How much has NC sea level risen in recent years? Answer varies from north to south

Posted by Bruce Siceloff on July 16, 2014 

It’s good to know that North Carolina coastal scientists are starting work on an official state forecast for sea-level rise in the future. But David Kesterson wonders about North Carolina’s past:

“How much has the sea level risen in recent years?”

Kesterson posed his question by email after reading my blog post about plans by the N.C. Coastal Resources Commission science panel to start work next week on a 30-year forecast for sea-level rise, and about a new prediction that sea-level rise will cause heavy flooding on the North Carolina coast in coming decades.

I relayed his question to a science panel member: Spencer Rogers, who is a coastal erosion and construction specialist for N.C. Sea Grant, a coastal research and education program.

The short answer to Kesterson’s question is this:

It depends on where you stand on the North Carolina shoreline. Sea levels have risen in the past 90 years about 15 inches at Duck on the northern Outer Banks, and about 7 inches at Wilmington.

The numbers have varied considerably on the North Carolina coast – more than many people would have guessed – partly because the northern end of our coast is part of a big land mass that has been sinking over past centuries (this is why Norfolk has a terrible flooding problem). But not our southern coast.

Here is the long answer from Spencer Rogers.

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Bruce,

The complicated qualifiers followed by the simplest answer.

The more precise numbers are in other reports that I can reference if necessary. Local sea level change depends on water level and ground elevation changes. Local tide gages measure both. For a gage to be useful it must be in place for more than 20 years. There are three gages in NC with such records: Wilmington, Morehead City and Duck. Durations range from about ~100 years to 30. The average monthly variation, removing storms can be over a foot for various reasons but if you look at the 20-year running averages over the lifetime of each gage, the long term trends are persistent rises without any obviously significant acceleration or deceleration during the varying gage records.

Over the 30-year record at Duck, the historical rate of rise from the Science Panel on Coastal Hazards reports was reported as about 15 inches over 90 years or about 0.17 inches/year (or two nickels/year.) Duck is the only NC gage that clearly has not been affected by dredging so was used in the initial NC report. The Morehead City and Wilmington gages have potentially been impact by major channel depth changes due to the state ports so were not used in the initial reports. The Morehead City gage will probably be determined to not be dredging-impacted in the next update. Wilmington impacts are not yet clear.

For local gage analysis see:

http://tidesandcurrents.noaa.gov/sltrends/

Most geologists believe that the Duck land mass is subsiding (lowering) with time at a relatively steady rate relative to the other NC gages south of Cape Lookout. As Duck appears to be subsiding, it is less clear so far whether the southern land areas are rising. The Morehead City gage indicates a long-term average local sea level rise of ~0.1 inches per year or about 9 inches over 90 years. Wilmington is rising at ~0.08 inches per year (about one nickel/year) or 7.2 inches over 90 years, about half the rate at Duck. Wilmington is still suspected of having been impacted by port dredging which along with the land mass changes will be one of the issues to be considered in the upcoming NC reanalysis.

So…. the tide gage records indicate sea level in NC over the last 30 to 100 years has risen at a relatively stable 20-year average local rate of between 0.08 to 0.17 inches per year (between one to two nickels per year) depending on the location. Port dredging and land subsidence/upheaval have affected the local rate relative to global sea level changes to an, as yet, unknown degree.

As a time reference I prefer the average lifetime of a wood-frame house, about 70 years. You could also phrase it over the average U.S. life expectancy. You can find the number on the internet somewhere.

Spencer

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Thanks to Spencer Rogers for the answer, and to David Kesterson for the question.

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