When most people think of sea level rise, they probably imagine a coastline slowly creeping across the land, so that the edge of the sea simply passes by a resort parking lot or a coastal road, perhaps encroaching on a coastal neighborhood and flooding homes and companies over time.
That is kind of scary, but also completely unrealistic. The reality of sea level rise, what it is and what it looks like, is very different – and much scarier. The water will not just rise gently over an unchanging landscape.
We recently saw this in horrifying detail. Mediterranean storm Daniel, which hit the African coast near Derna in Libya, shifted the coastline significantly inland – up to several meters in some places. The inland floods, the severity of which was increased by the construction of human dams, changed the configuration and extent of the eroded river basins. Several poorly maintained dams burst in the early hours of the morning while most residents were sleeping. Large areas of human settlement were wiped out by floods, causing widespread death and destruction.
Over the last century, global sea levels have risen by almost a foot (although this varies from place to place). As we changed the chemical composition of the atmosphere by adding excess CO2, mostly from the burning of fossil fuels, we wrote a promissory note on behalf of the countless homes on the sea cliffs, the offshore islands, the fishing villages and seaports, and many picturesque seaside villages. Rising sea levels will eventually reduce this claim due to constant wave and wind movements and occasional violent storms.
In most places, what we know as “land” consists of sediment that would be rapidly eroded by an encroaching shoreline. Behind most beaches there is a cliff, sometimes very short (about a meter), sometimes very high. Think of the California coast, Barbra Streisand’s clifftop house, or the high sea cliff at Cape Cod National Seashore on the East Coast. These cliffs were formed by rising post-Pleistocene seas that flooded what we now call the continental shelf – former land now eroded, drained and covered by sea.
This was not a gentle process, but rather a process of constant daily exposure to waves and wind, punctuated by violent storms, each destroying large areas of land over several thousand years. More recently, since the beginning of recorded history, the spread of sea over land has slowed or stopped in most places around the world, so we have no culturally maintained perception of this process. Until now, because with melting and collapsing glaciers the process has begun again – and it’s often in the news or, depending on where you live, in our front yards.
This crop from AFPTV footage dated September 13, 2023 shows an aerial view of the damage caused by flooding after Mediterranean Storm “Daniel” hit Libya’s eastern city of Derna.AFPTV/AFP via Getty Images
If a magical climate wizard waved his wand and prevented all storms, but still allowed the sea to continue rising with moderate waves and breezes, those waves would steadily erode the land horizontally, lowering the height of the dry land by several meters over time stretching several hundred meters across the land, although the speed would depend on the nature of the land itself.
A rocky coast like Maine’s would take thousands of years to erode just a few inches, while sandbars like the Cape Hatteras Islands of the Outer Banks or the poorly consolidated sediments of almost the entire state of Florida would erode almost as quickly as the sea rose . In California, Streisand’s house sits on a cliff high above the Pacific, but the Pacific only needs to rise a few tens of centimeters for the laws of physics and geology to force it to push back the cliff and take her house. It will be tragic and newsworthy.
The entire offshore continental shelf, wherever it exists, is covered with a vast layer of sand, the bones of eroded and deflated land. View a satellite view of the east coast of North America on Google Earth. You can see a light blue band of sea water. This was all dry land during the last Ice Age. As sea levels rose with melting ice, the sediments that made up the land were carried away and sorted out, with finer and lighter material floating away to be deposited in the great depths of the ocean, and less fine and heavy sediments, mainly sand, covers the underwater landscape of the shelf. (This has happened several times over the last 2 million years or so).
As sea levels continue to rise in the future, dry land areas will be occupied and redistributed in the same way, and the sea will advance much further inland than counting topographic lines suggests.
As a result, as sea levels rise, the horizontal extent of the ocean is much larger than we can imagine. Almost all of Louisiana is below the maximum possible level of sea level rise if there is no erosion. But if we imagine a 100-foot-tall cliff lining a future southern coastline after every piece of glacial ice melts, that could turn Missouri’s Mark Twain National Forest into beachfront property. And the conquest of this land by sea would probably come in a series of violent storms, rather than grain by grain.
So what does sea level rise look like? It looks like Derna, Libya.
Greg Laden, Ph.D., is a biological anthropologist and archaeologist who has conducted field research in North America, the Congo, and South Africa. He taught at Harvard University, the University of Minnesota and Century College, among others. He writes Greg Laden’s blog and is a freelance writer.
Source : themessenger.com