Are we risking catastrophe?

An international group of 28 scientists has proposed nine “planetary boundaries”—upper limits on measures ranging from climate change to chemical pollution that could help define safe limits for humans’ impact on the Earth.

The scientists tried to identify key variables that define the Holocene epoch—the unusually quiet 10,000 years in the planet’s history that allowed human civilizations to flourish in the first place. (All of human civilization falls within the Holocene. And we’ve just left the Holocene and entered the Anthropocene.) We’ve already exceeded the proposed safe limits for three of the nine boundaries: C02 concentration in the atmosphere, extinction rate, and available nitrogen (thanks to modern fertilizers). Exceeding any one of the variables’ boundaries risks triggering abrupt or irreversible environmental changes. And crossing one threshold also makes the others more vulnerable. For example, we would expect to lose more species, and faster, if our planet suddenly became much warmer.

John Foley, director of the University of Minnesota’s Institute on the Environment, says,

“The numbers we suggest—and they really are just suggestions—are merely guidelines. … But rather than wait and get to the last decimal place and really nail down every number, which may take years or decades to do, we felt it was really important to get this thinking out there, in the scientific community and the broader public, to think about and kind of raise the alarm a little bit.”

But all isn’t lost! Remember that hole in the ozone layer? That was also a planetary boundary that we exceeded. But O3 concentration is rising again because of 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,” says Diana Liverman of the University of Arizona’s Institute for the Environment and Society. “This provides some hope that we can manage our planetary impact if we choose.”