There is a fairly simple, inexpensive and guaranteed way to effectively counter the effects of climate change. What is it? It is certainly no secret: planting trees and restoring destroyed ecosystems. Forests are capable of absorbing large quantities of CO2, and restoring natural habitats would allow us to achieve one third of the greenhouse gas reduction needed to keep global warming below 1.5 degrees. Reducing current emissions and transitioning to clean energy, while urgent and necessary, are not enough on their own to meet the climate targets set at Cop 21 in Paris.

A massive reforestation effort is needed; the areas with the greatest potential in this regard are tropical rainforests, stripped bare by intensive oil palm plantations and timber production. But how many trees should we plant? That has been determined by Thomas Crowther, an ecologist at the Swiss Federal Institute of Technology in Zurich, and his team of researchers.

How many trees we need to plant

According to the British scientist, there is enough room on the planet for another 1.2 trillion trees. "The amount of carbon we could store if we planted 1.2 trillion trees would be far higher than that guaranteed by any other solution we might adopt to stop climate change," Crowther said. To arrive at this figure, researchers at the Swiss university calculated the global density of trees, identifying areas that could potentially be reforested.

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How much CO2 they could absorb

The researchers also calculated the amount of CO2 that could be stored by the new forests; however, this figure has not yet been released, as the study, which will be published in the journal Science, is currently under peer review. That amount is nonetheless estimated to exceed 90 billion tonnes.

Where to reforest

The locations identified by Crowther's team are found mainly on degraded and abandoned land, rather than on agricultural or urban areas. The choice of areas to restore must be carefully evaluated, as every ecosystem has its own particular characteristics. Carrying out intensive reforestation in northern Europe, for example, could prove counterproductive, since more trees could reduce the ability of snow-covered ground to reflect light and heat and, paradoxically, increase global warming.

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A long-term solution

Reforestation will not, however, deliver immediate results, given that many tree species need 30 to 40 years of growth before carbon storage reaches its full potential. It is therefore essential to halt deforestation immediately; according to Crowther, it still costs our planet around 15 billion trees every year.

The return of the forests

Reforestation projects have already been launched in various parts of the world. Last year China launched a massive reforestation plan aimed at increasing tree cover by 23 per cent by 2020; the British government announced the planting of 50 million trees over the next 25 years; Iceland launched an initiative to increase its forest cover from 2 to 12 per cent by 2100; while in Brazil the planting of 73 million trees is planned within the next 6 years (Bolsonaro permitting). The extraordinary power of forests to cool the planet's climate was recently demonstrated by a remarkable study, according to which the violent European colonisation of the Americas would have encouraged forest expansion and caused a drop in temperatures known as the "Little Ice Age".