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Rising seas, geologic faults present a shore uncertainty

Posted: June 26, 2013 - 12:02am

Hurricane Sandy has been described as a harbinger of what comes with rising seas: the inundation of coastal cities, devastating storm surges, destruction of coastal wetlands and abandonment of land. The story is simple: Glaciers melt and oceans warm, causing seas to rise.

The reality is anything but simple. The sea has been rising since the last ice age, but at a variable and poorly known rate. Long-term measurements are necessary to establish an accurate trend, but measurements were few in the 19th and early 20th centuries, especially in the Southern Hemisphere. More than two-thirds of the 322 world sea level records listed on the National Oceanic and Atmospheric Administration (NOAA) Web site are 50 years or less. Only 11 percent are more than 100 years, none of those south of the equator.

Sea rise is complicated because it is uneven and relative. It is uneven in time, space and certainty. All sea level measurements based on the shore are relative to that shore. Some shores are rising, some are sinking. As a 1986 editorial in the Journal of Coastal Research noted, “Many of the world’s great cities, including London, New Orleans, Tokyo, Houston, and Mexico City, are sinking.”

All news sources know that New Orleans has the most to fear from rising seas. They apparently don’t know that the concern is not new. Civil engineer E.L. Corthell reported in National Geographic in 1897 that a Spanish magazine built 200 years earlier below New Orleans was standing in 10 feet of water by 1877. He concluded from this information and an 1894 Mississippi River Commission report that the Mississippi Delta was sinking about 5 feet per century.

More than a century later, not much has changed. Some sections of the lower Mississippi Delta are sinking at rates of 0.8 inches or more per year; some parts of New Orleans are already 8 feet below sea level. The average sea rise of 1 to 2 mm per year (4-8 inches per 100 years, which the Intergovernmental Panel on Climate Change estimates for the 20th century) is hardly New Orleans’ biggest problem. It is sinking several times as fast.

The threats of wetland loss and coastal flooding caused by climate change are considered something new and devastating. But, as Corthell wrote in that century-old report, “It is a fact well-known to people living in the delta of the Mississippi that large tracts of land were long ago abandoned in consequence of overflow by Gulf waters, due to the sinking of the lands.”

The south Mississippi Delta was built over many centuries by sediment brought down from mid-continent by the river and deposited in the salt marsh. The sediment is loose or uncompacted and mixed with vegetation, but over the centuries it settles, compacts and sinks.

A similar process occurs in estuaries all over the Earth where land subsidence complicates sea-level rise. Subsidence on the Gulf Coast extends from the Panhandle of Florida to the south Texas coast. In the Chesapeake Bay, subsidence accounts for about half of the observed sea level rise.

An opposite complication of sea level measurement occurs over extensive areas of the earth where land is rising. This can occur along geologic faults, but also over large areas covered by thick ice during the last ice age. The Earth’s crust is not completely immovable; it floats on a molten core. As the ice age ended, tremendous weight was removed from the surface. The land rebounded and still is rebounding.

Coasts of some countries are losing land to the sea, others are gaining. The town of Churchill on Hudson Bay, large portions of Scandinavia and Finland coasts, and the southeast coast of Alaska have falling sea levels, relative to land surface. Skagway, Alaska, is rising as fast as the Mississippi Delta is sinking. The North Ireland coast has slightly falling seas. Twelve of 32 stations in China showed falling seas over the 20 to 80 years preceding 1993.

NOAA knows the variability. On its Web site it warns: “Over the next century, sea level rise within the United States is expected to vary between -6 to +4 feet, depending on location.” The causes and variability of sea level rise are so complicated that hundreds of scientific papers are written every year to clarify it. Unfortunately, as the discussion after Hurricane Sandy revealed, too often, “experts” and activists try to simplify the process to float their own agendas.

(University of Georgia Professor Emeritus R. Harold Brown is a Senior Fellow with the Georgia Public Policy Foundation.)

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