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'1,000-year' flood: Weather hyperbole or hard science?

The extreme flooding in South Carolina led many, including state Gov. Nikki Haley, to invoke the "1,000-year" flood terminology. It's not political rhetoric or apocalyptic hyperbole. It's actually the preferred term used by climate scientists to describe the math behind the tragic events that are still unfolding in the Palmetto State.

The label, used by the National Centers for Environmental Information, is a bit misleading, though. One might believe that a 1,000-year flood actually occurs once every 1,000 years. It doesn't, necessarily.

"It could happen twice in 10 years," said Michael Mann, a professor of meteorology at Penn State University. "And, then it might not happen for another 10,000 years."

The reference is to a statistical measure — 1 in 1,000. It means that there is a 1 in 1,000, or 0.1 percent, chance that such a rain event or flood will occur in any given year. There are also 500-year and 100-year rain events, which have probabilities of occurrence of 0.2 percent and 1 percent, respectively.

Based on historical data, it is possible to estimate how much rainfall would be associated with a very rare event in a given area, Mann said, though there is not a thousand years' worth of rainfall measurements; it's more like a century or so.

Coming up with data for a 1,000-year event, therefore, requires a mathematical function. It's the science version of educated guesswork.

"There's a lot of uncertainty about it," said Kenneth Kunkel, a professor focusing on climate variability at North Carolina State University.

A man kayaks on Tall Pines Circle in Columbia, South Carolina, on Oct. 4, 2015.
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A man kayaks on Tall Pines Circle in Columbia, South Carolina, on Oct. 4, 2015.

The estimates vary depending on whether or not a region typically sees a lot of rain. A community prone to heavy flooding will have a lower threshold, measured in inches of rainfall, for a 1,000-year event. The threshold for a 1,000-year event in Charleston County, South Carolina over a three-day period is 17.1 inches.

The recent flooding in South Carolina actually surpassed this figure. It is of such a high magnitude, (there's been more than 20 inches of rainfall in some areas and the total rainfall has been calculated at 4.4 trillion gallons of water), that its probability of occurring was less than a 10th of a percent.

A more common but related term is "100-year flood," created in the 1960s by the U.S. government as the basis for the National Flood Insurance Program. The United States Geological Service explains on its website that "because the 1 percent AEP flood has a 1-in-100 chance of being equaled or exceeded in any one year, and it has an average recurrence interval of 100 years, it often is referred to as the '100-year flood.' " It also explains the calculation in terms it hopes the public can understand here.

The end of 1,000-year events?

The rainfall estimates for these events are drawn from historical data that precede what many climate scientists consider a pertinent factor today: climate change.

A NOAA blog post written after this past May and June's flooding in Texas and Oklahoma by Thomas Di Liberto, a meteorologist who works with NOAA's Climate Prediction Center and Climate.gov blog, noted that El Niño alone cannot account for the record-breaking nature of the rains. There is also natural variability of the atmosphere — or, in other words, "extreme weather sometimes just happens" — as well as climate change, a warmer climate and a wetter atmosphere.

The NOAA blog post stated, "There has not been a shortage of 1-in-25, -50, -100, even -1,000-year events." It also stated that even though there is only 100 years' worth of observations, doesn't mean you can't do the math on these events. "Remember that just because you don't have all the letters in a 'Wheel of Fortune' puzzle doesn't mean you can't figure out the answer."

According to the National Climate Assessment, and data sourced to Kunkel's team at N.C. State as well as NOAA, the most extreme-precipitation events (those in the 99th percentile of intensity) have increased in every region of the contiguous states since the 1950s.

The South Carolina flooding is the sixth 1,000-year rain event since 2010, according to data provided to USA Today by reinsurance firm AON Benfield, including the Tennessee floods in May 2010, the Mid-Atlantic, Northeast and New England Hurricane Irene flooding in 2011, the Colorado Springs floods in 2013, Baltimore flooding in August 2014, and flooding earlier this year in Nebraska.

Mann said the excessive rainfall in South Carolina can be at least partially attributed to record ocean levels, which added moisture to the air.

That also means while there are more "1,000-year" events likely to come, any increase in frequency would probably put an end to the term now in use to describe them.

"Climate change is actually increasing the likelihood of these events," Mann said. "They are no longer 1,000-year events."