Want to take a guess at the number of trees growing on Earth? If you think it's in the billions, you're way off — researchers say our world is home to more than 3 trillion trees.

However, they add, there's some troubling news in that new estimate: the number of trees has been slashed by almost half since the beginning of human civilization.

In a study led by Yale University, researchers used images from satellites and existing forest inventories with modeling by supercomputers to arrive at their 3-trillion estimate — which is around eight times more than previously believed.

Humans cut down around 15 billion trees every year, causing an overall decrease in the world's forests by 46 percent since humanity began engaging in agriculture around 12,000 years ago, the researchers report in the journal Nature.

"Trees are among the most prominent and critical organisms on Earth, yet we are only recently beginning to comprehend their global extent and distribution," said study leader Thomas Crowther, a postdoctoral researcher at the Yale School of Forestry & Environmental Studies.

Crowther and his study colleagues analyzed data from ground-based counts on every continent except Antarctica, which allowed them to improve on previous tree-density estimates made using satellite imagery alone.

"It's not like we discovered new trees," Crowther explained. "Rather, we added another layer of information that allowed us to revise much of the previous estimates."

The highest tree densities were located in the boreal forests of North America, Russia and Scandinavia, which hold 24 percent of the global tree total.

Trees fill vital roles — storing massive amounts of carbon, cycling nutrients through ecosystems, improving air and water quality and providing a variety of human services, Crowther points out. Yet when asked to estimate how many trees are growing in the world, most people are off by an order of magnitude.

"I don't know what I would have guessed, but I was certainly surprised to find that we were talking about trillions," Crowther admitted.

A previous estimate of 400 billion would amount to around 61 trees for each person on Earth; the new figure of 3.04 trillion trees puts that at 422 trees for every person.

The researchers say the new estimate can help improve predictions about biodiversity and carbon storage around the globe. It also shows the impact of human activities – including deforestation, forest management and land-use changes – on the global tree population.

"We've nearly halved the number of trees on the planet, and we've seen the impacts on climate and human health as a result," Crowther added. "This study highlights how much more effort is needed if we are to restore healthy forests worldwide."