And more events seem to be in the news every month, from unprecedented floods in Riyadh, Saudi Arabia, to massive snowstorms that crippled the U.S. Northeast in early 2011, to the November 2010 to January 2011 torrents in Australia that flooded an area the size of Germany and France . This "disaster of biblical proportions," as local Australian officials called it, even caused global economic shock waves: The flooding of the country's enormously productive coal mines sent world coal prices soaring.
More stormy weather
More moisture and energy in the atmosphere, along with warmer ocean temperatures also mean more intense hurricanes, many scientists say. In fact, 2010 was the first year in decades in which two simultaneous category 4 hurricanes, Igor and Julia, formed in the Atlantic Ocean. In addition, the changed conditions bring an increased likelihood of more powerful thunderstorms with violent updrafts, like a July 23, 2010, tempest in Vivian, S.D., that produced hailstones that punched softball-size holes through roofs—and created a behemoth ball of ice measured at a U.S. record 8 inches (20 centimeters) in diameter even after it had partially melted. "I've never seen a storm like that before—and hope I'll never go through anything like it," says Les Scott, the Vivian farmer and rancher who found the hailstone .
Warming the planet alters large-scale circulation patterns as well. Scientists know that the sun heats moist air at the equator, causing the air to rise. As it rises, the air cools and sheds most of its moisture as tropical rain. Once six to 10 miles (9.5 to 16 kilometers) aloft, the now dry air travels toward the poles, descending when it reaches the subtropics, normally at the latitude of the Baja California peninsula. This circulation pattern, known as a Hadley cell, contributes to desertification, trade winds and the jet stream.
On a warmer planet, however, the dry air will travel farther north and south from the equator before it descends, climate models predict, making areas like the U.S. Southwest and the Mediterranean even drier. Such an expanded Hadley cell would also divert storms farther north. Are the models right? Richard Seager of Columbia University's Lamont–Doherty Earth Observatory has been looking for a climate change–induced drying trend in the Southwest, "and there seems to be some tentative evidence that it is beginning to happen," he says. "It gives us confidence in the models." In fact, other studies show that the Hadley cells have not only expanded, they've expanded more than the models predicted.
Such a change in atmospheric circulation could explain both the current 11-year drought in the Southwest and Minnesota's status as the number one U.S. state for tornadoes last year. On October 26, 2010, the Minneapolis area even experienced record low pressure in what Paul Douglas, founder and CEO of WeatherNation in Minnesota, dubbed a "landicane"—a hurricanelike storm that swept across the country. "I thought the windows of my home would blow in," Douglas recalls. "I've chased tornados and flown into hurricanes but never experienced anything like this before." Yet it makes sense in the context of climate change, he adds. "Every day, every week, another piece of the puzzle falls into place," he says. "More extreme weather seems to have become the rule, not just in the U.S. but in Europe and Asia."
The rise of climate attribution
Is humankind really responsible? That's where the burgeoning field of climate attribution, pioneered by Hadley's Peter Stott and other scientists, comes in. The idea is to look for trends in the temperature or precipitation data that provide evidence of overall changes in climate. When those trends exist, it then becomes possible to calculate how much climate change has contributed to extreme events. Or in more technical terms, the probability of a particular temperature or rainfall amount is shaped roughly like a bell curve. A change in climate shifts the whole curve. That, in turn, increases the likelihood of experiencing the more extreme weather at the tail end of the bell curve. Whereas day-to-day weather remains enormously variable, the underlying human-caused shift in climate increases the power and number of the events at the extreme. The National Oceanic and Atmospheric Administration's (NOAA) Deke Arndt puts it more colorfully: "Weather throws the punches, but climate trains the boxer," he says. By charting the overall shift, then, it's possible to calculate the increased chances of extreme events due to global warming.