Bases on tide gage the sea level has risen on the order of 10 to 20 centimeters. The threat of erosion on coastal communities would not be as great if it weren't for rising sea level. Changes in sea level keeps the shore in a continuous state of flux. Sediment is eroded from the foreshore and bays expand as the ocean encroaches inland. Both processes result in enormous losses for coastal communities.

Types of Sea Level Change

1. Eustatic sea level fluctuations are world-wide and result from a change in the volume of water in the ocean or in the size of the basin.
2. Isostatic sea level changes are isolated, local fluctuations resulting from local uplift or subsidence.
3. Relative sea changes result from the combine effects of both eustatic and local lsostatic changes. For example a 3.5 mm/yr rise in sea level in a deltatic region might result from a 1.5 mm/yr eustatic rise compounded by 2mm/yr local subsidence.

Causes of Sea Level Change

Tectonic Activity

• Eustatic changes occurring over the span of millions of year, and reflect rates of subduction and sea-floor spreading. More water is displaced onto the continents when rates are high and the ocean basin floor contains higher amounts of warm buoyant oceanic crust.
• Isostatic changes resulting from faulting can result in huge amounts of relative sea level change over a few seconds or minutes.

Climatic Activity

• Climatic warming
  • melting glaciers increase the amount of water in the oceans.
  • Warming also increases the volume of water in the oceans by expansion.
• Climatic cooling causes
  • The expansion of glaciers and formation of ice sheets
  • cooler denser ocean waters

In regions covered by advancing ice sheets the Earth's surface was isostatically depressed. However, because of the frontal forebulge and lower sea level, from water trapped in the glaciers, relative sea level was much lower. At glacial maxima the shoreline was closer to the seaward margin of the continental shelf.

In regions beyond the ice sheets most of the continental shelf was exposed. In South Florida Indians inhabited the continental shelve and didn't move inland until the Holocene sea level approached its present level.

Where thick ice retreated from the coast relative sea level rose to level higher than present. This condition prevailed until rebound caught up with eustatic sea level rise. Along the New England sea level was close to its present level around 6000 years ago.

Tides

• See Tides lecture

Climate-Related Short Term Fluctuations

• Storm Surges occur when winds drive water towards the shore, and low pressure causes the water's surface to bulge upward (about 1 inch for every inch drop in atmospheric pressure).
• El Niño generated Kelvin waves
• Read: El Niño Sea-Level Rise Wreaks Havoc in California's San Francisco Bay Region

• Coastal Erosion Along the U.S. West Coast During the 1997-98 El Niño: Expectations and Observations, 1999, Sallenger and other, U.S. Geological Survey Center for Coastal Geology
• El Niño Sea-Level Rise Wreaks Havoc in California's San Francisco Bay Region. U.S. Geological Survey
  Fact Sheet 175-99 Online Version 1.0
• FloridaEnvironment.com Site includes a series of informative audio programs
• Islands disappear under rising seas, June 1999, BBC
• Louisiana's Barrier Islands: A Vanishing Resource, 1995, List, U.S. Geological Survey Marine and Coastal Geology Program
• Thermal expansion of sea water associated with global warming, 1987, Wigley, T.M.L. and S.C.B. Raper, Nature 357: 293-300. Available online from CIESIN
• The Chesapeake Bay: Geologic Product of Rising Sea Level, 1998, Larsen, USGS Fact Sheet 102-98
• Global Warming Prompts Rethinking, 1997, Robert C. Cowen,The Christian Science Monitor
• Saving Venice, 2002, An ambitious plan seeks to prevent a modern Atlantis, Zeeya Merali, Scientific American
• Sea level Rise, 2002, Sarah Raper, Climatic Research Unit
• United Nations Environment Programme: Scenarios for Sea Level Rise / Potential impact of sea level rise: Nile River / Potential impact of sea-level rise on Bangladesh