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Humans are overstepping environmentally safe 'planetary boundaries.'

European, Australian and U.S. environmental scientists have identified nine "planetary boundaries" to the Earth system that define safe operating space for humans.  

Human activities may already have altered three of the planet's biophysical thresholds "with consequences that are detrimental or even catastrophic for large parts of the world," the 29 scientists conclude in an article in the Sept. 23 issue of Nature.

University of Arizona and Arizona State University scientists are among the co-authors of the groundbreaking report.

Human actions have already pushed processes of climate change, biodiversity loss and the nitrogen cycle beyond environmental conditions that enabled human civilizations to evolve and thrive during the past 10,000 years, the researchers wrote in their article, "A safe operating space for humanity."

Scientists have been warning for decades that the explosion of human activity since the industrial revolution is pushing the Earth's resources and natural systems to their limits.  The data confirm that 6 billion people are capable of generating a global geophysical force the equivalent to some of the great forces of nature – just by going about their daily lives.

This force has given rise to a new era – a geological epoch scientists are calling the "Anthropocene" – in which human actions have become the main driver of global environmental change.

"On a finite planet, at some point, we will tip the vital resources we rely upon into irreversible decline if our consumption is not balanced with regenerative and sustainable activity," said Sander van der Leeuw, who directs Arizona State University's School of Human Evolution and Social Change. Van der Leeuw, an archaeologist and anthropologist specializing in the long-term impacts of human activity on the landscape, is a co-author on the Nature article.

Defining planetary boundaries

It started with a fairly simple question: How much pressure can the Earth system take before it begins to crash?

"Until now, the scientific community has not attempted to synthesize Earth system data and make a proposal such as this," Van der Leeuw said. "We are sending these ideas out through the Nature article to be vetted by the scientific community at large."

The scientists identified nine boundaries: climate change, stratospheric ozone, land use change, freshwater use, biological diversity, ocean acidification, nitrogen and phosphorus inputs to the biosphere and oceans, aerosol loading and chemical pollution. Their study suggests that three of these boundaries – climate change, the rate of biodiversity loss and nitrogen input to the biosphere – may already have been transgressed.

Alarm bells for Arizona

"Our attempt to identify planetary boundaries that, if crossed, could have serious environmental and social consequences has a special resonance in the southwest, where pressures on biodiversity, land use, and water are likely to intersect with climate change to create tremendous challenges for landscapes and livelihoods," said professor Diana Liverman, co-director of the University of Arizona's Institute of the Environment.

"Three of the boundaries we identify – 350ppm of atmospheric carbon dioxide, biodiversity extinction rates more than 10 times the background rate, and no more than 35 million tons of nitrogen pollution per year – have already been exceeded with fossil fuel use, land use change, and agricultural pollution driving us to unsustainable levels that are producing real risks to our survival," Liverman said.

"Although the current focus is on the urgent need to control greenhouse gas emissions, we must not forget the other ways humans are altering the planet and its ecosystems," she added.

Liverman, who also is professor of environmental science and a senior fellow of Oxford University's Environmental Change Institute, is currently attending an international climate conference at Oxford, United Kingdom. Participants are discussing the implications for humans and Earth ecosystems of a 4 degree Celsius, or 7.2 degree Fahrenheit, global temperature rise.

"A safe operating space for humanity"

"Human pressure on the Earth system has reached a scale where abrupt global environmental change can no longer be exluded," said Johan Rockström of Stockholm University, lead author of the Nature article.

"To continue to live and operate safely, humanity has to stay away from critical 'hard-wired' thresholds in Earth's environment and respect the nature of the planet's climatic, geophysical, atmospheric and ecological processes.

"Transgressing planetary boundaries may be devastating for humanity, but if we respect them, we have a bright future for centuries ahead."

In addition to Liverman, Rockström and van der Leeuw, the group of authors includes Hans Joachim Schellnhuber, Will Steffen, Katherine Richardson, Jonathan Foley and Nobel laureate Paul Crutzen.