US-sized forest could be added to our planet

Trees are good for everything: they provide shade for picnics and habitat for animals. But they also help fight global warming by absorbing carbon dioxide from the air.
A new article published in Science on Thursday shows how important they are in mitigating climate change. Right now there are about 17 million square miles of forest cover on Earth, and there is enough space to add another 3.5 million square miles of trees — a US-sized chunk of land, according to the new study.
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Although trees are almost everywhere, figuring out how many trees there are on the planet is quite a challenge. The Food and Agriculture Organization of the United Nations defines forests as any area with more than 10 percent tree cover. And the best way to really see how many trees there are is to use satellite data. And the study is based exactly on that.
Using software the researchers collected satellite images and studied 78,774 photos of forested area. They looked specifically at protected areas and places with limited human activity to avoid including urban parks, farms and other land uses that might look like forests but are not actually forests. They provided all that data as well as 10 other variables describing the climate and soil using a model to estimate current tree cover as well as areas where the tree cover could be created. The results show an area roughly equivalent to Russia, Canada, the United States, and Australia — or nearly a third of the world’s land area — is covered in forest.
According to the study, more than 12 million square miles of land could contain more forests, but given that we need that land for crops and places to live, only 3.5 million square miles of that land is actually suitable for forest cover.
The first four places suitable for reforestation are Russia, the US, Canada, and Australia, all developed countries, and in the case of the first three, they are all home to a piece of the vast forest that surrounds the North Pole. Brazil and China are also on the list, and together those six countries contain 50 percent of the area where forests could grow again.
Based on what we know about forests, another 205 gigatons of carbon would be absorbed as those trees grew to maturity. For comparison, last year, the world emitted 37.1 gigatons of carbon dioxide. The newly filled out forests would help to cope with new emissions, which could be part of the fight against climate change.
But here’s the thing — the scientists modeled two climate scenarios: one where emissions rise rapidly and the other where they peak by mid-century and begin to decline — to see how habitable those areas would really be for trees. It turns out that while the new forest is likely to grow in the north, tree cover is likely to decrease in the tropics. The Amazon is particularly at risk.

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