Planting trees: a fix for climate change?
In a world looking for solutions to global warming, tree-planting is increasingly taking centre stage
Why is tree-planting so popular?
Trees are the most effective carbon capture machines on the planet. As they grow, they suck earth-warming carbon dioxide out of the air and release oxygen. They lock carbon up in their leaves, branches, trunks and roots: about half of the dry weight of a tree is carbon. Trees also lower air temperatures, increase rainfall, filter out pollutants and dust, and create habitats for wildlife. Ever since it was first established that global warming was a serious problem, tree-planting has been recognised as a simple, cheap and safe tool for mitigating its effects, and there are schemes dating back decades. But over the past year in particular, the idea has caught the attention of a world starved of hopeful climate news.
What sort of projects are currently under way?
The biggest is the Bonn Challenge, a global effort to restore 350 million hectares of the world’s degraded and deforested lands – an area bigger than India – led by the German government and the International Union for Conservation of Nature. Under this and linked schemes, 54 mainly tropical and subtropical nations have committed 172 million hectares so far: Vietnam alone is allowing 15 million hectares to return to natural forest. By 2017, Pakistan had already hit its target of planting a billion trees in Khyber province. China says it has planted 66 billion trees since 1978 (partly to arrest desertification). There is a 5,000-mile wall of trees being built across Africa from Senegal to Djibouti – the Great Green Wall – which aims to be the largest living structure on the planet; it is now 15% complete, with 11.4 million trees planted in Senegal alone. Private and public projects have recently sprung up across the West: Boris Johnson pledged £640m prior to last year’s election to fund the planting of 30 million trees a year.
Why has there been a recent surge in interest?
One powerful influence was the study published in the journal Science last year, by researchers from ETH Zürich university, that made headlines around the world. The researchers found that some 0.9 billion hectares of land, an area almost the size of the US, was suitable and, in theory, available for planting about a trillion new trees (i.e. it wasn’t farmed or built on) – mostly in Russia, the US, Canada, Australia, Brazil and China. When mature, in 50 to 100 years, these trees could theoretically store up to 205 gigatonnes of carbon – about two-thirds of the carbon released by human activities since the Industrial Revolution. “What blows my mind is the scale,” said Prof Tom Crowther, who led the research. “I thought restoration would be in the top ten, but it is overwhelmingly more powerful than all of the other climate change solutions proposed.”
Did everyone agree with them?
No: the report has since been heavily criticised by other scientists. Crowther and his colleagues were accused of heavily overestimating the carbon storage potential of new forests; of ignoring the carbon already stored in, for instance, grasslands; and of forgetting about the albedo effect (where snow reflects solar heat from the earth; in planting trees in northern latitudes we could actually reduce this effect, thus warming the planet rather than cooling it). Prof Simon Lewis, a climate change expert at UCL, described the 205 gigatonnes figure as “shockingly bad”. The study was accused of using “overly hopeful” figures that might “misguide the development of climate policy”.
How could policy be “misguided”?
Climate scientists are concerned that treeplanting could be promoted as a silver bullet: an alternative to cutting emissions. As such, it would be totally inadequate: even 205 gigatonnes only equates to less than ten years of global anthropogenic CO2 emissions at current rates. And in reality, newly planted trees need to grow for many decades to come close to their carbon storage potential, and there’s no guarantee that will happen. Treeplanting can be useless: it was reported two weeks ago that 90% of the 11 million saplings planted last year by Turkey have in fact died. Almost half of the areas pledged under the Bonn Challenge are actually plantations, of non-native commercial species such as eucalyptus for paper or rubber trees, rather than natural forests. These are much poorer at storing carbon, and are harvested at regular intervals, releasing much of their CO2 back into the atmosphere. Some are even damaging.
How can tree-planting be damaging?
Plantations can destroy natural habitats rather than enhancing them. Commercial forestry often involves using weedkillers and carbon-intensive fertilisers, and planting uniform monocultures – “pines in lines” – using plastic tree guards. In the race to grow trees, invasive species and diseases can be introduced via transplanted saplings. Environmentalists are concerned that some precious habitats – grassland, for instance – are being turned over to woodland. Many think that tree-planting is a bad idea, and that it’s better to rewild: to let nature take its course, producing richer habitats. “Tree-planting is not synonymous with conservation; it is an admission that conservation has failed,” said the late woodland ecologist Oliver Rackham.
So what should we do?
According to the Intergovernmental Panel on Climate Change, some 730 gigatonnes of CO2 will have to be taken out of the atmosphere by the end of the century if we are to limit global warming to 1.5˚C above preindustrial levels. Emissions will have to be cut drastically, and vast amounts of carbon will have to be captured too; forests will play a vital part. Further deforestation will have to be prevented (see box), along with large-scale reforestation. Sensitivelymanaged reforestation projects which also provide local employment, from new native broad-leaved forests in England to mangrove-planting in Madagascar, have been shown to be greatly beneficial. Planting trees is a necessary step, but it’s no panacea.