SciAm: Storm Warnings: Extreme Weather Is a Product of Climate Change, Part I

Storm Warnings: Extreme Weather Is a Product of Climate Change

More violent and frequent storms, once merely a prediction of climate models, are now a matter of observation. Part 1 of a three-part series

by John Carey, Scientific American, June 28, 2011

In North Dakota the waters kept rising. Swollen by more than a month of record rains in Saskatchewan, the Souris River topped its all time record high, set back in 1881. The floodwaters poured into Minot, North Dakota's fourth-largest city, and spread across thousands of acres of farms and forests. More than 12,000 people were forced to evacuate. Many lost their homes to the floodwaters.

Yet the disaster unfolding in North Dakota might be bringing even bigger headlines if such extreme events hadn't suddenly seemed more common. In this year alone massive blizzards have struck the U.S. Northeast, tornadoes have ripped through the nation, mighty rivers like the Mississippi and Missouri have flowed over their banks, and floodwaters have covered huge swaths of Australia as well as displaced more than five million people in China  and devastated Colombia. And this year's natural disasters follow on the heels of a staggering litany of extreme weather in 2010, from record floods in Nashville, Tenn., and Pakistan, to Russia's crippling heat wave.

These patterns have caught the attention of scientists at the National Climatic Data Center in Asheville, N.C., part of the National Oceanic and Atmospheric Administration (NOAA). They've been following the recent deluges' stunning radar pictures and growing rainfall totals with concern and intense interest. Normally, floods of the magnitude now being seen in North Dakota and elsewhere around the world are expected to happen only once in 100 years. But one of the predictions of climate change models is that extreme weather—floods, heat waves, droughts, even blizzards—will become far more common. "Big rain events and higher overnight lows are two things we would expect with [a] warming world," says Deke Arndt, chief of the center's Climate Monitoring Branch. Arndt's group had already documented a stunning rise in overnight low temperatures across the U.S. So are the floods and spate of other recent extreme events also examples of predictions turned into cold, hard reality?

Increasingly, the answer is yes. Scientists used to say, cautiously, that extreme weather events were "consistent" with the predictions of climate change. No more. "Now we can make the statement that particular events would not have happened the same way without global warming," says Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

That's a profound change—the difference between predicting something and actually seeing it happen. The reason is simple: The signal of climate change is emerging from the "noise"—the huge amount of natural variability in weather.

Extreme signals
There are two key lines of evidence. First, it's not just that we've become more aware of disasters like North Dakota or last year's Nashville flood, which caused $13 billion in damage, or the massive 2010 summer monsoon in Pakistan that killed 1,500 people and left 20 million more homeless. The data show that the number of such events is rising. Munich Re, one of the world's largest reinsurance companies, has compiled the world's most comprehensive database of natural disasters, reaching all the way back to the eruption of Mount Vesuvius in A.D. 79. Researchers at the company, which obviously has a keen financial interest in trends that increase insurance risks, add 700-1,000 natural catastrophes to the database each year, explains Mark Bove, senior research meteorologist in Munich Re's catastrophe risk management office in Princeton, N.J. The data indicate a small increase in geologic events like earthquakes since 1980 because of better reporting. But the increase in the number of climate disasters is far larger. "Our figures indicate a trend towards an increase in extreme weather events that can only be fully explained by climate change," says Peter Höppe, head of Munich Re's Geo Risks Research/Corporate Climate Center: "It's as if the weather machine had changed up a gear.

The second line of evidence comes from a nascent branch of science called climate attribution. The idea is to examine individual events like a detective investigating a crime, searching for telltale fingerprints of climate change. Those fingerprints are showing up—in the autumn floods of 2000 in England and Wales that were the worst on record, in the 2003 European heat wave that caused 14,000 deaths in France, in Hurricane Katrina—and, yes, probably even in Nashville. This doesn't mean that the storms or hot spells wouldn't have happened at all without climate change, but as scientists like Trenberth say, they wouldn't have been as severe if humankind hadn't already altered the planet's climate. This new science is still controversial. There's an active debate among researchers about whether the Russian heat wave bears the characteristic signature of climate change or whether it was just natural variability, for instance. Some scientists worry that trying to attribute individual events to climate change is counterproductive in the larger political debate, because it's so easy to dismiss the claim by saying that the planet has always experienced extreme weather. And some researchers who privately are convinced of the link are reluctant to say so publicly, because global warming has become such a target of many in Congress.

