Editor's note: The original online version of this story was posted on September 23, 2009.

The scale of humanity’s impact on the globe is becoming ever more apparent: we have wiped out species at a rate to rival great extinction events of all geologic time as well as contributing to a rapidly acidifying ocean, dwindling ice caps and even sinking river deltas. Now an international group of 29 scientists has taken a preliminary stab at setting some concrete environmental thresholds for the planet.

Johan Rockström of Stockholm University and his colleagues have proposed nine “planetary boundaries” online in the September 23 Nature. (Scientific American is part of the Nature Publishing Group.) The boundaries, dealing with climate change, ocean acidification, chemical pollution and others, are meant to set thresholds, or safe limits, for natural systems with respect to human impact, although exact numbers have not yet been determined for some.

“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 387 parts per million (ppm), well above the preindustrial figure of 280 ppm. 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 square meter (current warming is roughly 1.5 watts per square meter).

“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 and 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.”

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 affect one another as well. “Crossing one threshold makes the others more vulnerable,” Foley adds. For example, biodiversity loss “on a really hot planet is accelerated.”

Several scientists laud the effort but criticize the precise thresholds set. 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. (One cubic kilometer of water equals about 264 trillion gallons.*)

Even the 350-ppm limit for carbon dioxide is “questionable,” says physicist Myles Allen of the Climate Dynamics Group at the University of Oxford. Instead he thinks that focusing on keeping cumulative emissions below one trillion metric tons might make more sense—although that means humanity has already used up more than half of its overall emissions budget.

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—namely, diminishing levels of stratospheric ozone caused by emissions of ozone-destroying chemicals (the “ozone hole”). We pulled back thanks to 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,” points out environmental scientist Diana Liverman of the University of Arizona, one of the authors of the new thresholds. “This provides some hope that we can manage our planetary impact if we choose.”

Note: This article was originally printed with the title, "Setting Boundaries."

*Erratum (1/25/10): The correct conversion is 264 billion gallons.