

In the classic English fable Jack and the Beanstalk, the intrepid protagonist risks being devoured on sight in order to repeatedly raid the home of a flesh-eating giant for gold. All goes well until the snoring giant awakens and gives furious chase. But Jack beats him back down the magic beanstalk and chops it down with an axe, toppling the descending cannibal to its death. Jack thus wins back his life plus substantial economic profit from his spoils.

Industrialized society has also reaped enormous economic and social benefit from fossil fuels, so far without rousing any giants. But as geoscientists, my colleagues and I devote much of our time to worrying about whether they might be slumbering in the Earth's climate system.

We used to think climate worked like a dial — slow to heat up and slow to cool down — but we've since learned it can also act like a switch. Twenty years ago anyone who hypothesized an abrupt, show-stopping event — a centuries-long plunge in air temperature, say, or the sudden die-off of forests — would have been laughed off. But today, an immense body of empirical and theoretical research tells us that sudden awakenings are dismayingly common in climate behavior.

Ancient records preserved in tree rings, sediments, glacial ice layers, cave stalactites, and other natural archives tells us that for much of the past 10,000 years — the time when our modern agricultural society evolved — our climate was remarkably stable. Before then it was it was capable of wild fluctuations, even leaping eighteen degrees Fahrenheit in ten years. That's as if the average temperature in Minneapolis warmed to that of San Diego in a single decade.

Even during the relative calm of recent centuries, we find sudden lurches that exceed anything in modern memory. Tree rings tell us that in the past 1,000 years, the western United States has seen three droughts at least as bad as the Dust Bowl but lasting three to seven times longer. Two of them may have helped collapse past societies of the Anasazi and Fremont people.

The mechanisms behind such lurches are complex but decipherable. Many are related to shifting ocean currents that slosh around pools of warm or cool seawater in quasi-predictable ways. The El Niño/La Niña phenomenon, which redirects rainfall patterns around the globe, is one well-known example. Another major player is the Atlantic thermohaline circulation (THC), a massive density-driven "heat conveyor belt" that carries tropical warmth northwards via the Gulf Stream. The THC is what gifts Europe with relatively balminess despite being as far north as some of Canada's best polar bear habitat.

If the THC were to weaken or halt, the eastern U.S. and Europe would become something like Alaska. While over-sensationalized by The Day After Tomorrow film and a scary 2003 Pentagon document imagining famines, refugees, and wars, a THC shutdown nonetheless remains an unlikely but plausible threat. It is the original sleeping giant of my field.

Unfortunately, we are discovering more giants that are probably lighter sleepers than the THC. Seven others — all of them potential game-changers — are now under scrutiny: (1) the disappearance of summer sea-ice over the Arctic Ocean, (2) increased melting and glacier flow of the Greenland ice sheet, (3) "unsticking" of the frozen West Antarctic Ice Sheet from its bed, (4) rapid die-back of Amazon forests, (5) disruption of the Indian Monsoon, (6) release of methane, an even more potent greenhouse gas than carbon dioxide, from thawing frozen soils, and (7) a shift to a permanent El Niño-like state. Like the THC, should any of these occur there would be profound ramifications — like our food production, the extinction and expansion of species, and the inundation of coastal cities.

To illustrate, consider the Greenland and Antarctic ice sheets. The water stored in them is enormous, enough to drown the planet under more than 200 feet of water. That will not happen anytime soon but even a tiny reduction in their extent — say, five percent — would significantly alter our coastline. Global sea level is already rising about one-third of a centimeter every year and will reach at least 18 to 60 centimeters higher just one long human lifetime from now, if the speeds at which glaciers are currently flowing from land to ocean remain constant. But at least two warming-induced triggers might speed them up: percolation of lubricating meltwater down to the glaciers' beds; and the disintegration of floating ice shelves that presently pin glaciers onto the continent. If these giants awaken happen our best current guess is 80 to 200 centimeters of sea level rise. That's a lot of water. Most of Miami would either be surrounded by dikes or underwater.

Unfortunately, the presence of sleeping giants makes the steady, predictable growth of anthropogenic greenhouse warming more dangerous, not less. Alarm clocks may be set to go off, but we don't what their temperature settings are. The science is too new, and besides we'll never know for sure until it happens. While some economists predicted that rising credit-default swaps and other highly leveraged financial products might eventually bring about an economic collapse, who could have foreseen the exact timing and magnitude of late 2008? Like most threshold phenomena it is extremely difficult to know just how much poking is needed to disturb sleeping giants. Forced to guess, I'd mutter something about decades, or centuries, or never. On the other hand, one might be stirring already: In September 2007, then again in 2008, for the first time in memory nearly 40% of the later-summer sea ice in the Arctic Ocean abruptly disappeared.

Unlike Jack, the eyes of scientists are slow to adjust to the gloom. But we are beginning to see some outlines and unfortunately, discern not one but many sleeping forms. What is certain is that our inexorable loading of the atmosphere with heat-trapping greenhouse gases increases the likelihood that one or more of them will wake up.