If humans cause the sixth mass extinction on Earth–something that may already be underway–how long will it take life on the planet to recover? CO2 stays in the atmosphere for thousands of years, but a new study suggests that it will take much, much longer than that for biodiversity to recover.
“We think that we’re approaching a mass extinction driven by us from habitat loss and from a multitude of factors, including climate change,” says Andrew Fraass, a paleobiologist from the University of Bristol and one of the authors of a study published in Nature Ecology and Evolution. “This work, and the work it’s based on, basically suggests that it’s going to take on the scale of millions of years for the biosphere to recover.”
The researchers studied the fossil record after the last mass extinction, when an asteroid crashed into the Yucatan Peninsula, causing a tsunami, spewing vaporized debris into the atmosphere, heating up the atmosphere, and wiping out the dinosaurs and roughly 75% of species on Earth. “That’s the one thing that basically happens faster than modern climate change, because it happens in one day, and then chunks of North America catch on fire and all this death and destruction happens,” he says.
Tiny plankton called foraminifera show up in detail in the fossil record, so the researchers were able to see how they fared. After the Cretaceous mass extinction, the number of species of the plankton dropped from dozens to a handful. It took around 10 million years for diversity to rebound, with species slowly becoming more specialized. That’s how long we could expect it to take again, should we experience a similar level of species loss.
Fraass says that it’s difficult to determine if the world is already experiencing another mass extinction now–or if it’s merely on the verge–in part because scientists have little information about species in past eras, so it’s difficult to compare. (There is a much stronger fossil record of the plankton than other species.) But there are ample signs of precipitous decline. One recent study found that 40% of insect species are threatened with extinction. A 2015 study that looked at mammals, amphibians, birds, and reptiles found that the average rate of extinction is now as much as 100 times higher than normal. As coral reefs die, marine species are also dying. On land, 1,700 species risk extinction in as little as 50 years because of habitat loss alone. Another study found that humans may drive so many mammals extinct in 50 years that it will take at least 3 million years for biodiversity to recover. But it may take even longer.