The scale of mankind's impact on the globe is becoming more and more apparent: We have achieved a species extinction rate to rival great extinction events of all geologic time as well as a rapidly acidifying ocean, dwindling ice caps, and even sinking river deltas, a new study from scientists at the University of Colorado at Boulder reveals. No wonder then that some geologists and other scientists have dubbed the modern epoch the Anthropocene. And now an international group of 28 scientists has taken a preliminary stab at setting some concrete environmental thresholds for the planet.

Johan Rockström of Stockholm University and his colleagues are proposing nine "planetary boundaries" in this week's Nature. (Scientific American is part of the Nature Publishing Group.) Ranging from climate change to chemical pollution, the boundaries are meant to set thresholds, or safe limits, for natural systems with respect to human impact.

"We have reached the planetary stage of sustainability, where we are fiddling with hard-wired processes at the global Earth-system scale," Rockström says. "What are the Earth-system processes that determine the ability of the [planet] to remain in a stable state?"

The research takes as its desired stable state the Holocene epoch, the 10,000 years since the last ice age during which human civilization has flourished, and attempts to identify the key variables that might push planetary cycles past safe thresholds.

So, for example, the key variable for climate change is atmospheric carbon dioxide concentration as well as its attendant rise in the amount of trapped heat. At present, atmospheric CO2 has reached more than 387 parts per million (ppm), well above the preindustrial figure of 280 ppm. So, the estimated safe threshold identified by the scientists, including NASA climatologist James Hansen, is 350 ppm, or a total increased warming of one watt per meter squared (current warming is roughly 1.5 watts per meter squared).

"We begin to quantify, very roughly, where we think these thresholds might be. All have huge error bars," says ecologist Jonathan Foley director of the University of Minnesota's Institute on the Environment, one of the authors. "We don't know exactly how many parts per million it would take to stop climate change, but we think it starts at about 350 ppm."

Along with the climate change boundary, humanity has already pushed past the safe threshold in two more of the nine identified boundaries—biodiversity loss and available nitrogen (thanks to modern fertilizers). And, unfortunately, many of the processes impact each other, as well. "Crossing one threshold makes the others more vulnerable," Foley adds. For example, "biodiversity [loss] on a really hot planet is accelerated."

Earth System             Threshold Measure       Boundary       Current Level    Preindustrial

Climate Change         CO2 Concentration       350 ppm         387 ppm           280 ppm
Biodiversity Loss        Extinction Rate             10 pm             >100 pm*          0.1-one pm
Nitrogen Cycle            N2 Tonnage                  35 mmt**       121 mmt            0
Phosphorous Cycle    Level in Ocean              11 mmt          8.5-9.5 mmt      –1 mmt
Ozone Layer               O3 Concentration         276 DU#        283 DU            290 DU
Ocean Acidification     Aragonite^^ Levels       2.75               2.90                  3.44
Freshwater Usage      Consumption                4,000 km3^    2,600 km3       415 km3
Land Use Change      Cropland Conversion    15 km3          11.7 km3          Low
Aerosols                     Soot Concentration      TBD                TBD                TBD
Chemical Pollution     TBD                              TBD                TBD                TBD

*pm=per million
**mmt=millions of metric tons
#DU=dobson unit
^km3=cubic kilometers
^^Aragonite is a form of calcium carbonate. Measurement is in global mean saturation state.

In associated commentaries published online this week in Nature Reports Climate Change, several scientists criticize the precise thresholds set, although they laud the effort. Biogeochemist William Schlesinger of the Cary Institute of Ecosystem Studies argues that the limits on phosphorus fertilizer are too lenient and can allow "pernicious, slow and diffuse degradation to persist nearly indefinitely." Allowing human water use, largely for agriculture, to expand from 2,600 cubic kilometers today to 4,000 cubic kilometers in the future will allow further degradation at such environmental disaster sites as the drying Aral Sea in Asia and seven major rivers, including the Colorado in the U.S., that no longer reach the sea, notes David Molden, deputy director general for research at the International Water Management Institute in Sri Lanka.

Even the 350-ppm limit for carbon dioxide is "questionable," says physicist Myles Allen of the Climate Dynamics Group at the University of Oxford, and focusing instead on keeping cumulative emissions below one trillion metric tons might make more sense, which would mean humanity has already used up more than half of its overall emissions budget.

And such efforts to set thresholds have a mixed track record. For instance, the "limits to growth" argument put forward by the Club of Rome in 1972 failed to materialize, thanks in part to some of the innovations listed here, such as increased nitrogen use in industrial agriculture. "A big part of this is feeding 6.7 billion people," Foley says. "We are heading towards nine billion who are going to want to eat more like people in the developed world, and there's the specter of biofuels. Those boundaries look really fragile."

Most importantly, however, regardless of impacts on the planet, the human condition has likely never been better in terms of material prosperity. The question is: "How do you continue to improve the human condition?" Foley asks. "How can we sustain a world that will reach nine billion people without destroying the planet? At least knowing a bit where the danger zones are is a really important first step."

There are grounds for hope. Humanity has crossed one of these thresholds before—diminishing levels of stratospheric ozone due to emissions of ozone-destroying chemicals  (the "ozone hole")—and pulled back through international cooperation and the 1989 Montreal Protocol. "We did manage to move ourselves away from the ozone boundary and have made serious efforts at regional levels to protect biodiversity; reduce agricultural pollution, aerosols and water demand; and slow land conversion," says environmental scientist Diana Liverman of the University of Arizona's Institute for the Environment and Society, one of the new thresholds authors. "This provides some hope that we can manage our planetary impact if we choose."