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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?”
Foley’s team put the atmospheric carbon dioxide threshold at 350 parts per million, a level the Intergovernmental Panel for Climate Change says should keep Earth’s average temperature from rising by more than four degrees Fahrenheit. Current atmospheric CO2 levels are already approaching 400 parts per million.
Also exceeded are limits for species loss, which the scientists set at 10 per year per million species, and nitrogen use, pegged at 35 million tons per year. The current extinction rate is ten times higher than advised, ostensibly compromising the ability of ecosystems to process nutrients. The use of nitrogen — which is needed for fertilizer, but causes oxygen-choking algae blooms — is nearly four times higher than recommended.
On the positive side, atmospheric levels of ultraviolet radiation-blocking ozone are safe, thanks to a 1987 ban on ozone-destroying chemicals. Total rates of ocean acidification, freshwater consumption and land use are also acceptable, but those thresholds are expected to be exceeded in coming decades.
The seven boundary points are certain to be controversial, and Nature commissioned seven separate critiques by leading experts in each field.
William Schlesinger, president of the Cary Institute of Ecosystem Studies, said the recommended nitrogen limit “seems arbitrary.” Echoing his words was Steve Bass of the International Institute for Environment and Development, who said the 15 percent cap on land devoted to agriculture could as easily be 10 or 20 percent.
International Water Management Institute researcher David Molden said the 4,000 cubic kilometer ceiling on freshwater use — roughly one-third of all freshwater — “may be too high.” Myles Allen, an Oxford University climatologist, argued that CO2 emissions should be counted in a different way. Cristian Samper, director of the U.S. Natural History Museum, said that taxonomic family loss is a more relevant measure than species loss.
According to Foley, who called his team’s threshold values a “cave painting” version of the true limits, the paper is less important for its details than its approach. And though the critics argued over the numbers, all agreed that exceeding them will be disastrous.
“Planetary boundaries are a welcome new approach,” wrote Molden. “It is imperative that we act now on several fronts to avert a calamity far greater than what we envision from climate change.”
Peter Brewer, an ocean chemist at the Monterey Bay Aquarium Research Institute, criticized the paper’s lack of proposed solutions. Given the ongoing failure of governments and citizens to follow their scientists’ advice on climate change, more than dire warnings is clearly needed.
“Is it truly useful to create a list of environmental limits without serious plans for how they may be achieved?” Brewer wrote. “Without recognition of what would be needed economically and politically to enforce such limits, they may become just another stick to beat citizens with.”
“It’s unsatisfactory, I agree. We don’t answer the question of how to keep humanity from crossing the boundaries,” said Johan Rockstrom, director of the Stockholm Environment Institute and a lead author of the Nature paper. “That’s the next challenge. To stay within planetary boundaries, we need tremendous social transformation.”
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Note: The Nature paper is an edited version of the full article, which is available from the Stockholm Resilience Institute.
Citations: “A safe operating space for humanity.” By Johan Rockström, Will Steffen, Kevin Noone, Åsa Persson, F. Stuart Chapin, III, Eric F. Lambin, Timothy M. Lenton, Marten Scheffer, Carl Folke, Hans Joachim Schellnhuber, Björn Nykvist, Cynthia A. de Wit, Terry Hughes, Sander van der Leeuw, Henning Rodhe, Sverker Sörlin, Peter K. Snyder, Robert Costanza, Uno Svedin, Malin Falkenmark, Louise Karlberg, Robert W. Corell, Victoria J. Fabry, James Hansen, Brian Walker, Diana Liverman, Katherine Richardson, Paul Crutzen, Jonathan A. Foley. Nature, Vol. 461 No. 7263, September 24, 2009.
“Thresholds risk prolonged degradation.” By William Schlesinger. Nature, Vol. 461 No. 7263, September 24, 2009.
“Keep off the grass.” By Steve Bass. Nature, Vol. 461 No. 7263, September 24, 2009.
“Tangible targets are critical.” By Myles Allen. Nature, Vol. 461 No. 7263, September 24, 2009.
“Identifying abrupt change.” By Mario J. Molina. Nature, Vol. 461 No. 7263, September 24, 2009.
“The devil is in the detail.” By David Molden. Nature, Vol. 461 No. 7263, September 24, 2009.
“Consider all consequences.” By Peter Brewer. Nature, Vol. 461 No. 7263, September 24, 2009.
“Rethinking biodiversity.” By Cristian Samper. Nature, Vol. 461 No. 7263, September 24, 2009.