Editor's note: This article is the second of a three-part series by John Carey. Part 1, posted on June 28, is "Storm Warning: Extreme Weather Is a Product of Climate Change".

Extreme floods, prolonged droughts, searing heat waves, massive rainstorms and the like don't just seem like they've become the new normal in the last few years—they have become more common, according to data collected by reinsurance company Munich Re (see Part 1 of this series). But has this increase resulted from human-caused climate change or just from natural climatic variations? After all, recorded floods and droughts go back to the earliest days of mankind, before coal, oil and natural gas made the modern industrial world possible.

Until recently scientists had only been able to say that more extreme weather is "consistent" with climate change caused by greenhouse gases that humans are emitting into the atmosphere. Now, however, they can begin to say that the odds of having extreme weather have increased because of human-caused atmospheric changes—and that many individual events would not have happened in the same way without global warming. The reason: The signal of climate change is finally emerging from the "noise"—the huge amount of natural variability in weather.

Scientists compare the normal variation in weather with rolls of the dice. Adding greenhouse gases to the atmosphere loads the dice, increasing odds of such extreme weather events. It's not just that the weather dice are altered, however. As Steve Sherwood, co-director of the Climate Change Research Center at the University of New South Wales in Australia, puts it, "it is more like painting an extra spot on each face of one of the dice, so that it goes from 2 to 7 instead of 1 to 6. This increases the odds of rolling 11 or 12, but also makes it possible to roll 13."

Why? Basic physics is at work: The planet has already warmed roughly 1 degree Celsius since preindustrial times, thanks to CO2and other greenhouse gases emitted into the atmosphere. And for every 1-degree C (1.8 degrees Fahrenheit) rise in temperature, the amount of moisture that the atmosphere can contain rises by 7 percent, explains Peter Stott, head of climate monitoring and attribution at the U.K. Met Office's Hadley Center for Climate Change. "That's quite dramatic," he says. In some places, the increase has been much larger. Data gathered by Gene Takle, professor of meteorology at Iowa State University in Ames, show a 13 percent rise in summer moisture over the past 50 years in the state capital, Des Moines.

The physics of too much rain
The increased moisture in the atmosphere inevitably means more rain. That's obvious. But not just any kind of rain, the climate models predict. Because of the large-scale energy balance of the planet, "the upshot is that overall rainfall increases only 2 to 3 percent per degree of warming, whereas extreme rainfall increases 6 to 7 percent," Stott says. The reason again comes from physics. Rain happens when the atmosphere cools enough for water vapor to condense into liquid. "However, because of the increasing amount of greenhouse gases in the troposphere, the radiative cooling is less efficient, as less radiation can escape to space," Stott explains. "Therefore the global precipitation increases less, at about 2 to 3 percent per degree of warming." But because of the extra moisture, when precipitation does occur (in both rain and snow), it's more likely to be in bigger events.

Iowa is one of many places that fits the pattern. Takle documented a three- to seven-fold increase in high rainfall events in the state, including the 500-year Mississippi River flood in 1993, the 2008 Cedar Rapids flood as well as the 500-year event in 2010 in Ames, which inundated the Hilton Coliseum basketball court in eight feet (2.5 meters) of water . "We can't say with confidence that the 2010 Ames flood was caused by climate change, but we can say that the dice are loaded to bring more of these events," Takle says.

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.

This idea was already in the air in 2003 when Stott traveled though the worst heat wave in recorded European history on a wedding anniversary trip to Italy and Switzerland. One of the striking consequences he noticed was that the Swiss mountains were missing their usual melodious tinkling of cowbells. "There was no water in the mountains, and the farmers had to take all their cows down in the valley," he says. He decided to see if he could pin part of the blame on climate change after he returned to his office in Exeter, England. "I didn't expect to get a positive result," he says

But he did. In fact, the signal of a warming climate was quite clear in Europe, even using data up to only 2000. In a landmark paper in Nature Stott and colleagues concluded that the chances of a heat wave like the 2003 event have more than doubled because of climate change. (Scientific American is part of Nature Publishing Group.) Data collected since then show that the odds are at least four times higher compared with pre-industrial days. "We are very aware of the risks of misattribution," Stott says. "We don't want to point to specific events and say that they are part of climate change when they really are due to natural variability. But for some events, like the 2003 heat wave, we have the robust evidence to back it up."

Case in point: Hurricane Katrina
Another event with a clear global warming component, says Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., was Hurricane Katrina. Trenberth calculated that the combination of overall planetary warming, elevated moisture in the atmosphere, and higher sea-surface temperatures meant that "4 to 6 percent of the precipitation—an extra inch [2.5 centimeters] of rain—in Katrina was due to global warming," he says. "That may not sound like much, but it could be the straw that breaks the camel's back or causes a levee to fail." It was also a very conservative estimate. "The extra heat produced as moisture condenses can invigorate a storm, and at a certain point, the storm just takes off," he says. "That would certainly apply to Nashville." So climate change's contribution to Katrina could have been twice as high as his calculations show, he says. Add in higher winds to the extra energy, and it is easy to see how storms can become more damaging.

This science of attribution is not without controversies. Another case in point: the 2010 Russian heat wave, which wiped out one quarter of the nation's wheat crop and darkened the skies of Moscow with smoke from fires. The actual meteorological cause is not in doubt. "There was a blocking of the atmospheric circulation," explains Martin Hoerling, a research meteorologist at the NOAA's Earth System Research Laboratory, also in Boulder. "The jet stream shifted north, bringing a longer period of high pressure and stagnant weather conditions." But what caused the blocking? Hoerling looked for an underlying long-term temperature trend in western Russia that might have increased the odds of a heat wave, as Stott had done for the 2003 European event. He found nothing. "The best explanation is a rogue black swan—something that came out of the blue," he says.

Wrong, retorts NCAR's Trenberth. He sees a clear expansion of the hot, dry Mediterranean climate into western Russia that is consistent with climate change predictions—and that also intensified the Pakistan monsoon. "I completely repudiate Marty—and it doesn't help to have him saying you can't attribute the heat wave to climate change," he says. "What we can say is that, as with Katrina, this would not have happened the same way without global warming."

Yet even this dispute is smaller than it first appears. What is not in doubt is that the Russian heat wave is a portent—a glimpse of the future predicted by climate models. Even Hoerling sees it as a preview of coming natural disasters. By 2080, such events are expected to happen, on average, once every five years, he says: "It's a good wake-up call. This type of phenomenon will become radically more common."

Tomorrow: Part 3: "Our Extreme Future: Predicting and Coping with a Changing Climate."

Reporting for this story was funded by Pew Center on Global Climate Change