But the evidence is growing for a link between the emissions of modern civilization and extreme weather events. And that has the potential to profoundly alter the perception of the threats posed by climate change. No longer is global warming an abstract concept, affecting faraway species, distant lands or generations far in the future. Instead, climate change becomes personal. Its hand can be seen in the corn crop of a Maryland farmer ruined when soaring temperatures shut down pollination or the $13 billion in damage in Nashville, with the Grand Ole Opry flooded and sodden homes reeking of rot. "All of a sudden we're not talking about polar bears or the Maldives any more," says Nashville-based author and environmental journalist Amanda  Little. "Climate change translates into mold on my baby's crib. We're talking about homes and schools and churches and all the places that got hit."

Drenched in Nashville
Indeed, the record floods in Nashville in May 2010 shows how quickly extreme weather can turn ordinary life into a nightmare. The weekend began innocuously. The forecast was a 50% chance of rain. Musician Eric Normand and his wife Kelly were grateful that the weather event they feared, a tornado, wasn't anticipated. Eric's Saturday concert in a town south of Nashville should go off without a hitch, he figured.

He was wrong. On Saturday, it rained—and rained. "It was a different kind of rain than any I had experienced in my whole life," says Nashville resident Rich Hays. Imagine the torrent from an intense summer thunderstorm, the sort of deluge that prompts you to duck under an underpass for a few minutes until the rain stops and it's safe to go on, Little says. It was like that, she recalls—except that on this weekend in May 2010 it didn't stop. Riding in the bus with his fellow musicians, Normand "looked through a window at a rain-soaked canopy of green and gray," he wrote later. Scores of cars were underwater on the roads they had just traveled. A short 14-hour bus gig turned out to be "one of the most stressful and terrifying we had ever experienced," Normand says.

And still it rained—more than 13 inches (33 centimeters) that weekend. The water rose in Little's basement—one foot, two feet, three feet (one meter) deep. "You get this panicky feeling that things are out of control," she says. Over at Hays's home, fissures appeared in the basement floor, and streams of water turned into a "full-on river," Hays recalls. Then in the middle of night, "I heard this massive crack, almost like an explosion," he says. The force of the water had fractured the house's concrete foundation. He and his wife spent the rest of the night in fear that the house might collapse.

Sunday morning, Normand went out in the deluge to ask his neighbor if he knew when the power might go back on—it was then he realized that his normal world had vanished. A small creek at the bottom of the hill was now a lake one-half mile (0.8 kilometer) wide, submerging homes almost up to their second stories. "My first reaction was disbelief," Normand says. He and his family were trapped, without power and surrounded by flooded roads. "We were just freaked out," he recalls.

And all across the flooded city the scenes were surreal, almost hallucinatory, Little says. "There were absurdities heaped upon absurdities. Churches lifted off foundations and floating down streets. Cars floating in a herd down highways." In her own basement her family's belongings bobbed like debris in a pond.

By time the deluge ended, more than 13 inches (33 centimeters) of rain had fallen, as recorded at Nashville's airport. The toll: 31 people dead, more than $3 billion in damage—and an end to the cherished perception that Nashville was safe from major weather disasters. "A community that had never been vulnerable to this incredible force of nature was literally taken by storm," Little says.

But can the Nashville deluge, the North Dakota floods and the many other extreme weather events around the world be connected with the greenhouse gases that humans have spewed into the atmosphere? Increasingly the answer seems to be yes. Whereas it will never be possible to say that any particular event was caused by climate change, new science is teasing out both the contributions that it makes to individual events—and the increase in the odds of extreme weather occurring as a result of climate change.

Tomorrow: Part 2, Global Warming and the Science of Extreme Weather

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

http://www.scientificamerican.com/article.cfm?id=extreme-weather-caused-by-climate-change