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The Tipping Points

Abrupt Climate Changes Present Another Reason to Cut Emissions

Petermann Floating Tongue in north Greenland SOURCE: NASA/JPL The Petermann Floating Tongue in north Greenland, where ice is melting much faster than expected.

Climate scientists are, almost by definition, a conservative bunch. Never ones to stir the pot (with some notable exceptions), it is not uncommon to hear them preface their findings with a fair bit of hedging—throwing in some percentages and equivocations for good measure. Not that this is meant as a criticism. Indeed, the uncertainty that underlies much of climate science, compounded by researchers’ need to rely on imperfect models or data sets, makes any accurate short-term prediction difficult, if not impossible. And that assumes you don’t consider the real wild card in the equation—the possibility of abrupt climate change.

The U.S. Climate Change Science Program defines abrupt climate change as: “a change in the climate that takes place over a few decades or less, persists for at least a few decades, and causes substantial disruptions in human and natural systems.” A good way to wrap your head around this is to think through an analogy. Say you’re sitting in a canoe and you start leaning over the side. At first, you’ll only cause a slight tilt. Continue to lean over, however, and you’ll suddenly cause the canoe to flip over—with you ending up in the water. Now imagine the canoe represents Earth and your leaning over represents fossil fuel use, or any other climate-affecting anthropogenic activity. Push against Earth only slightly, and you may experience rising temperatures or increased aridity; push just a bit harder though, and you may trigger an abrupt climate change. Like flipping an upside-down canoe back over, returning to a normal climate system isn’t easy—and it takes time.

Abrupt climate changes do happen, but at the present moment, scientists do not have a complete picture of how far we can lean over the side of our Earth-shaped canoe before tipping into a spiral of dramatic climactic disruptions. And that makes it all the more importance to curb greenhouse gas emissions sooner rather than later, when they have reached atmospheric concentrations well beyond the horizon of current climate models.

Most of the scientific evidence we have of past abrupt climate changes comes from a variety of sources, including tree rings, pollens, ice caps, and the sediment record. Arguably the best-studied abrupt climate change event, the Younger Dryas, which took place around 12,800 years before present, resulted in the planet being plunged into frigid conditions. Named after Dryas Octopetala, a plant commonly found in colder climates, the period lasted roughly 1,200 years before ending abruptly—following a sudden 10°C temperature jump over the span of only 10 years. While there remains disagreement over the exact cause of the Younger Dryas, the two most oft-cited explanations are a shutdown of the ocean conveyor belt, also called thermohaline circulation, or THC, and the interruption of the El Niño-Southern Oscillation.

The first conjecture is the reason why scientists are so concerned about the Antarctic and Greenland ice sheets melting. The sudden massive influx of freshwater could potentially overwhelm the ocean conveyor belt, plunging the planet once more into a period of near-glacial conditions. Wallace Broecker of the Lamont-Doherty Earth Observatory at Columbia University, one of the world’s most renowned climate experts, and the man who coined the term “global warming,” has called THC the “Achilles heel of our climate system.” In a speech delivered to the World Economic Forum in 2003, Robert Gagosian, the former president of the Woods Hole Oceanographic Institution, warned that its shutdown could cool certain parts of the planet by 3°C to 5°C and cause prolonged droughts elsewhere. The areas most affected by this shutdown would be those bordering the North Atlantic—in effect, some of the world’s most developed countries.

A more recent example of abrupt climate change can be traced back to the 1920s, when a warming of 4°C on the Atlantic side of the Arctic helped precipitate an extended drought during the following decade—now more commonly known as the Dust Bowl. Or take the drought that afflicted Africa’s Sahel region for over two decades, into the early 1980s, and caused a widespread famine that killed millions. The National Oceanographic and Atmospheric Adminstiration’s Geophysical Fluid Dynamics Laboratory, which studied its climatic implications, concluded that changes in sea surface temperatures over large areas—the result of natural climate variations and anthropogenic activities—were to blame.

The ecological impacts of abrupt climate change, as might be expected, can be devastating. Unlike periods of slow, gradual climate change, which afford humans and other organisms some time to adapt to their changing conditions, abrupt climate changes can strike on short notice—leaving long-lived, sedentary organisms at a severe disadvantage. Though lacking in some respects, especially compared to the body of research on the effects of climate change, evidence from the sedimentary record indicates that local extinctions and large-scale ecosystem disturbances were common. Many North American mammal extinctions almost coincide with the advent of the Younger Dryas event, while ecosystem shifts in the northeastern and central Appalachian United States were recorded as taking place less than 50 years following its conclusion. With world population levels now bearing down on 7 billion, an abrupt climate change event would have devastating ecological and economic impacts.

A recent report released by the U.S. Climate Change Science Program considered the potential impacts of four types of abrupt climate change could have on the planet if they took place in the near future: the rapid melting of glaciers and ice sheets, the widespread and sustained changes to the water cycle, changes in the thermohaline circulation, and the release of methane trapped in permafrost and on ocean floors. While the report readily acknowledges that rapid changes in the Earth’s climate will likely persist for the foreseeable future, it concludes that an abrupt climate change is highly unlikely—at least in the short-term. This reflects the views of a majority of climate scientists, who peg the probability of an abrupt climate change event at less than one in five.

What worries some, however—and is one of the main reasons why it remains such a contentious topic—is that our understanding and modeling of key global processes, such as deep water formation, give us an incomplete picture. Few scientists expected the Arctic ice cap to melt as fast as it has—some now predict it could be gone by 2013—and many are troubled by the speed at which methane, a greenhouse gas that is more than 20 times as powerful as carbon dioxide, is being released from thawing permafrost in the Arctic. The only way to avert abrupt climate change, some are now arguing, is to use geoengineering—a set of controversial schemes aimed at tweaking our planet out of harm’s way. Others point to the need for more effective mitigation strategies, paired with a stronger emphasis on research. In the end, it may be that only a combination of these distinct approaches proves sufficient to prevent the next abrupt climate change.

Maybe the National Research Council put it best when it stated the following in its 2002 report: “It is important not to be fatalistic about the threats posed by abrupt climate change.”

Jeremy Jacquot is a graduate student in marine environmental biology at the University of Southern California and is the Los Angeles correspondent for TreeHugger.com.

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