University of Vermont

For the last 10,000 years on Earth, it's been a smooth ride. Compared to much of the planet's wildly swinging history — ice age to hothouse — temperatures have been steady, freshwater plentiful, and many biological and chemical cycles reliable. This period of stability, known to geologists as the Holocene, has allowed civilizations to rise and agriculture to flourish.

"But now we're heading off a cliff," says the University of Vermont's Robert Costanza. He's one of the authors on a paper published Sept. 24 in the journal Nature, arguing that, actually, there are at least nine cliffs, or "planetary boundaries," that we shouldn't cross at risk of "disastrous consequences for humanity." The paper has been receiving broad attention in the media, from Time magazine to Wired.com, for its new, quantified approach to defining the conditions that have allowed for human development — and its warning that some of these planetary boundaries have been overstepped.

"There are three where we're already over the edge," says Costanza, director of UVM's Gund Institute for Ecological Economics. "It's the Wile E. Coyote thing."

The rate of species extinctions, climate change, and human interference with the nitrogen cycle threaten to knock Earth out of its stable pattern into irreversible and abrupt environmental change, Costanza and his colleagues contend in "A Safe Operating Space For Humanity."

Rising seas, collapsing ecosystems, shifting rains, and ocean dead zones are but a few of the potential consequences that could, as their paper tersely notes, lead to a chaotic "state less conducive to human development," — instead of several thousand more years of the Holocene.

Through rapidly expanding use of fossil fuels and industrial agriculture, humans have become the "main driver of global environmental change," the paper notes. There's broad scientific consensus that rising carbon dioxide levels are driving up temperatures and land use changes are driving out species. The new paper makes a preliminary attempt to put numbers on levels that are more sustainable: bringing atmospheric carbon dioxide down from its current 387 parts per million to 350, species driven to extinction down from hundreds each year (at least) to under 10, and amounts of nitrogen extracted from the air for fertilizer from 121 million tons each year to 35 million.

The paper — led by Johan Rockstrom at the Stockholm Resilience Center in Sweden — also describes seven other planet-wide problems: phosphorous pollution, ozone depletion, ocean acidification, freshwater overuse, conversion of natural areas to cropland, atmospheric aerosols, and chemical pollution — and proposes several other boundaries. The paper also makes clear that these boundaries are tightly linked: forest clearing in the Amazon can threaten water resources in Tibet. "We do not have the luxury of concentrating our efforts on any one of them in isolation from the others," the authors write. "If one boundary is transgressed, then other boundaries are also under serious risk."

"This paper goes against the mainstream economic thinking that there are no limits to growth," Costanza says. But just what are those limits on a planetary scale? These have been fuzzy, at best, in the literature of ecological economics. "We're finally better defining what they are," he says, "and what we would need to do to stay away from the cliffs and still have plenty of room for people to operate."