Human beings are having an overwhelming impact on Earth's ecosystems, whether we're pouring plastic into the ocean or filling the skies with carbon. But it's not just modern society that's to blame — our environmental legacy stretches way back into history. Since dawn of civilisation, we've caused nearly half of the world's trees to disappear.
That's the depressing conclusion of a massive ecological study published today in the journal Nature, which offers the very first data-driven global tree census. According to the study, there approximately 3.04 trillion trees planet Earth today — roughly 422 per person. The good news is, that's nearly seven times more trees than we reckoned in our previous global estimate. The bad news? The number of trees has declined 46 per cent since humans started tilling the land.
To arrive at these numbers, the researchers collated 429,775 ground-sourced measurements of tree density from every continent on Earth except Antarctica. Combining these field measurements with satellite data on climate, topography, and human land use, they constructed a series of models that predict tree density worldwide at the resolution of a single square kilometre.
By combining tree density predictions with the spatial maps of forest cover loss, the study's authors estimate that humans are currently removing some 15.3 billion trees from the planet every year. Rates of forest loss are highest in the tropics, which also happen hold the lion's share of the planet's trees, roughly 1.39 trillion. The net loss is closer 10 billion trees per year, thanks to forest regrowth in some of the world's temperate regions.
Global map of tree density at the square-kilometre pixel scale. Image credit: Crowther, et al 2015 "I didn't expect human activity to come out as the strongest control on tree density across all of the biomes [habitat types], lead study author Thomas Crowther told The Guardian. "It was one of the dominant regulators of the number of trees in almost all of the world. It really highlights how big an impact humans are having on the Earth at a global scale."
Should that impact worry us? Absolutely. Trees offer critical ecosystem services, whether they're cleaning our water, building fertile soils, or providing us with food and raw materials. Trees also buffer us from the effects of climate change, by soaking up a tremendous share of human carbon emissions every year. As forests disappear, so does the planet's natural ability to sequester carbon and maintain a stable climate.
Simply put, a future with fewer trees is a future less secure for humans.
"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," lead study author Thomas Crowther said. "This study highlights how much more effort is needed if we are to restore healthy forests worldwide."