Volume 3, Issue 7
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Volume 3, Issue 7 |
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Archives Natural global warming, that is. It is estimated that sea level has risen as much as 330 feet since the peak of the Last Ice Age, also known as the Pleistocene Epoch (2 million to approx. 12,000 years ago). During the 20th century, however, sea level only increased by perhaps four to six inches. There are a variety of reasons for periods of Earth climate variations. These include variations in solar activity, variations in the axial inclination of the Earth, and variations in the Earth’s orbit. Once the Earth enters into a period of global cooling, regardless of the reason, the growing ice caps tend to reflect more sunlight back into space, further increasing the rate of cooling. Also, with the decreasing sea levels, there is less shallow water and cooler temperatures result in less evaporation and less humidity, and less of the Greenhouse Effect. This we call a Positive Feedback Loop, when an environmental change triggers another environmental change that “speeds up” the initial change. The two primary controls on world-wide sea levels are the size of the polar ice caps and the rate of seafloor spreading. Warmer conditions over the last 12,000 years have lead to polar ice cap shrinkage and higher sea level. When the mid-ocean ridges are more active and seafloor spreading is occurring at a faster rate, the mid-ocean ridge expands and displaces sea-water, resulting in higher sea level. When sea level rises, flooding the continental margins, this creates more shallow water. The shallow water heats more rapidly than the deeper water and this leads to more evaporation and more of the Greenhouse Effect. Also, the seawater expands as it warms, furthering the rise of sea level. These are more examples of Positive Feedback Loops. The greater extent of shallow sea water also means more shallow water habitat for marine creatures. There is ample evidence of human habitation of North America prior to the end of the last Ice Age. Aside from the usual threats to archeological sites from farming, development, and other human activities and natural events, e.g., flooding, erosion, changes in river channels, there is evidence that many useful, important archeological sites now lie on the Atlantic Ocean continental shelf, off the Eastern Seaboard and on the Gulf of Mexico continental shelf, as well. The average inclination to the surface of the continental shelf (sea bottom) is 10 feet per mile, away from the land. Using this average figure, if sea level was 330 feet lower during the Pleistocene, there is approximately 33 miles of continental shelf that is now covered by the Atlantic Ocean. Existing rivers had to travel 33 miles further to reach the ocean, at that time. Evidence of this is the presence of mastodon bones found around Gray’s Reef, 17.5 nautical miles east of Sapelo Island, Georgia, under approximately 60 to 70 feet of water. How many coastal archeological sites now lie beneath tens of feet of the Atlantic Ocean? How much of paleo-American history has been lost? I will leave it to professional Archeologists to offer an estimate. I recall that a few years ago, the Florida Museum of Natural History sponsored an archeological dive off the Florida Gulf Coast to study a now-submerged site. The post-Pleistocene sea level rise has not been constant. There have been intermittent periods of global warming and global cooling. The most recent period of global cooling, the Little Ice Age, lasted from approximately 1350 to 1850 AD (or even 1900 by some estimates). So perhaps all but a few inches of this sea level increase took place before the Industrial Revolution. What archeological evidence has been lost – at the hands of “Mother Nature”? Note: For those who would like to follow up on this topic I recomend Tom Koppel's book The Lost World. Also Joseph has some related thoughts on November 4th post on his own blog. - John Sweat |
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