2 What causes sea level change, and why is it rising now?
North Carolina’s coastal zone preserves evidence of both the current rise in sea level and the long decline that preceded it (see the Coastal Wetlands field trip included in this series). Evidence of declining sea level is found in the series of old shorelines that can be found across our coastal plain. These shorelines control the courses of our rivers, the topography and soil types of our land, and the types of natural communities that occur upon them. A complete description of the processes that control sea level change is beyond the scope of this project, but suffice it to say that sea level researchers have identified at least six separate processes that influence sea level. The details of these processes and the time and scales at which they operate are not really needed to understand the sea level change record preserved in our coastal zone.
The largest time-space scale of sea level control involves tectonic processes operating over millions of years to change the volume of ocean basins in ways that force sea water up onto the continents. This largest-scale process is important to us because ninety million years ago it forced sea water over the continent as far west as Raleigh. The processes that brought sea level down from that height are related to climate. As the earth cooled before the last major glaciation, sea water volume decreased, and an increasing fraction of the earth’s water was stored in glaciers on land. At their peak these glaciers were thousands of feet thick and occupied land as far south as Chicago and New York. At that time, about 20,000 years ago, sea level was about 300 feet lower than it is now — the shoreline was at the edge of the continental shelf. Since that time, the climate has warmed, many of the glaciers have melted, and sea level has risen.
This portion of worldwide sea level change is plotted in Figure 1. North Carolina has experienced a rise even larger than the worldwide average, because our coastal lands have been sinking as the glaciers melted. That elevation change occurred when land that had been pushed up by the weight of the thick glaciers sank back down as the glaciers disappeared. The current rate of sea level rise along the North Carolina coast is about twice the worldwide average.