In South Florida, “we’ve had nine inches of sea-level rise since the 1920s.”

Not long after President Barack Obama gave a speech outlining his plan for attacking climate change, Rep. Debbie Wasserman Schultz, D-Fla., appeared on Fox News to discuss the future of energy and the environment.

“We’ve had nine inches of sea-level rise since the 1920s,” said Wasserman Schultz in a June 28, 2013, interview with Tucker Carlson. “What that means is that communities like mine in South Florida and coastal communities all across the country are facing dangerous sea-level rise, which will ultimately cause homes to be under water in just a few short years.”

In this item, we won’t analyze her projections for future sea-level rises; such estimates are based on a variety of theoretical models and come with lots of uncertainty attached. However, we did wonder whether Wasserman Schultz was correct about past changes — specifically, whether South Florida has “had nine inches of sea-level rise since the 1920s.”

First, some background on sea-level rises. “Sea level” refers to the height of the ocean’s surface as measured either by a mechanical tide gauge (a method used since the 19th century) or, since the late 1990s, by satellite measurements. The kind of changes Wasserman Schultz mentioned are not short-term changes such as high tide and low tide, but rather average sea-level measurements taken over the course of many years.

We located sea level data from the National Oceanic and Atmospheric Administration for a variety of locations, including Miami Beach, Fla. Using tide gauge data from 1931 to 1981, NOAA found a change equivalent to 0.78 feet in 100 years. That’s roughly nine inches since the 1920s.

We checked with two scientists who specialize in sea-level measurements — Gary B Griggs, director of the Institute of Marine Sciences at the University of California-Santa Cruz, and Gary Mitchum, a physical oceanographer at the University of South Florida — and both agreed that the NOAA data was an appropriate source for Wasserman Schultz to rely on. Both scientists considered her claim to be accurate.

We will, however, offer two notes of caution when thinking about sea-level rise.

One complication for making these measurements is that the land-based reference point for a tide gauge is not as fixed as it would seem to the naked eye. In reality, the land itself may be rising or falling, masking changes in sea level.

For instance, in some places, such as Alaska or Scandinavia, the land is rising because it’s still rebounding from the retreat of heavy glaciers. In other locations, the slow action of plate tectonics is pushing the land upwards. The effect of land rises can be strong: In places like Alaska, the rise in the land has been faster than the rise in sea level, meaning that sea level is actually dropping in relative terms.

By contrast, in Louisiana, the land is sinking due to a variety of landscape changes made by humans. With sea levels rising and the land sinking, the relative change in sea level is especially dramatic.

South Florida fits somewhere between these two extremes. But because of the variability in land rise from place to place, it’s not appropriate to assume that just because sea level has risen by nine inches in Miami Beach that it has risen by the same amount everywhere.

The second caveat is that past changes in sea level are a whole lot clearer than future changes.

In a 2011 paper, Mitchum estimated that sea level in South Florida would rise 32 inches by 2100, with a smaller possibility of a 40-inch rise. However, other estimates have varied, and Mitchum cautions against assigning too much certainty to long-term estimates of sea-level rise.

Our ruling

Wasserman Schultz said that in South Florida, “we’ve had nine inches of sea-level rise since the 1920s.” That essentially matches the data collected by the National Oceantic and Atmospheric Administration. While there is considerable uncertainty about the future course of sea-level rises, Wasserman Schultz’s estimate of the historical rise appears to be on target. We rate her statement True.