Scientist: Earth has passed 4 of the 9 boundaries for hospitable life

In a paper published in Science, Johan Rockstrom, an Earth resilience strategist, posits that humans have already passed four of the nine limits for hospitable life.

By Rockstrom's estimates, we should maintain 90 percent of Earth's biodiversity, but in some parts of the world, biodiversity has dropped to 84 percent. And while Rockstrom believes humans should maintain 75 percent of Earth's original forests, we've only maintained 62 percent. The other two limits Rockstrom believes we've passed are because of humanity's increased use of phosphorus and nitrogen in ecosystems and increased carbon dioxide levels, which have resulted in climate change.

"The planet has been our best friend by buffering our actions and showing its resilience," Rockstrom said in a statement. "But for the first time ever, we might shift the planet from friend to foe."

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As for the remaining five boundaries, Rockstrom believes Earth is approaching those limits, too. While we aren't nearing the limits for freshwater use and ocean acidification, we're close to the limits for emitting poisonous aerosols and depleting the ozone. It's not all bad news, though: Rockstrom believes that if we cut down on things like carbon emissions, and increase efforts at maintaining biodiversity, Earth will become more hospitable once again.