�ٺ���Ƶ

Humans may be forcing an irreversible, planetary-scale tipping point that could severely impact fisheries, agriculture, clean water and much of what Earth needs to sustain its inhabitants.

That is the consensus of a team of 22 scientists from the United States, Canada, South America and Europe, including two University of California, Davis, professors. The team reports its conclusions in the June 7 issue of the journal Nature.

Such a change has not been seen since the shift from the Ice Age to an interglacial age 11,700 years ago—a time of mass extinctions and extreme climate shifts, according to the authors, who estimate that Earth may experience the next major tipping point within a few generations.

The authors call for accelerated global cooperation to reduce world population growth and per-capita resource use, replace fossil fuels with sustainable sources, develop more efficient food production and distribution systems, and better protect land and ocean areas not already dominated by humans. They also urge focused research to identify early warning signs of a global transition.

“Once you go past the tipping point, putting it back together is much more difficult than preventing something from happening,” said Alan Hastings, professor of environmental science and policy at �ٺ���Ƶ and one of the study’s authors.

Hastings uses mathematical models to understand natural systems. For example, he studied a coral reef that had been overrun with seaweed after a 1983 plague killed nearly all of the sea urchins that served as natural grazers for the coral. Grazing parrotfish filled in for the sea urchins, forming a second line of defense, until they became overfished. Hastings calculated that once the reef went past its tipping point, a drastic increase in parrotfish would be required to bring it back to a healthy state. 

The researchers analyzed how similar scenarios might play out on a global scale, as well as what data from the end of the last glacial period might reveal about future tipping points.

“This isn’t the first time a tipping point has happened,” said �ٺ���Ƶ paleontologist Geerat Vermeij, who brought a geological perspective to the review. He analyzed fossil records for clues about the nature, speed, causes and predictability of past tipping points. “We’d like to know what these earlier ones were like and if they tell us anything about future ones.”

The paper describes an urgent need for better predictive models that are based on a detailed understanding of how the biosphere reacted in the distant past to rapidly changing conditions, including climate and human population growth. Better forecasting could lead to better decisions about protecting natural resources.

“We really do have to be thinking about these global-scale tipping points, because even the parts of Earth we are not messing with directly could be prone to some very major changes,” said UC Berkeley professor and lead author Anthony Barnosky. “My underlying philosophy is that we want to keep Earth, our life support system, at least as healthy as it is today, in terms of supporting humanity, and forecast when we are going in directions that would reduce our quality of life so that we can avoid that.”

While tipping points are not inherently irreversible, recovering after reaching a tipping point is difficult, if not impossible. The result of a major shift in the biosphere would be mixed, with some plant and animal species disappearing, new mixes of species remaining, and major disruptions in agriculture, according to the study.

At least 43 percent of Earth’s land has already undergone wholesale transformation, a rate of roughly 2.27 transformed acres per person, the report notes.

If this rate does not change, half of Earth’s landmass “will have exhibited state-shifts when world population hits 8.2 billion, estimated to occur by the year 2025—a scant 13 years from now,” the report warns.

Vermeij thinks that in some ways, Earth has already reached a tipping point.

“Many people who have written about our ecological future have expressed a level of optimism that I simply don’t share,” he said. “I think it’s de rigueur to write optimistically because people don’t like pessimism, and rightly so. But sometimes you have to be realistic. Sometimes you have to say things the way they are.”

Hastings also warns that solutions will be difficult. “Irrespective of the impact on global warming, we don’t have simple ways to generate enough energy to power the world the way it’s been going for that much longer,” he said. “Things are going to get ugly.”

The Nature paper appears in an issue devoted to the environment in advance of the June 20-22 United Nations Rio+20 Earth Summit in Rio de Janeiro, Brazil.