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.