How Many Trees Are There in the World?

There’s an old saying that there are more stars in the sky than there are grains of sand on the earth. Both have such magnificent numbers that it’s really hard to even fathom; our brains aren’t really wired to deal with such vast volumes. As it turns out, we can add trees to the list of profound concepts to grapple with. Because there are a lot of trees on this planet. Like, a lot a lot.

Several years ago while working at the Yale School of Forestry and Environmental Studies, Thomas Crowther came across a bit of a mystery presented by a friend who was working on the UN’s Billion Tree Campaign. The aim of the initiative is to plant a billion trees in an effort to fight global warming, but they had a problem: they weren’t really sure if a billion was the right number. Was it too many? Not enough? They had no idea.

"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, friend-guy asked Crowther a question, a seemingly simple one: How many trees are on our planet?

"I assumed that this was somewhere out there, it's information that someone will know," says Crowther. But in fact, no. "Having spoken to a lot of forestry experts, it doesn't seem like anyone had any idea."

One estimate put the number at around 400 billion trees worldwide based on satellite images. Another, based on ground-truthed measurements, suggested there were 390 billion trees in the Amazon basin alone.

And thus, Crowther’s mission was set. He knew there was a method to get better numbers, and so he did.

"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 further fine-tune the numbers, his team also relied on detailed forest inventories that a number of countries have produced. "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.

Compiling information from around 400,000 forest plots, the researchers meticulously crunched countless calculations and fed the data to a computer, which arrived at the number.

"We all gathered in a room, it was a very exciting time," remembers Crowther. "We'd been working toward it for two years."

The result: A staggering three trillion trees.

Three trillion trees!

It’s a number so huge that it becomes abstract; it almost goes in one ear and out the other. But consider this, three trillion seconds equals 94,638 years. It also equals roughly 422 trees per person.

He says the number astonished them, and also caused a bit of concern.

"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."

Compared to the time before human civilization took over the planet like the crazy invasive species that we are, there were twice as many trees. He notes that the gross number of trees lost annually because of humans is now about 15 billion.

The highest densities of trees were found in subarctic regions of Scandinavia, Russia, and North America, where 24% of the world's trees reside. The tropics have the largest forested areas, with 43% of global trees. The researchers found that biomes could predict tree density. Trees tend to prefer wet ground, as it helps them to grow; but that's also where humans prefer to grow agricultural crops, which leads to deforestation.

Other researchers have tackled this question in the years since Crowther's study. A more recent study analyzed numbers of tree species (as opposed to total number of trees) and estimated that there are roughly 73,000 species worldwide, and—researchers think—approximately 9,000 species yet to be discovered, 40% of which are in South America. These yet-to-be-discovered ones are likely rare, with small populations and limited distribution.

As for friend-guy who asked Crowther the question in the first place, did the tree tally have an effect on the aim of the tree-planting campaign?

"Based on this, they really want to upscale their efforts hugely," says Crowther, who says that the new count has amped up their efforts. "Their goal is now to plant a trillion trees."

Translated to seconds? That would be 31,546 years.