Be warned humans, five more strikes and you might be looking for a new planet to call home. A team of scientists that once laid out nine "planetary boundaries" that should not be crossed says humans have managed to cross four of them already, reports Reuters. The bad grades come in the categories of rapid species loss, deforestation, the level of carbon dioxide in the atmosphere, and the amount of nitrogen and phosphorous (think fertilizer) flowing into oceans. We're still OK on ozone depletion, ocean acidification, freshwater use, microscopic particles in the atmosphere, and chemical pollution, write the 18 international researchers in Science. But they warn that the long-term trend—going back to 1950 and the start of rapid economic growth around the world—doesn't bode well.
"For the first time in human history, we need to relate to the risk of destabilizing the entire planet," says one of the authors. Bottom line? Lead author Will Steffen tells the Washington Post that at our current pace, the "Earth System" could unravel in anywhere from "decades out to a century." The researchers concede their boundaries are squishy, and the Post illustrates an example involving carbon dioxide levels: The boundary we weren't supposed to cross was 350 parts per million. But we're now well past that and into a "zone of uncertainty" that extends to 450ppm. And after that? "The unknown—planetary conditions unfamiliar to us." Steffen tells Scientific American that the boundaries and our understanding of them continue to evolve, and the next step is to get a better handle on how they interact with each other. (Meanwhile, 2014 was the hottest year in history.)