Remember the good old days, when all we had to worry about was a rapidly warming planet and its associated climatological disasters? Well, sure, we still need to work on transitioning away from fossil fuels and somehow bringing the CO2 in the air down to manageable levels. But it turns out, says Nature, that ecologically speaking there are a few other boundaries humanity was never meant to cross. And right now we’re driving with the petal to the metal and no brakes.
Climate change threatens to turn the planet into a stormy, overheated mess: That much we know. But according to 28 leading scientists, greenhouse gas pollution is but one of nine environmental factors critical to humanity’s future. If their boundaries are stretched too far, Earth’s environment could be catastrophically altered — and three have already been broken, with several others soon to follow.
This grim diagnosis, published Wednesday in Nature, is the most ambitious assessment of planetary health to date. It’s a first-draft users’ manual for an era that scientists dub the “anthropocene,”  in which nearly seven billion resource-hungry humans have come to dominate ecological change on Earth. The scientists’ quantifications are open to argument, but not the necessity of their perspective.
“It’s a crude attempt to map the environmental space in which we can operate,” said Jon Foley, director of the University of Minnesota’s Institute on the Environment and one of the paper’s lead authors. “We need to keep our activities in a certain range, or the planet could tip into a state we haven’t seen in the history of our civilization.”
Thresholds for atmospheric carbon dioxide and ozone have already been described, and are widely known to the public. But the scientists say five other factors are just as important: ocean acidification, nitrogen and phosphorus pollution, land use, freshwater use and biodiversity. They say chemical pollution and atmospheric aerosols may also be essential, but can’t yet be quantified.
Values for the proposed boundaries are still just estimates, and don’t account for how pushing one could affect another — how, for example, acidification that kills plankton could make it harder for the ocean to absorb CO2 and rebound from nitrogen pollution. Ecological models still can’t capture the entirety of Earth’s biological, geological and chemical processes, and it’s impossible to run whole-Earth experiments — except, arguably, for the experiment that’s going on now.
Despite those uncertainties, one aspect of Earth’s behavior is becoming clear. Records of global transitions between geological ages, and of regional changes between environmental stages, suggest that planet-wide change could happen relatively quickly. It might not take thousands or millions of years for Earth’s environment to be altered. It could happen in centuries, perhaps even decades.
Exactly what Earth would look like is difficult to predict in detail, but it could be radically different from the mild environment that has prevailed for the last 10,000 years. It was temperate stability that nurtured the rise of civilization, and it should continue for thousands of years  to come, unless humanity keeps pushing the limits.
“The Earth of the last 10,000 years has been more recognizable than the Earth we may have 100 years from now. It won’t be Mars, but it won’t be the Earth that you and I know,” said Foley. “This is the single most defining problem of our time. Will we have the wisdom to be stewards of a world we’ve come to dominate?”