Is There a Climate-Change Tipping Point?

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NASA / Corbis

Global warming — the very term sounds gentle, like a bath that grows pleasantly hotter under the tap. Many people might assume that's how climate change works too, the globe gradually increasing in temperature until we decide to stop it by cutting our carbon emissions. It's a comforting notion, one that gives us time to gauge the steady impact of warming before taking action.

There's just one problem: that's not how climate change is likely to unfold. Instead, scientists worry about potential tipping points — triggers that, once reached, could lead to sudden and irrevocable changes in the climate, almost without warning. It's the same phenomenon of sudden collapse that can be seen in any number of complex systems that seem perfectly stable, until they're not — ecosystems, financial markets, even epileptic seizures. The trick is to identify the warning signs that indicate a tipping point — and collapse — are about to be reached and to take action to avoid them.

A new article in the Sept. 3 issue of Nature shows there may be ways to do this, since certain warning signals appear to be similar across a variety of complex systems. Researchers from Wageningen University, the University of Wisconsin and Scripps Institution of Oceanography found that an assortment of systems they studied all had critical thresholds that could trigger change from one state to another — changes that tend to be abrupt, not gradual. "Such threshold events don't happen that often, but they are extraordinarily important," says study co-author Stephen Carpenter of the University of Wisconsin. "They are the portals to change."

So, how do we know that change is at hand? The Nature researchers noticed one potential signal: the sudden variance between two distinct states within one system, known by the less technical term squealing. In an ecological system like a forest, for example, squealing might look like an alternation between two stable states — barren versus fertile — before a drought takes its final toll on the woodland and transforms it into a desert, at which point even monsoons won't bring the field back to life. Fish populations seem to collapse suddenly as well — overfishing causes fluctuations in fish stocks until it passes a threshold, at which point there are simply too few fish left to bring back the population, even if fishing completely ceases. And even in financial markets, sudden collapses tend to be preceded by heightened trading volatility — a good sign to pull your money out of the market. "Heart attacks, algae blooms in lakes, epileptic attacks — every one shows this type of change," says Carpenter. "It's remarkable."

In climate terms, squealing may involve increased variability of the weather — sudden shifts from hot temperatures to colder ones and back again. General instability ensues and, at some point, the center ceases to hold. "Before we reached a climate tipping point we'd expect to see lots of record heat and record cold," says Carpenter. "Every example of sudden climate change we've seen in the historical record was preceded by this sort of squealing."

The hard part will be putting this new knowledge into action. It's true that we have a sense of where some of the tipping points for climate change might lie — the loss of Arctic sea ice, or the release of methane from the melting permafrost of Siberia. But that knowledge is still incomplete, even as the world comes together to try, finally, to address the threat collectively. "Managing the environment is like driving a foggy road at night by a cliff," says Carpenter. "You know it's there, but you don't know where exactly." The warning signs give us an idea of where that cliff might be — but we'll need to pay attention.