Tree Counter Is Astonished By How Many Trees There Are

LINDA WERTHEIMER, HOST:

Aside from your rapidly desiccating Christmas tree, how many trees do you estimate there are on the planet? Most people have no idea. NPR's Nell Greenfieldboyce reports that researchers have completed a rigorous census of all the trees on Earth. And they were astonished by the results.

NELL GREENFIELDBOYCE, BYLINE: A couple of years ago, Thomas Crowther was working at the Yale School of Forestry and Environmental Sciences. He had a friend who was involved with a group that had an ambitious goal - trying to fight global warming by planting a billion trees. A billion trees sounded like a lot, but was it really?

THOMAS CROWTHER: They didn't know if planting a billion trees was going to add 1 percent of the world's trees, add 50 percent of the world's trees. They didn't even know if it was even possible to fit a billion trees on Earth.

GREENFIELDBOYCE: His pal asked him a simple question. How many trees are on our planet?

CROWTHER: I assumed that this was somewhere out there. It's information that someone will know.

GREENFIELDBOYCE: Well, he was wrong.

CROWTHER: Having spoken to a lot of forestry experts, it doesn't seem like anyone had any idea.

GREENFIELDBOYCE: There was one estimate based on satellite images. It said there were about 400 billion trees or 61 trees for every person. Crowther and his colleagues did some back-of-the-envelope calculations and realized this estimate had to be way off. The problem was its assumptions about how many trees would be in areas of forest. Crowther knew there was a way to get much better numbers.

CROWTHER: There's a lot of national forest inventories. Like, the one in the U.S. is a fantastic resource where people have actually gone out and studied their forests within those countries. And that information's all publicly available. And so we were able to access all that information and start to generate our models.

GREENFIELDBOYCE: They took on-the-ground tree counts made by real people from around 400,000 forest plots worldwide and combined them with what the satellites told them. Their computer models crunched a bunch of numbers. Then it was time to see the final total.

CROWTHER: We all gathered in a room. It was a very exciting time. We'd been working towards it for two years.

GREENFIELDBOYCE: And...

CROWTHER: The total number of trees is close to about 3.04 trillion.

GREENFIELDBOYCE: Three trillion - that's, like, eight times more than the previous estimate. If you were to plant a tree every second...

CROWTHER: It would take you somewhere in the order of 96,000 years to plant that three trillion trees. So it's a huge astronomical number that I don't think I could comprehend before this study.

GREENFIELDBOYCE: The work appears in the journal Nature and it impressed Matthew Hansen. He's a geographer at the University of Maryland who maps land cover.

MATTHEW HANSEN: It's quite rigorous. It's using all of the available - best available data that we have at the global scale. And I think it's a nice advance.

GREENFIELDBOYCE: Now that scientists have this number, what should we make of it? Well, Crowther knows what he doesn't want us to think.

CROWTHER: My theory is that a lot of people might think, OK, well there's loads of trees so who cares about the environment? There's plenty left, no worries. What I'd highlight is that it's not like we've discovered new trees.

GREENFIELDBOYCE: In fact, Crowther's group looked back in time and calculated that the Earth has actually lost nearly half its trees since the start of human civilization. He says the number of trees being cut down each year is astonishing.

CROWTHER: We're losing 10 billion trees every year and that's a net number

GREENFIELDBOYCE: So how did the group with the lofty goal of planting a billion trees react to these numbers? Crowther says they took it pretty well.

CROWTHER: Based on this, they really want to upscale their efforts hugely by 1,000 times.

GREENFIELDBOYCE: Their new goal is to plant a trillion trees. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.