杏吧原创

A massive expansion of the world鈥檚 forests by almost a third could be the most effective way for humanity to tackle climate change.

Jean-Francois Bastin at ETH Zurich says he and his team have produced the most realistic estimate yet of how many more trees the world can support 鈥� and the number is much bigger than previous suggestions, at 0.9 billion hectares of forest cover, roughly the size of the United States. The team estimates that would lock up around 205 gigatonnes of carbon dioxide, a huge amount given .

The scale of forest growth is broadly in line with what the UN climate science panel will be needed by 2050 to hold temperature rises to 1.5掳C.

鈥淚t鈥檚 showing that it鈥檚 achievable,鈥� says Bastin. 鈥淲e estimated that if you managed to restore all the forest we talked about it would buy us about 18 or 20 years [to meet climate targets]. It gives us time to adapt our way of living, to adapt our economy.鈥�

The team analysed 79,000 plots of forest across the world鈥檚 protected areas extracted from Google Earth. Combined with data on what soil and climates could support trees, they built a machine-learning algorithm to predict where trees could grow.

Once land with existing buildings and farmlands was stripped out, that left a potential 0.9 billion hectares of potential extra forest cover, in addition to the existing 2.8 billion hectares. Bastin says the analysis is more accurate than earlier efforts because of the resolution of today鈥檚 forest cover and only looking at places where trees could feasibly grow.

While the resulting map of potential new forests shows they could be planted all over the world, restoring forests in six countries would account for about half of the expansion. Those six 鈥� the United States, Canada, Russia, China, Australia and Brazil 鈥� are partly top simply because of their size and the amount of forest they have destroyed already. In some places, such as Madagascar, more could be done relative to the size of the country.

Bastin says that while new forests could take centuries to reach maturity, they would take up carbon dioxide much more quickly, with 80 per cent of the 205 gigatonnes locked up with 30 years, and the rest within another 30 years. The researchers鈥� model, however, found that future global warming would cut that potential storage, because new areas in the north becoming suitable for trees would be outweighed by losses of suitable land in the tropics.

Simon Lewis of the University of Leeds says the estimate of 0.9 billion hectares of forest cover may be a reasonable 鈥渂road brush stroke鈥� but points out land for planting trees is limited at local levels. 鈥淟andscapes need to fulfil many functions, not just carbon storage.鈥�

Lewis says that the potential carbon storage of the forests is 鈥渢oo high and not supported by either previous studies or climate models.鈥� Estimating how much carbon a new forest can lock up is difficult, as it depends on the species that are planted or regenerate naturally, how long they are protected from human disturbance into the future, and how climate change will affect them.

Data published this week also shows the world is still going in the wrong direction. Deforestation rates in Brazil鈥檚 part of the rainforest , to 920 square kilometres.