One Tree, Two Trees, Three Trees…

How many trees are there? It’s OK to admit that you don’t know—until today, no one really did.

“Trees are among the most prominent and critical organisms on earth…yet you ask people to estimate, within an order of magnitude, how many there are, and they don’t know where to begin,” says Thomas Crowther, a postdoctoral fellow at the Yale School of Forestry & Environmental Studies and lead author of the newest, most accurate tree census ever undertaken, published today in Nature.

Crowther has a point. We know roughly how many people there are (7.2 billion and counting), how many whales there are (around 1.7 million), and how many cars there are (1.2 billion). Consulting companies expect job applicants to be able to accurately estimate how many piano tuners are in Chicago (290?). But until this study, our estimates on the tree population were ludicrously inaccurate.

According to the research, there are just over three trillion trees on earth. To reach this figure, Crowther’s team combined and extrapolated more than 400,000 measurements of tree density on every continent except Antarctica. (FYI: There are no trees in Antarctica.) Previous researchers had placed the global tree population at just 400 billion—actually, it turns out there are nearly 400 billion trees in just the Amazon basin alone.

This is great news, in some ways. We have far more trees than we thought. The research also turned up some very worrying trends, though. Three trillion is a very small number in historical context. When humans settled into cities around 12,000 years ago, there were more than 6.5 trillion trees—at least 1.3 million trees for every person on the planet. Since then, we have reduced the global tree count by 46 percent while massively expanding the human population, bringing the ratio to a pitiful 429 trees for each man, woman, and child. Each year, we remove 15 billion trees from the face of the earth without replacing them.

Crowther shows the extent to which humans have upset the natural relationship between trees and climate. In the absence of humans, trees grow more densely in wetter, warmer regions—the Amazon basin is a good example of this correlation. Globally, however, trees are not concentrated in these climates, because people also prefer warm, wet climates for agriculture. As a result, trees are now more common in dry regions—the opposite of their preferred habitat—such as large swaths of the American West. Human influence on tree populations is so pervasive, the study’s authors concluded, that development is the only factor that is consistently (and negatively) correlated to tree populations on all parts of the planet.

This steep decline over the course of the Anthropocene has put us into a bizarre position: Having killed off nearly half of the world’s trees, we are now actively considering populating the world with “artificial trees” —machines that will draw carbon out of the atmosphere and help save us from climate change.

I don’t believe his PhD was in climatology, but we should have listened to Dr. Seuss when he warned us about this.