National Geographic : 2012 Sep
• more of damage each, far exceeding the previous record of nine such disasters in 2008. What's going on? Are these extreme events signals of a dangerous, human-made shi in Earth's climate? Or are we just going through a natural stretch of bad luck? e short answer is: probably both. e pri- mary forces driving recent disasters have been natural climate cycles, especially El Niño and La Niña. Scientists have learned a lot during the past few decades about how that strange seesaw in the equatorial Paci c a ects weather world- wide. During an El Niño a giant pool of warm water that normally sits in the central Paci c surges east all the way to South America; during a La Niña it shrinks and retreats into the west- ern Paci c. Heat and water vapor coming o the warm pool generate thunderstorms so powerful and towering that their in uence extends out of the tropics to the jet streams that blow across the middle latitudes. As the warm pool shi s back equipment, including components for a 36-by- 60-foot video screen that had been assembled for Brad Paisley's upcoming concert tour, which was set to begin in less than three weeks. "Every amp, every guitar I'm used to, was destroyed," Paisley says. "I felt powerless in a way I've never felt before with weather." e experience changed him. "Here in Nash- ville our weather is manageable, normally," he says. "But since that ood, I've never once taken normalcy for granted." ' in the weather. Ex- treme events like the Nashville ood---described by o cials as a once-in-a-millennium occur- rence---are happening more frequently than they used to. A month before Nashville, torrential downpours dumped 11 inches of rain on Rio de Janeiro in 24 hours, triggering mud slides that buried hundreds. About three months af- ter Nashville, record rains in Pakistan caused ARE WE SEEING A DANGEROUS SHIFT IN CLIMATE? OR JUST A NATURAL STRETCH OF BAD LUCK? ooding that a ected more than 20 million peo- ple. In late 2011 oods in ailand submerged hundreds of factories near Bangkok, creating a worldwide shortage of computer hard drives. And it's not just heavy rains that are making headlines. During the past decade we've also seen severe droughts in places like Texas, Australia, and Russia, as well as in East Africa, where tens of thousands have taken refuge in camps. Deadly heat waves have hit Europe, and record numbers of tornadoes have ripped across the United States. Losses from such events helped push the cost of weather disasters in 2011 to an estimated $150 billion worldwide, a roughly 25 percent jump from the previous year. In the U.S. last year a record 14 events caused a billion dollars or and forth along the Equator, the wavy paths of the jet streams shi north and south---which changes the tracks that storms follow across the continents. An El Niño tends to push drenching storms over the southern U.S. and Peru while visiting drought and re on Australia. In a La Niña the rains ood Australia and fail in the American Southwest and Texas---and in even more distant places like East Africa. ose outcomes aren't mechanical and invari- able; the atmosphere and ocean are chaotic uids, and other oscillations in uence the weather at a given time and place. e tropical Paci c is especially in uential, though, because it pumps so much heat and water vapor into the atmosphere. Extreme El Niños or La Niñas thus set the stage for extreme events elsewhere. But natural cycles can't by themselves explain the recent streak of record-breaking disasters. Senior Editor Peter Miller wrote the January cover story on the scienti c study of twins.