Planting trees alone will not stop global warming

A bidding war has developed over tree planting. Across the world, politicians are promising their constituents they will plant new forests. In Britain’s recent general election the parties promised billions of new trees: the leftwing Labour party promised 2 billion by 2040, while the Conservatives promised 30,000 a year. As part of its re-election bid, Canada’s liberal party promised 2bn new trees over the next 10 years. Ethiopia, earlier this year, claimed it managed 350m in one day.

It is not only governments that have got in on the act: EasyJet, a low-cost airline, wants to plant trees to operate net-zero carbon flights, while Royal Dutch Shell, the oil major, is planning three new forests. Charities, too, are pledging to reforest the earth, Elon Musk donated $1m to a YouTuber’s campaign to plant 19m trees.

Trees make effective environmental superheroes. They are attractive and carry positive associations. They provide a habitat for wildlife and can be sold as part of a wider agenda of conserving the natural world. Perhaps most importantly they do not require sacrifice: no need to raise fuel taxes, give up meat or shut down industrial and mining jobs. They seem to offer the promise of a solution to global warming without dramatic change in lifestyles or a steep reduction in consumption.

The science underpinning the idea is well understood: as trees grow they work as carbon sinks, absorbing the greenhouse gas from the atmosphere and storing it within their bodies. One study estimated that planting a trillion trees across the world would be enough to remove two-thirds of carbon emissions in the atmosphere thanks to human action. This could be done, the scientists argue, without infringing on the land used for agriculture and animal husbandry.

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Unfortunately, the reality is more complex. Like all models, the efficacy of forests as a carbon sink depends on the assumptions made: agricultural land stores carbon too, albeit less than woodland. All estimates come with uncertainty. Planting trees changes the colour of the earth’s surface. Darker objects absorb more heat and tree cover in the northern hemisphere — particularly on tundra — can actually warm the ground closest to the earth, offsetting the beneficial effects.

Some “volatile organic compounds” produced by trees as pesticides or to attract pollinators can be greenhouse gases themselves, others react with compounds in the atmosphere to produce even worse emissions. Interaction with car fumes can produce ozone, which, at ground level, keeps heat trapped on earth. These chemicals are a particular problem in North America: clouds of VOCs produced by the forests in the Great Smoky Mountains, in Tennessee, give the national park its name.

Other problems come from maintenance: roughly half the trees planted to line the route of Britain’s planned High Speed 2 rail route died during a hot summer when they were not watered. Nor do trees always command public support. In Ireland, where the government is relying on new forests to offset agriculture, locals have objected to the choice of tree: fast growing non-native Sitka spruce that, they say, creates ugly monocultures.

Trees undoubtedly play a role in fighting climate change, but they have to be deployed smartly. That means shifting the debate away from numbers and towards getting the policies right. In truth, the only possible way of stopping the planet warming is to burn fewer fossil fuels. If measures to offset emissions from airlines and oil companies encourage us to keep using their services then the surge of tree planting will have failed completely.

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