More than three trillion trees now grow on Earth, seven times more than scientists previously thought. But it’s also trillions fewer than there used to be.

A United Nations-affiliated youth group had a goal of planting one billion trees. Yale forestry researcher Thomas Crowther, was asked if planting that many trees would do anything to help combat human-made climate change. Trees capture and store heat-trapping carbon dioxide.

Crowther said first he had to figure out how many trees are on Earth. He did, published in the journal Nature, and that number was far more than anyone expected: 3.04 trillion trees. The previous estimate had been 400 billion trees based on satellite images. Crowther and colleagues used 429,775 ground-based measurements along with satellite measurements and computer models to get a more accurate figure.

“These things really dominate our planet,” Crowther said. “They are the most prominent organisms on our planet and there are 3 trillion of them.”

But Earth used to be covered with far more trees. Using computer models, Crowther and colleagues estimated that before human civilization Earth had about 5.6 trillion trees. So the number of trees on Earth has been chopped nearly in half. Crowther mostly blames people. He found that 15 billion trees are cut down each year by people, with another 5 billion trees replanted. At that rate, all of Earth’s trees will be gone in about 300 years. And he found that nearly 1.4 trillion of Earth’s trees are in tropical and subtropical forests, precisely where the rate of forest loss is the highest.

The upshot: The professor did conclude that given the number of trees on Earth (3 trillion), that a group dedicated to planting a billion trees won’t do much to fight climate change. But he said that didn’t stop the tree planters group; reportedly they just upped their goal. Plant for the Planet says its objective now is to plant 18 billion trees.

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