Sea level change

George M. Cole,
Aucilla Research Institute
Contributor

Due to the sometimes misleading discussions of sea level change in the popular media, it is helpful to take a good look at the actual facts. This writing attempts to provide that look, using an examination of evidence in the coastal plain of Jefferson County as a starting point.
Over the history of the Earth, there have been a continual series of long-term climate cycles. These have included periods with cooler temperatures, commonly known as ice ages, followed by periods with warmer temperatures. These periods lasted for thousands of years. During the cooler periods, a large percentage of the waters of the Earth was locked up in glaciers resulting in lower sea levels. Then, during the warmer interglacial periods, the melting glaciers resulted in higher sea levels. Evidence of these periods is visible in Jefferson County.

High Water Lines in Jefferson

The Florida Geological Survey (FGS) has identified three past high-water marks visible in Jefferson County. The most prominent of these is the very visible Cody Escarpment (Scarp) which has changes in elevation as much as 100 feet in places. This feature lies about 15 miles north of the current shoreline. The base of the scarp is roughly 40-45 feet above the current mean sea level in our area and was formed during the interglacial period between two past ice ages.
Moving closer to the current shoreline, another, somewhat smaller, scarp, identified by the FGS as the Pamlico Shoreline, lies about 10 miles north of the current shoreline and was also formed during a past interglacial period. This feature is especially noticeable near the Fanlew Community where it occurs at an elevation of about 26 to 30 feet above current sea level with about 8 or 10 feet of relief.
The FGS has also identified another, far more recent, scarp named the Silver Bluff. That feature occurs at an elevation of about 10 feet above the current sea level and lies about 4 to 6 miles north of the current shoreline. Similar to the other two features, this scarp also represents the high water mark of a past sea level stand during a warm period.

Sea Level Change Since the Last Ice Age
During the last ice age which ended about 20,000 years ago, sea level was much lower than today. At that time, the Gulf of Mexico shoreline is believed to have been as much as 90 miles southerly of its current location. A study of sea level rise after that time, based on geological evidence, has also been conducted by the FGS. A plot of the data from that study is shown below.

The data from that study indicates that, after rising at a slower rate for several thousand years, sea level began to rise fairly rapidly as the glaciers melted. Periods with rises as high as 20 – 40 mm/year were experienced at times. Then, the rate of rise gradually slowed to an average rate of about 2 mm/year in the last 5000 years or so. During those periods, there were numerous short-term trends with sea level trends considerably different from the averages. As examples: a downward trend in sea level, between 12,835 and 11,735 years before present, has been associated with a sudden short-term cooling of the earth, believed to have been triggered by the impact of an asteroid with Earth. That cooling is believed to have caused the sudden extinction of the mammoths and a decline of the Clovis population in North America. Data from that study also suggests that sea level rose above the current levels for short periods in recent times. Other fairly recent unusual trends indicated by this study include a period of rapid rise between 1000 – 700 years before present (950 – 1250 AD) during the so-called “medieval warm period”; and another of rapid decline during the “Little Ice Age” from 650 – 100 years before present (1300 - 1850 AD) when much of the world was subjected to cooler weather.

Recent Sea Levels
In the last two centuries, technology has allowed the direct measurement of sea level trends. The U.S. Coast and Geodetic Survey (now part of NOAA) began installing mechanical tide gauges in 1854. In addition, other nations have operated gauges. While there are no long-term gauges along the Jefferson County coastline, data from nearby stations give some idea of the current rate. To the southeast, the NOAA gauge at Cedar Key shows a current sea level rise of 2.3 mm/year and the gauge at Apalachicola shows a rate of 2.8 mm/year. This suggests that the rate along the Jefferson County shoreline is somewhere in that range.
Interestingly, the data from the tide gauges show considerable variation in the rates of sea level change at the various locations. As one example, at a NOAA gauge along the Louisiana coast, there is an apparent rise in sea level of 9.2 mm/year. Yet, at Juneau, Alaska, a gauge there reveals a decline in apparent mean sea level of 13.4 mm/year. By the use of GPS observations, it has been determined that the coastal area around the Louisiana gauge is sinking. Similarly, it has been determined that the ground along the Alaskan shore is rising. After corrections are applied, both stations have been determined to have sea level rise of around 2 mm/year. These examples illustrate that local geologic mechanics are often a significant factor in apparent sea level rise. Based on average world-wide tide gauge records, the average current rate of sea level rise is currently in the 2-3 mm/year range.
The bottom line of this writing is that both sea levels and the crust of the Earth have always and continue to be in constant motion. Some scientists believe that human activity has affected the current rate. Nevertheless, the current rate of sea level change has been relatively constant and is considerable less than some rates experienced in the past, well before extensive human activity. What a study of pre-historic and current sea level changes does teach us, however, is that long-term development should be avoided in coastal zones such as in southern Jefferson County.
This story is based on a recently published book by the writer, Sea Levels and Lines.