Original URL: http://www.theregister.co.uk/2012/06/08/climate_disruption_tipping_point/

Climate scientists see 'tipping point' ahead

'The Day After Tomorrow' may come the day after tomorrow

By Rik Myslewski

Posted in Science, 8th June 2012 00:17 GMT

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.

The authors' suggestion? Not what you think

The paper argues that it's humans who are currently the prime biosphere stressors. The authors note, for example, that the atmospheric CO2 concentrations that are now about 35 per cent higher than pre-industrial levels – hitting 400ppm over the arctic this spring – have caused the earth's oceans to become more acidic. Increased CO2, the paper contends, contributes to "a higher rate of global warming than occurred at the last global-scale state shift," and will result in the mean global temperature by 2070 or earlier being "higher than it has been since the human species evolved."

Also, plants are migrating due to climate disruption. "Modelling suggests that for ~30 per cent of Earth," the authors write, "the speed at which plant species will have to migrate to keep pace with projected climate change is greater than their dispersal rate when Earth last shifted from a glacial to an interglacial climate, and that dispersal will be thwarted by highly fragmented landscapes."

Among other anthropogenic biosperical changes, the paper cites ocean "dead zones" and nutrient-cycle disruptions due to pollutants from agricultural run-off and urban areas.

With all this Sturm und Drang, you could understandably imagine that the central thesis of "Approaching a state shift in Earth's biosphere" might be an impassioned plea for humankind to put the brakes on anthopogenic climate disruption. You'd be wrong.

What Barnosky and his international team are arguing for is an increased focus by the scientific community on developing processes and procedures for analyzing the many and varied factors that might push the biosphere into a global-scale state change. "The plausibility of a planetary-scale 'tipping point'," they argue, "highlights the need to improve biological forecasting by detecting early warning signs of critical transitions on global as well as local scales, and by detecting feedbacks that promote such transitions."

Recent research in state-shift theory, they argue, shows that mathematical risk-assessment models can to be developed to illuminate various and sundry data sets and divine what they can tell us about the approaches of biospherical systems towards "tipping points".

More work also needs to be done, they argue, to understand how global changes affect local changes – and, of course, vice versa. This is a particularlty sticky problem, they admit, seeing as how it is fiendishly difficult to define what might be considered an "unusual" change in a system, "because biological systems are dynamic and shifting baselines have given rise to many different definitions of 'normal', each of which can be specified as unusual within a given temporal context."

And then there's the complexities of what they refer to as "scale-jumping" effects – that is, when one set of biospherical changes affects another in unforseen ways, such as, for example, how changes in rainfall amounts can lead to changes in plant pollination cycles.

"These 'scale-jumping' effects, and the mechanisms that drive them," they write, "have become apparent only in hindsight, but even so they take on critical importance in revealing interaction effects that can now be incorporated into the next generation of biological forecasts."

The problem is an immense one, seeing as how the global ecosystem is composed of a myrid of interlocking smaller-scale ecosystems, each of which interact in complex dances of cause and effect – and because of that interaction, state shifts in small-scale systems can propagate to lead to a state shift in the entire system.

Although the paper's main focus is on the need for better tools to monitor, predict, and perhaps defer a negative biospherical tipping point, the authors are intellectually honest enough to put their cards on the table and provide their own recommendations as to what could be done now to lower the probability that such a tipping point – aka "rapid and unpredictable transformations within a few human generations" – might occur:

Of course, those who dismiss climate disruption as a "hoax" perpetrated by grant-seeking scientists – or, more kindly, as simply an unfounded set of errors in measurement, analysis, or judgment – will see little reason to undertake what Barnosky and his team refer to as these "admittedly huge tasks."

But even if the science underpinning climate change isn't nailed down as tight as a drum – as little emerging science is – and although reasonable observers can have reasonable differences of opinion, it's difficult to mount a plausible argument against developing mathematical models that could help predict a negative biospherical tipping point, and convincingly illuminate steps that might be taken to avoid it.

It's equally difficult to argue that we puny but ubiquitous humans have had little effect on our planet. As the paper's authors note, "Humans have already changed the biosphere substantially, so much so that some argue for recognizing the time in which we live as a new geologic epoch, the Anthropocene." ®