Wave energy eroding and transporting sediment on the coasts is confined to a vertical zone of 20 m
- storm waves and tides reach no more than 10 m above mean sea level; below 10m, waves have become too weak to transport sediments

Sea level has fluctuated repeatedly in the last 2 M years with successive ice ages
- low global temperatures - water is frozen into polar icecaps and shelves and continental ice sheets; temperature increases and the glaciers discharge water back into the oceans; sea level can change more that 100m

High sea level and energy conditions correspond with the greatest rates of coastal erosion
 

Changes in Regional Sea Levels

Tectonic Activity

Seismic activity may cause a sudden shift in regional sea level by sinking or uplifting the shore
- e.g. Alaskan earthquake of 1964 (8.6) - blocks of shoreland rose or subsided 2 m (max of 6 m); off shore islands - 12 m

Severe earthquakes are fairly common wherever plates collide (convergent margins)
 

Climate Fluctuations

Seasonal differences can be as much as 10-30 cm - direct result of annual weather patterns: temperature and wind
- water expands in warm temperatures and winds push water piling it up and creating a small depression behind
        - upwelling and downwelling

El Nino

- a warm current off the western coast of S. America
- appears in late December
- normally, cold Peru Current travels northward up coast along with a wind-driven upwelling of even colder nutrient-rich waters
- every 4-7 years, trade winds diminish and allow warm waters to move south and drive the Peru Current to the west prematurely; warm surface water blocks upwelling; may last for more than a year
- wintertime temperature differences between the water and air cause storms with erosion and flooding, and changes in the wind and current patterns
- as El Nino moves south, the climatic effects and warmer waters raise the sea level; long waves originating across the Pacific Ocean pile up along the coast causing further erosion
 

Subsidence due to Compaction and Fluid Withdrawal

e.g. the sea level of the coastal zone of Louisiana is rising about 9-10 mm per year (about 3x the global average); about 6-7 mm of this is due to subsidence caused by compaction of delta sediments
    - the addition of sediment in an active delta region compresses underlying sediments and drives out water underneath; because the remaining sediment from the mud adds less volume than was lost from compaction the net effect is a regional rise in sea level
    - dams have reduced the amount of sediment, which accelerates the rate of subsidence

e.g. the coastal zone of Texas subsiding due to the withdrawal of fluids by human activities; nearly 100,000 oil wells and countless water wells for domestic and industrial use; e.g. coasts near Galveston have subsided nearly 2 m this past century
    - water is being pumped back into the ground to lessen the threat of subsidence

e.g. construction of high-rise buildings in a city on unstable sediments; Houston and New Orleans are now below sea level
 

Isostasy:  subsidence and rebound of the lithosphere

Istostasy is the condition of equalibrium that is achieved by constant rebalancing of forces that tend to elevate and depress the lithosphere; adjustments in the relative position of the lithosphere in the asthenosphere are called isostatic adjustments

e.g. a glacier can press a continental lithosphere downward by as much as 200-300 m; when the glacier melts, the lithosphere rises again

e.g. an increase in density due to cooling of a hot lithosphere will cause a rise in sea level

e.g. prolonged accumulation of sediments or volcanic rock add weight that causes the lithosphere to be out of balance and subside raising the sea level - Gulf and Atlantic Coasts; island of the Pacific Ocean
 

Changes in the Volume of the World Ocean

A global (eustatic) change in sea level can occur:

- change in temperature causing an expansion or contraction of volume
- tectonic forces widening or narrowing oceans - long term

Cold climate forms ice sheets that trap water in glaciers while at the same time lower the temperature contracting the volume of water
e.g. during the last ice age, glaciers covered 30% of the land area of the N. Hemisphere - nearly all of the continental shelf was exposed; caused by a 2-3 C temperature decrease from today's mean annual temperature; if the current trend continues, the process will be reversed
    - difficult to access the rate of global climate change - data only recorded over the short term (last 100 years for sea level records, less for climate measurements)
    - human generated rise in global temperature - cannot refute release of greenhouse gases; some climatologists predict a rise of 3 C by 2030 - may raise the sea level 30-40 m in a few centuries
 

Advance and Retreat of  Ice Sheets

Driving force is the growth and shrinkage of continental ice sheets and polar icecaps of Greenland and Antarctica

Each of the glacial periods lasts 10s of 1000's years; the last ice age was the Wisconsin ice age - began 120K years ago and ended 20K years ago
- created the conditions that molded our current shorelines
- the tremendous increase in water volume during the retreat of the glaciers increased the sea level despite the subsidence of the ocean floor caused by the weight of the glaciers (rebounding effect is still occurring in some places)
- the lowest position of sea level was about 20K years ago when teh ice sheets of eht Wisonsin ice age reached their maximum development - called the lowstand: about 120 m below present sea level - and rose rapidly to its present level until about 6-7000 years ago
    - the shoreline moved so rapidly that sandbars and barrier islands had no time to develop; became tide-dominate with widespread estuaries and tidal flats; at the end of rapid sea level rise, the shorelines became more stable and waves became dominate - beaches and barrier islands were formed
 

Current and Future Sea Level Changes

Reliable data for the last 100 years indicate that the sea level is rising at an increasing rate; REMINDER... the duration of data collection is insufficient to predict long-term trends
- the N. American east coast ranges from the slow rise of the northern coasts where isostatic rebound is still going on to a more rapid rise in the south
- the N. American west coast ranges from low rates of rise along the southern coasts to a drop in sea level along the Alaskan coast due to rapid tectonic uplift along the continental margin - up to 14 mm per year
 

Implications for Coastal Environments

Most of the coasts today only go back a few thousand years; they represent one of the fastest changing parts of the Earth's surface; a continued rise in sea level of 3-4 mm per year for a century or more will devastate many densely populated parts of the world
- e.g. Mississippi River delta region sea level annual rise is 9 mm and causes a loss of over 40 acres of coastal land each month
- e.g. Pakistan: the Ganges-Brahmaputra Rivers flood every monsoon season; if there is a 0.5 m rise in sea level in the next 50 years, more than 1000 km2, or 0.1% of the country's area will be covered