In a banner announcement for shade-lovers everywhere, a new study out of Yale University finds that there are more trees in the world than previously estimated — way, way more.
The research, published this week in Nature, drew on global satellite imagery and more than 400,000 sample counts from forests around the world in order to estimate that there are currently 3.04 trillion trees on earth. This is 750 percent more than the previous best estimate, which was 400 billion.
“When I started getting my results I was like, what the. . . this is an order of magnitude difference,” says Thomas Crowther, lead author of the study and a postdoctoral fellow at Yale University during the time he conducted this research.
Two years ago Crowther, who is now at the Netherlands Institute of Ecology, was contacted by representatives of Plant for the Planet, an organization involved with a United Nations-led effort to plant 1 billion new trees each year. The trees are meant to capture carbon from the atmosphere and reduce the effects of climate change. Plant for the Planet wanted to know how much of a difference 1 billion trees would really make and asked Crowther to provide context by updating the existing global tree count.
That number was established in 2005 by Nalini Nadkarni, an ecologist at the University of Utah, based on satellite images and calculations about the density of trees in different kinds of forests. In an e-mail, Nadkarni explained that at the time she made her estimate, the data she used “appeared to be the best and most quantitative available.” Crowther improved on Nadkarni’s work by combining it with hundreds of thousands of actual tree counts that have been carried out by governments around the world for the purpose of managing their own forests. (In these counts, trees were usually defined as having a trunk diameter of 10 centimeters at breast height. Given the way forests thin as trees grow, the total number of trees would fall by far more than half if the cut-off were, say, 20 centimeters.)
Crowther explains that his research will help scientists and conservationists recalibrate their statistical models to better reflect the number of trees actually living are on earth. Other experts in the field, however, think that an accurate estimate of the global tree population is less important than it might seem.
“When I think about forests and what they have to offer, the number of trees doesn’t really matter,” says Aaron Weiskittel of the School of Forest Resources at the University of Maine. A more useful statistic, he argues, is the amount of biomass in a forest, which includes trunks, branches, and leaves. Biomass is a better indicator of a forest’s carbon-capturing capacity and it’s not all that closely tied to the total number of trees. In Maine, for instance, the number of trees per acre of forest varies from 2,000 to 3,000, but the amount of biomass is fairly constant.
Still, the total tree population is a statistic that makes people pay attention, and it would seem to be only a good thing that we have trillions more trees than we thought. That is, of course, unless you had been hoping to really make a big impact by planting 1 billion new trees. That effort now seems scant, but Crowther explains that Plant for the Planet was far from discouraged when he informed them that 1 billion trees might not go as far as they’d hoped.
“Now based on these results they’ve decided to upscale it to 1 trillion trees,” he says.