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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.