A land of many wetlands

The Atlantic Ocean began to form about 200 million years ago. Click the image to see the full sequence of globes. (From This Dynamic Earth: The Story of Plate Tectonics, published by the U.S. Geological Survey. More about the illustration)

Eastern North Carolina is a land of many wetlands. More than forty different types have been identified by botanists with the state’s Natural Heritage Program.

Geographically, this wetland heritage was achieved in the most straightforward way: all of the land east of Raleigh was once flooded by the Atlantic Ocean. The Atlantic formed when North America separated from Africa more than 200 million years ago. At the time of this separation, its level was more or less where it is now, but before it reached half its current age, it had risen over 200 meters. The highest sea level was reached about 90 million years ago, and its long-term trend has been downward ever since. This has been shown by a number of studies, most persuasively by K.O. Emery and David Aubrey in their 1991 book entitled Sea Levels, Land Levels and Tide Gauges.

“But wait,” you say, “all the newspapers report that sea level is rising!” Both you and the newspapers are right. In the “short term,” sea level has been rising since the world’s last major glaciation event ended about 20,000 years ago. This rise has been caused both by the ocean becoming warmer, and by the transformation of glacial ice into liquid sea water.

When these last glaciers formed, sea level fell dramatically all across the world. At the peak of the glaciation, sea level was more than 300 feet lower than it is now. At that time, mastodons and other large herbivores ranged across what is now the sea floor of our continental shelf. As the climate warmed, however, sea level began to rise. At first, it rose quite quickly, about 10 mm per year. Then, about 3,500 years ago, the rate of global sea level rise fell to about 1.4 mm per year. This slower rise seems to have continued to the present, although many scientists think the rate of rise will increase if global average temperature rises in the coming decades. Both the fall and the rise have had important consequences for development of coastal wetlands.

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