When the Earth warms, as it has since the Little Ice Age ended in about 1850, waters expand and ice melts. These factors cause global sea levels to rise. Over the Twentieth Century, the Planet?s seas rose at an average rate of 0.8 inches per decade, and since 1993 this rise has accelerated to 1.3 inches per decade. Subsurface layers of rock or sediment where groundwater flows freely are known as aquifers. In coastal aquifers, a layer of freshwater sits on top of denser saltwater. As the sea level rises, the shore moves further inland, and in the groundwater, the layer of saltwater comes closer to the surface of the aquifer and moves further inland. This results in mixing of the fresh and salt waters and contamination of groundwater resources. Also, until recently, it was assumed that the magnitude of the saltwater intrusion into the aquifer would be the same as the inland movement of the shore at the surface. Depending on the make-up of the coastline (i.e. fine or coarse sand, rock, and how these types of sand and rock are layered), however, the intrusion of the saltwater into the aquifer can be between ten and fifteen percent greater than the inland movement of the shore. In other words, if the shore advances ten yards inland, the saltwater layer in the aquifer can move between 11 and 15 yards inland. America’s coastal areas are its most densely populated regions, and an estimated fifty percent of our Nation’s water supply comes from groundwater.
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(Source: The United Nations: Intergovernmental Panel on Climate Change. Fourth Assessment. Working Group 2: Climate Change Impacts, Adaptation, and Vulnerability, Chapter 6 and Rahmstorf, S. et al. “Recent Climate Observations Compared to Projections.” Science 316 (2007): 709 and Burkett, Virginia, et al. Potential Consequences of Climate Variability and Change for the Southeastern United States. (Washington, DC) US Climate Change Science Program / US Global Change Research Program  November 3, 2006 http://www.usgcrp.gov/usgcrp/Library/nationalassessment/05SE.pdf and “Climate Change Threatens Druinking Water, As Rising Sea Penetrates Coastal Aquifers.” Science Daily 7 November 2007. Accessed online 18 March 2008 < http://www.sciencedaily.com/releases/2007/11/071106164744.htm>.)