Ozone "hole" (purple) over Antarctica can be used as one measure of humanity's impact on Earth.
Image: NASA
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The Best Science Writing Online 2012
Showcasing more than fifty of the most provocative, original, and significant online essays from 2011, The Best Science Writing Online 2012 will change the way...
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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.
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9 Comments
Add CommentWhere is the "complete listing of the planetary boundaries and thresholds,...." promised in the foot note to Setting Boundaries?
Reply | Report Abuse | Link to thisAsking myself the same question, David.
Reply | Report Abuse | Link to thisSame here, Doug.
Reply | Report Abuse | Link to thisHere's a link which has the list of 9 (plus two that haven't been quantified yet).
Reply | Report Abuse | Link to thishttp://www.nature.com/nature/journal/v461/n7263/fig_tab/461472a_T1.html
And you can find our version of the list here in the online version:
Reply | Report Abuse | Link to thishttp://www.scientificamerican.com/article.cfm?id=scientists-identify-safe-limits-for-human-impacts
Thank you for your 10:43, David.
Reply | Report Abuse | Link to thisFact check: Energy aficionados who know that a "cubic mile of oil" is roughly a trillion gallons immediately spot the error in the parenthetic comment: (one cubic kilometer of water equls 264 trillion gallons). Should be 10^9, not 10^12.
Reply | Report Abuse | Link to thisI beg to differ with scientists on higher use of water for irrigating agri-fields. In fact, it recharges water table in the area and moreover prevents water from going to the oceans and becoming a waste.
Reply | Report Abuse | Link to thisMy submission to the scientist is"the main cause of concern to save the earth is human population explosion on the earth.All the problems,whether it is biodiversity or ecological imbalance are caused by human population growth internationaly .I'm here agree with Mr.Foley that how can we sustain a world that will reach nine billion people without destroying the planet? -- Niranjan email-niranjanprasadsingh@gmail.com
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