North Carolina sea levels rising 3mm a year? UC sea level data says differently

Below: North Carolina’s Albemarle Sound.

Note marker at 36N -76W.

First the Press Release from the University of Pennsylvania:

North Carolina Sea Levels Rising Three Times Faster Than in Previous 500 Years, Penn Study Says

October 28, 2009

PHILADELPHIA –- An international team of environmental scientists led by the University of Pennsylvania has shown that sea-level rise, at least in North Carolina, is accelerating. Researchers found 20th-century sea-level rise to be three times higher than the rate of sea-level rise during the last 500 years. In addition, this jump appears to occur between 1879 and 1915, a time of industrial change that may provide a direct link to human-induced climate change.

The results appear in the current issue of the journal Geology.

The rate of relative sea-level rise, or RSLR, during the 20th century was 3 to 3.3 millimeters per year, higher than the usual rate of one per year. Furthermore, the acceleration appears consistent with other studies from the Atlantic coast, though the magnitude of the acceleration in North Carolina is larger than at sites farther north along the U.S. and Canadian Atlantic coast and may be indicative of a latitudinal trend related to the melting of the Greenland ice sheet.

Understanding the timing and magnitude of this possible acceleration in the rate of RSLR is critical for testing models of global climate change and for providing a context for 21st-century predictions.

“Tide gauge records are largely inadequate for accurately recognizing the onset of any acceleration of relative sea-level rise occurring before the 18th century, mainly because too few records exist as a comparison,” Andrew Kemp, the paper’s lead author, said. “Accurate estimates of sea-level rise in the pre-satellite era are needed to provide an appropriate context for 21st-century projections and to validate geophysical and climate models.”

The research team studied two North Carolina salt marshes that form continuous accumulations of organic sediment, a natural archive that provides scientists with an accurate way to reconstruct relative sea levels using radiometric isotopes and stratigraphic age markers. The research provided a record of relative sea-level change since the year 1500 at the Sand Point and Tump Point salt marshes in the Albemarle-Pamlico estuarine system of North Carolina. The two marshes provided an ideal setting for producing high-resolution records because thick sequences of high marsh sediment are present and the estuarine system is microtidal, which reduces the vertical uncertainty of

paleosea-level estimates. The study provides for the first time replicated sea-level reconstructions from two nearby sites.

In addition, comparison with 20th-century tide-gauge records validates the use of this approach and suggests that salt-marsh records with decadal and decimeter resolution can supplement tide-gauge records by extending record length and compensating for the strong spatial bias in the global distribution of longer instrumental records.

The study was funded by the National Oceanic and Atmospheric Administration Coastal Ocean Program, North Carolina Coastal Geology Cooperative Program, U.S. Geological Survey and National Science Foundation.

The study was conducted by Kemp and Benjamin P. Horton of the Sea-Level Research Laboratory at Penn, Stephen J. Culver and D. Reide Corbett of the Department of Geological Sciences at East Carolina University, Orson van de Plassche of Vrije Universiteit, W. Roland Gehrels of the University of Plymouth, Bruce C. Douglas of Florida International University and Andrew C. Parnell of University College Dublin.

I was curious, because this seemed a bit “off” to me based on other data that I’ve seen. So I went to the University of Colorado Sea Level data server and entered the coordinates for Albemarle Sound (36N -76W or in their usage 36N 284W).

The graph they serve up looks like this:

It’s low resolution, but does look rather flat. Fortunately they provide the data with the plot. You can read all about the Topex/Poseidon data preparation here.

I took that raw data and plotted it here in an expanded size and did a trend line, shown below:

The result was surprising. A slight negative trend.

I chose a different location to get closer to Pamlico Sound, also cited in the study. Unfortunately the interactive tool at UC is coarse on lat/lon and the closest I could get was 35N -76W, just off the outer banks.

The data from that point is plotted below. The source data for 35N -76W  is available here.

Apologies for the slight cosmetic differences in line size between the two graphs. I had a computer reset between sessions and lost some settings.

So, if there is 3mm rise per year recently, since 1992, we certainly can’t see it. I can’t say anything for the other years in the study.

But in the press release they say:

The rate of relative sea-level rise, or RSLR, during the 20th century was 3 to 3.3 millimeters per year, higher than the usual rate of one per year.

If that is true, then the rate appears to have slowed significantly in the late 20th century to present. For 35N, -76W, the 1.12mm/yr rate certainly looks like the “…usual rate of one per year”.