Gov. Nikki Haley said on Sunday that, “We haven’t seen this level of rain in the Lowcountry in a thousand years, that’s how big this is.” In one sense, Gov. Haley is right: This is big. But her statement about the weather is not true.
Regular, reliable weather observations in the United States are barely 150 years old. There are no weather data that take us back to the year 1015. The term “1,000-year flood” refers to probabilities; it is a flood that has a 0.1 percent chance of occurring in any given year. Experts devote their professional lives to calculating those probabilities, and insurance companies use them to determine their risk liability.
To put it in its simplest terms, Gov. Haley’s statement about a 1,000-year level of rainfall bears no relationship to any recorded event. Rather, it is based on the same sort of predictive modeling that tells us global climate change is underway.
The 2014 National Climate Assessment describes how the number of annual extreme weather events has trended upward sharply since the 1960s. It also describes extreme rainfall events as increasing by 30 percent over the same period. “The risks from future floods are significant,” it says, and coastal South Carolina should expect a 20 percent increase in rainfall by 2099. Sea levels will rise by nearly 4 feet.
The same statistical methods that describe 1,000-year events for insurance companies and governors also predict these catastrophic changes that probably lie just down the road for us.
In 2014, Gov. Haley opposed EPA regulations that would require South Carolina to cut in half by 2030 its carbon emissions — the pollutants thought to contribute most to climate change. “This is exactly what we don’t need,” she said. “This is exactly what hurts us.”
The data tell us how much more frequently we should expect flooding events like what happened across the state this weekend. We will need to rethink what a 1,000-year flood means.
And as we survey the economic and human cost of what has happened, perhaps finally we will rethink exactly what it is that we need and just what hurts us.
Steven P. Millies