Here is a pop quiz: How many trees are on the planet?
Most people have no idea.
A new study says the answer is more than 3 trillion trees — that's trillion with a T, and that number is about eight times more than a previous estimate.
Thomas Crowther was inspired to do this tree census a couple of years ago, when he was working at the Yale School of Forestry and Environmental Studies. He had a friend who was working with a group with an ambitious goal: trying to fight global warming by planting a billion trees. A billion trees sounded like a lot. But was it really?
"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," recalls Crowther. "They didn't even know if it was even possible to fit a billion trees on Earth."
So his pal asked him a simple question: How many trees are growing on our planet? "I assumed that this was somewhere out there, it's information that someone will know," says Crowther.
That turned out to be wrong, he says. "Having spoken to a lot of forestry experts, it doesn't seem like anyone had any idea."
There was one estimate based on satellite images: about 400 billion trees worldwide, or 61 trees for every person.
But there were doubts about that number because another recent estimate, based on ground-truthed measurements, found 390 billion trees in the Amazon basin alone.
Crowther knew there was a way to get much better numbers.
"We used ground-sourced information," says Crowther. "All of the information that went into our models was generated from people standing on the ground counting numbers of trees in a given area. And so we could relate this information to what the satellites are telling us."
To get a better estimate, his team took advantage of the fact that countries produce detailed forest inventories. "It definitely couldn't have been done without all of those huge national forest inventories and thousands of people going out, collecting tree information around the world," says Crowther.
Using information from around 400,000 forest plots, the researchers painstakingly crunched a ton of numbers. And then it was time for the computer to spit out the final total.
"We all gathered in a room, it was a very exciting time," remembers Crowther. "We'd been working toward it for two years."
He says the huge number astonished them. And then he got a little worried.
"My fear 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," he says. "We've just generated a new number that will help us to understand the global forest."
The results are being published by the journal Nature.
"It's quite rigorous. It's using all of the best available data that we have at a global scale, so I think it's a nice advance," says Matthew Hansen, a geographer at the University of Maryland who maps land cover.
Hansen says in the future, he'd like to see new instruments in space that could do increasingly detailed observations of forests. That would let scientists do a more direct census of trees over and over, to track how the total number changes.
Crowther says their work suggests that, compared with the days before human civilization, the world has lost roughly half its trees. And the gross number of trees lost each year because of humans is now about 15 billion.
So did all of this news discourage that group that wanted to plant a billion trees?
"Based on this, they really want to upscale their efforts hugely," says Crowther, who explains that the new analysis has spurred them on. "Their goal is now to plant a trillion trees."
ROBERT SIEGEL, HOST:
Here's a trivia question. How many trees are there on the planet? Most people have no idea. NPR's Nell Greenfieldboyce reports that researchers recently did a rigorous census of all the trees on earth, and they were astonished by the result.
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 publically 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.
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 3 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 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 of 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 a thousand times.
GREENFIELDBOYCE: Their new goal is to plant a trillion trees. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.