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An international group of climate scientists warns that a "tipping point" in the earth's life-support systems may be rapidly approaching, and that should we step over that as-yet-undetermined threshold, it may be too late to reverse course.

"The science tells us that we are heading toward major changes in the biosphere," UC Berkeley biologist Anthony Barnosky, lead author of "Approaching a state shift in Earth's biosphere", published Thursday in Nature, told the San Francisco Chronicle.

"And given all the pressures we are putting on the world," he added, "if we do nothing different, I believe we are looking at a time scale of a century or even a few decades for a tipping point to arrive."

Before the climate-change deniers among our beloved Reg readership get their knickers in a bunch, know that the increasing levels of atmospheric CO2 are only one of many factors that are of concern to Barnosky and his 21 coauthors.

The factors that the bio-boffins discuss in their paper are, however, primarily anthropogenic – that's "human-caused", for those of you who haven't been party to the climate-disruption party.

According to the paper, humans have already vastly transformed the earth's biosphere, with 43 per cent of the earth's land having been converted to agricultural and urban use in support of its current 7 billion inhabitants – which is four times the number of earthlings just a century ago.

Referring to that land use, the paper notes: "This exceeds the physical transformation that occurred at the last global-scale critical [climate] transition, when ~30 per cent of Earth's surface went from being covered by glacial ice to being ice-free."

This transformation is rapid and increasing, as is the population of humans who are performing it. The paper reports that if fertility rates remain at the rate they were at from 2005 to 2010, population projections for 2100 top off at a staggering 27 billion.

That number of humans would, of course, put an equally staggering stress on our planet's biological support systems. But simply the ability to feed, house, clothe, and provide energy to more and more billions of folks isn't what's really worrying the paper's authors. What they're concerned about is whether the earth's biosphere is about to experience what they refer to as a "global-scale state shift", when the environment – and its inhabitants – undergo rapid and irreversible change.

Well, irreversible in human timeframes, that is. Geological time is a wee bit more expansive – just look at the "Cambrian Explosion", for example, a massive global-scale state shift that began around the mid-500 million years ago and lasted for 30 million years or so.

"State shifts resulting from threshold effects can be difficult to anticipate," Barnosky and his coauthors write, "because the critical threshold is reached as incremental changes accumulate and the threshold value generally is not known in advance."

What's worse, state shifts can exhibit hysteresis, meaning that the shifts themselves can be separated in time from their causes – in other words, we may not know exactly what hit us until it's too late. "The net effect," the paper contends, "is that once a critical transition occurs, it is extremely difficult or even impossible for the system to return to its previous state."

The most recent major global-scale state shift was what the paper describes as the "warm-cold-warm fluctuation in climate" that occurred between 14,300 and 11,000 years ago. During a mere 1,600 years of that period – not even a blink of an eye in geological time – between 12,900 and 11,300 years ago, the biosphere's applecart was quite thoroughly upset, the paper notes, including:

The extinction of about half of the species of large-bodied mammals, several species of large birds and reptiles, and a few species of small animals; a significant decrease in local and regional biodiversity as geographic ranges shifted individualistically, which also resulted in novel species assemblages; and a global increase in human biomass and spread of humans to all continents.

That time around, the state shift was orbitally induced, with cyclic variations in solar energies that caused rapid global warming. In addition, "Direct and indirect of effects of humans probably contributed to extinctions of megafauna and subsequent ecological restructuring," the paper adds in fine boffinese.

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