the atmosphere) will take away more arable land, we arrive at a crucial ‘trilemma’. Should we use our spare soil for agriculture, reforestation or bioenergy? Such a question would make sense in a multilateral, concordant world; not on a planet where the president of the richest country scraps an environment-saving treaty, thus encouraging the president of the most tropically-forested nation to unleash a tree-cutting spree. Last year, 36,000 sq km of forest was felled. Wouldn’t it be better to start by stopping deforestation altogether? Animal farming takes 77 per cent of the world’s arable land and provides us with 18 per cent of the calories. Shouldn’t we globally cut back on meat consumption? Modern bioenergy (such as liquid biofuel from bagasse, or biogas from residues) is already available. Shouldn’t we banish first-generation biofuels, which are distilled from food crops? ● September 2019 • 9

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WORLDWATCH CARBON

Planting the Seeds

The lead author of a scientific study, which claimed that planting trees has mind-blowing potential to tackle the climate crisis, reflects on the mixed response to the report

Not every scientific study gets the attention it deserves, but every now and then one captures the public imagination. That’s what happened when a group of scientists demonstrated that the Earth is home to 1.7bn hectares of treeless land (around the size of the US), which they say could support an additional 500 billion native trees without affecting cities or agriculture. If planted, the researchers, led by Dr Thomas Crowther of ETH Zurich, claim that once mature, these trees could remove around two-thirds of all the carbon emissions produced by humans in the industrial era. The estimates were produced by combining satellite images of Earth’s surface with environmental data about where trees can grow and how much carbon they can store. While the frenzy that accompanied the release of the report was largely positive (Crowther says that donations to restoration projects spiked in its wake),

8 • Geographical there were criticisms. Geographical caught up with Crowther to discuss the response the report generated... Some commenters to the report suggested that planting 500 billion trees isn’t economically viable. ‘Well, the study was never intended to include an economic analysis but that’s certainly the next step. I’m optimistic that it can be done. We really want to start looking into the economics of global-scale restoration. I hope this will inspire more government support, but I don’t think that’s what’s going to make the difference. I really think its going to be industry and the general public donating.’ Even if that many trees were planted, it’s likely to be a long time before the amount of carbon predicted could realistically be sequestered. ‘Certainly. As with all climate change solutions we have at the moment, it’s a very long-term solution. It’s only by the end of the century if not into the next century that we’d start to see that amount of carbon sequestered. The critical thing is that we need to get going yesterday.’

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