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 March 29, 2016
What Florida's ancient past tells us about sea-level rise today

 The looming threat of sea-level rise is a cause for anxiety throughout much of the coastal United States, and Florida is one of the unlucky states most at risk. Miami, alone, is considered one of the most vulnerable cities in the nation --- it's already subject to frequent flooding and has plans to spend hundreds of millions of dollars on flood control in the near future.

But today's Sunshine State residents are by no means the first Floridians forced to deal with the rising tides. In fact, some of the state's earliest inhabitants were also forced to move and adapt in response to changing water levels thousands of years ago --- and their history may provide some helpful insights into the struggles faced by today's coastal dwellers.

This is the conclusion presented by a new study, published last week in the journal Geoarchaeology by researcher Paulette McFadden, a postdoctoral researcher at the Florida Museum of Natural History at the University of Florida. Since 2009, McFadden has been involved in field investigations along Florida's northern Gulf shore, both on the mainland and on small islands in the adjacent marshes. Some of these sites contain evidence of human occupation stretching as far back as 4,000 years. McFadden became interested in how the placement of these sites may have been influenced by the changing environment throughout the millennia.

Regional studies have suggested that sea levels began to rise along the Gulf Coast following the Last Glacial Maximum, when the climate in the Northern Hemisphere began to substantially warm, and ice sheets started to melt.

"I was very interested in trying to understand how people in the past dealt with some of the same issues that we're grappling with in our present life --- mainly sea level rise," McFadden told The Washington Post. "One of the things that I did learn during my research is that environmental change on the coast is a normal part of life, or it certainly was for the people in the past. So I wanted to know what kinds of strategies did they use when sea-level rise on the shoreline began to move inland."

McFadden's research focused on an area of Florida's Gulf Coast known as Horseshoe Cove, which is located in the Big Bend, the general region where the panhandle meets the peninsula. The area is marked by wide marshes, a network of tidal creeks and numerous small islands, which contain archaeological deposits from human settlements as far back as 4,000 years.

To reconstruct the region's coastal evolution over the past several thousand years, McFadden took sediment core samples from both exposed and submerged land along the coast and analyzed them to see what kinds of materials they contained and how old they were. This information helped her figure out what areas were underwater, and when, throughout history.

She also conducted archaeological excavations in the same region and used radiocarbon dating to figure out how old the human settlements in any given location were. Using this information, paired with her reconstruction of the coastline's history, she was able to make inferences about how the early Floridians might have moved around in response to their changing environment.

She found that there was a strong correlation between environmental change and the timing and locations of human settlements throughout the centuries. Reconstructed maps of Horseshoe Cove from 2800 BC to after AD 200, using McFadden's data, show the way the coastline --- once dominated by solid land --- gradually flooded and became characterized by salt marshes. At the same time, the sites chosen for human occupation throughout the centuries appeared to be dictated by where land was available and what the environment looked like at the time.

"They had very specific strategies," McFadden said of the region's early inhabitants. "When it was time for them to move because the sea level was coming up, they were targeting very specific areas on the landscape." Settlements tended to crop up in areas bordering the marshes that would be protected from flooding and storm surges, but that also offered easy access to fishing and other marine resources.

"So as the sea level came up, they probably had ingrained in their culture these strategies and a knowledge that eventually they were going to have to move back away from the water," McFadden said.

Interestingly, she noted that the region's human inhabitants were also able to exert some long-lasting influences on the environment as well.

"The modern coastline in the Big Bend region is largely shaped the way it is because people lived there in the past," she said. "They piled up shell and they piled up debris as they lived in an area, and some of those islands that I did the archaeological work on only survived today because the elevation was increased by people living in them."

The work underscores the idea that human civilization and the changing environment have been closely tied up with one another for centuries --- and that the struggles faced by modern humans, while heightened by the accelerating effects of anthropogenic climate change, have existed throughout the ages.

Moreover, this kind of archaeological work could even help modern humans better adapt to the changing landscape, McFadden suggested. For one thing, local archaeological studies can provide important insight into how different regions have responded to environmental changes over time, and that could help us make better predictions about how they'll change in the future.

"For instance, my study area has remained relatively stable over about the past 1,500 years, and if you look at the east coast of Florida you see it's eroding very quickly," McFadden said. "Those kinds of wave-dominated areas are vulnerable to sea-level rise much quicker than the marsh-type areas. So what this tells us is we need to maybe focus our efforts toward those more vulnerable coasts right now."

Indeed, local studies are the best way to get a good sense of any given area's history and potential future, said Neill Wallis, an assistant curator of Florida archaeology at the Florida Museum of Natural History and one of McFadden's supervisors. Scientists sometimes use data from one part of the world as a proxy for past sea-level rise all over the world, he said, which may not always be accurate.

"Those don't really tell us always what happens in any particular location, because sea level can rise globally and we might not see much change in one area," he said. "In that sense, I think all archaeological research that relates to climate change has to be done locally."