Quick-Change Planet: Do Global Climate Tipping Points Exist?

An academic debate ponders whether Earth's climate could change precipitously, and how unmitigated regional stressors could irrevocably alter the planet

By Dave Levitan

PLANET TIPPING: Researchers continue to debate whether global tipping points exist or not. Image: Courtesy of NASA

Is there a chance that human intervention?rising temperatures, massive land-use changes, biodiversity loss and so on?could ?tip? the entire world into a new climatic state? And if so, does that change what we should do about it?

As far back as 2008 NASA?s James Hansen argued that we had crossed a ?tipping point? in the Arctic with regard to summer sea ice. The diminishing ice cover had moved past a critical threshold, and from then on levels would drop precipitously toward zero, with little hope of recovery. Other experts now say that recent years have confirmed that particular cliff-fall, and the September 2012 record minimum?an astonishing 18 percent lower than 2007?s previous record?was likely no fluke.

Sea ice represents a big system, but it is generally thought to be self-contained enough to follow such a tipping-point pattern. The question that has started to pop up increasingly in the last year, however, is whether that sort of phase transition, where a system shifts rapidly?in nonlinear fashion, as scientists say?from one state to another without recovery in a timescale meaningful to humanity, is possible on a truly global scale.

?You?re pushing an egg toward the end of the table,? says Tony Barnosky, a professor at the University of California, Berkeley. At first, he says, ?not much happens. Then it goes off the edge and it breaks. That egg is now in a fundamentally different state, you can?t get it back to what it was.? Barnosky was the lead author on a much-discussed paper in Nature[DL1]? last summer that suggested the world?s biosphere was nearing a ?state shift??a planetary-scale tipping point where seemingly disconnected systems all shifted simultaneously into a ?new normal.? (Scientific American is part of Nature Publishing Group.)

Claims of catastrophic temperature shifts are unlikely to go down without an argument. A new paper published recently in Trends in Ecology & Evolution by Barry Brook of the University of Adelaide in Australia and colleagues argues that there is no real grounding to the idea that the world could display true tipping-point characteristics. The only way such a massive shift could occur, Brook says, is if ecosystems around the world respond to human forcings in essentially identical ways. Generally, there would need to be ?strong connections between continents that allow for rapid diffusion of impacts across the planet.?

This sort of connection is unlikely to exist, he says. Oceans and mountain ranges cut off different ecosystems from each other, and the response of a given region is likely to be strongly influenced by local circumstances. For example, burning trees in the Amazon can increase CO2 in the atmosphere and help raise temperatures worldwide, but the fate of similar rainforests in Malaysia probably depend more on what?s happening locally than by those global effects of Amazonian deforestation. Brook and colleagues looked at four major drivers of terrestrial ecosystem change: climate change, land-use change, habitat fragmentation and biodiversity loss; they found that truly global nonlinear responses basically won?t happen. Instead, global-scale transitions are likely to be smooth.

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