Could human activity push Earth's biological systems to a planet-wide tipping point, causing changes as radical as the Ice Age's end – but with less pleasant results, and with billions of people along for a bumpy ride?

It's by no means a settled scientific proposition, but many researchers say it's worth considering – and not just as an apocalyptic warning or far-fetched speculation, but as a legitimate question raised by emerging science.

"There are some biological realities we can't ignore," said paleoecologist Anthony Barnosky of the University of California, Berkeley. "What I'd like to avoid is getting caught by surprise."

In "Approaching a state shift in Earth’s biosphere," published June 6 in Nature, Barnosky and 21 co-authors cite 100 papers in summarizing what's known about environmental tipping points.

While the concept was popularized by Malcolm Gladwell's accounts of sudden, widespread changes in society, the underlying mathematics – which won physicist Kenneth Wilson a Nobel Prize in 1982 – have far-reaching implications.

In the last few decades, scientists have found tipping behaviors in various natural environments, from locale-scale ponds and coral reefs to regional systems like the Sahara desert, which until 5,500 years ago was a fertile grassland, and perhaps even the Amazon basin.

Common to these examples is a type of transformation not described in traditional ideas of nature as existing in a static balance, with change occurring gradually. Instead, the systems seem to be dynamic, ebbing and flowing within a range of biological parameters.

Stress those parameters – with fast-rising temperatures, say, or a burst of nutrients – and systems are capable of sudden, feedback loop-fueled reconfiguration.

According to some researchers, that's what happened when life's diversity exploded in an eyeblink 540 million years ago, or much more recently when a glacier-chilled Earth became in a couple thousand years the temperate garden that cradled human civilization.

But while the Cambrian explosion and Holocene warming were sparked by natural, planet-wide changes to ocean chemistry and solar intensity, say Barnosky and colleagues, there's a new force to consider: 7 billion people who exert a combined influence usually associated with planetary processes.

Human activity now dominates 43 percent of Earth's land surface and affects twice that area. One-third of all available fresh water is diverted to human use. A full 20 percent of Earth’s net terrestrial primary production, the sheer volume of life produced on land every year, is harvested for human purposes. Extinction rates compare to those recorded during the demise of dinosaurs and average temperatures will likely be higher in 2070 than at any point in human evolution.

Scientists informally call our current geological age the "Anthropocene," and to Barnosky's group this means we're strong enough to tip the planet, radically changing regional climates and ecologies.

"Everything that happened the last time around is happening now, only more of it," said Barnosky of the last ice age's end and ongoing changes to Earth's climate and biosphere. "I think the evidence makes it pretty clear that another critical transition or tipping point is very plausible within the next century."

Yet while Barnosky and colleagues write that the plausibility of a planetary shift is high, they say "considerable uncertainty remains about whether it is inevitable and, if so, how far in the future it may be." Other scientists echoed the caution.

"We have quite good evidence for the Earth having tipping elements. They can be very small, like a pond, or large like a monsoon system. Those we understand very well. But the bigger ones are harder to understand," said ecologist Marten Scheffer of Wageningen University, a tipping point research pioneer. Scheffer said he is "not so convinced" that a single, Earth-wide shift is imminent.

In contrast, ecologist Aaron Ellison of Harvard University, who studies the dynamics of tipping points, said the new paper "states the obvious. We're in a rapidly changing world and things are happening really quickly."

One important aspect of the new review, said ecologist Steve Carpenter of the University of Wisconsin, is its focus on changing land use patterns.

Most historical large-scale tips were apparently driven by changes in Earth's biogeochemistry, such as the bacterial oxygenation of primeval seas that afterwards could support multicellular life. But humans are rapidly changing local species compositions and ecosystem functions, causing small-scale changes that could combine and cascade into planet-wide shifts.

Brad Cardinale, an ecologist at the University of Michigan, said the science is suggestive but still not conclusive, likening the trajectory of research to that followed by chaos theory in the late 20th century.

"We discovered in mathematical models that chaos should exist and, if it did, it would have major implications for our ability to predict ecological changes on the planet. A few empirical case studies emerged to suggest chaos actually occurs in ecosystems. But the interpretation of some of these was controversial, and subsequent studies ultimately failed to show that chaos was the generality," he said.

Continued Cardinale, "Ten years from now, the Barnosky et al. paper will have one of two fates. We'll either look back and think this was a visionary warning about how people are changing the planet. Or we'll look back and say that state shifts was a 'sexy' idea that was over-sold and didn't pan out. Only time will tell."

The pressing question, then, is one of risk analysis: Given incomplete but troubling information, what should people do? Barnosky and colleagues call for innovations and changes – more-efficient food production, fossil fuel alternatives, better ecosystem management and reduced population growth. Ellison hopes some disruptive change will cause a tipping point in human sustainability.

"These are admittedly huge tasks, but are vital if the goal of science and society is to steer the biosphere towards conditions we desire, rather than those that are thrust upon us unwittingly," wrote Barnosky and colleagues.

"There have been big, planetary shifts before," Barnosky said. "We can see it coming. That's the difference. The dinosaurs couldn't see it coming."

Citation: "Approaching a state shift in Earth’s biosphere." By Anthony D. Barnosky, Elizabeth A. Hadly, Jordi Bascompte, Eric L. Berlow, James H. Brown, Mikael Fortelius, Wayne M. Getz, John Harte, Alan Hastings, Pablo A. Marquet,, Neo D. Martinez, Arne Mooers, Peter Roopnarine, Geerat Vermeij, John W. Williams, Rosemary Gillespie, Justin Kitzes, Charles Marshall, Nicholas Matzke, David P. Mindell, Eloy Revilla & Adam B. Smith. Nature, Vol. 486, No. 7402, June 6, 2012.