Sea Level Rise

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Sea Level Rise

• Rising sea level will
• Impact 100 million people live within 1 m of sea
  level (Tuvalu)
  
• Threaten existence of some island states and
  deltaic coasts
  
• Threaten coastal wetlands if rise is faster than
  rate which plants can move
  
• Increased erosion of sandy beaches.
• Sub-surface water table encroachment.
• Sea level rise in the geologic record
• Since last glacial maximum (21,000 y BP), sea
  level has risen 120 m
  
• Melting of ice sheets was complete 5000-6000 y
  BP
  
• GSL rise was small, perhaps zero, 3000-4000 y BP
• Records indicate that GSL began to abruptly
  increase in the middle of the 19th century
  

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• Mean sea levels at New York City in the 20th
  century. Black dots represent the annual relative
  sea level (RSL) measured in centimeters. The red
  line represents the running 20-year mean rate of
  RSL rise measured in millimeters per year.
  Interannual variations are clearly so large that
  portions of the record a few decades in length do
  not at all reflect the long-term trend of sea
  level rise, which is represented here by the
  black line. All tide-gauge records display this
  phenomenon.

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Two scales of sea level change

• 1. Global (eustatic) sea level rise
• (a)Thermal expansion of sea water.
• The density of water decreases with
  increasing temperature.
  
• Warming atmosphere causes thermal
  expansion.
  
• 1oC increase equals 2 meters of rise
• 80 of sea level rise in last 130 yrs (0.6oC)
  accounted for by thermal expansion.
  

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Two scales of sea level change

• 1. Global (eustatic) sea level rise
• (b) Change in ocean basin capacity
• Dependent on spreading rate of mid-ocean
  ridges
  
• Fast spreading rate more pronounced
  profile of ridge (higher ridge volume)
  
• Sea water displaced upwards, raising sea
  level at coasts.
  
• Changes sea level 1,000 m/1 million yrs

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Two scales of sea level change

• 1. Global (eustatic) sea level rise
• (c) The role of glaciers
• Glacial melting increases sea level.
• Sea level has risen 120 m since last glacial
  maximum (21,000 y BP).
  
• If remaining glaciers melt, sea level may rise
  another 60 m.
  

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• Land and sea ice cover over the Northern
  Hemisphere at the last glacial maximum (LGM) 21
  000 years ago, as modeled in references 13 and
  14. The color contour over the oceans represents
  the sea-surface temperatures predicted for LGM
  conditions using a coupled atmosphere-ocean
  general circulation model of climate system
  evolution. The extent of sea ice is represented
  in white for summer and as a gray extension for
  winter.

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Two scales of sea level change

• 1. Global (eustatic) sea level rise
• (d) Global sea level also affected by formation
  or destruction of large, inland lakes.
  
• (e) How is global sea level measured?
• Tidal gauges at various locations
• Satellite altimetry

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Two scales of sea level change

• 2. Local, or relative, sea level change
• Isostatic rebound
• Caused by the melting of glaciers
• Lithosphere rises to find equilibrium in absence
  of glacial load, results in observed drop in sea
  level
  
• Same process is causing New Orleans to subside

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• Relative sea level rise for Fennoscandia in the
  20th century derived from 70 very long-term
  tide-gauge records. The contours, measured in
  millimeters per year, roughly match the position
  and mass of the ice sheet that covered the region
  during the last glacial maximum.

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• Relative sea level trends as a function of
  latitude at sites from Key West, Florida, to
  Churchill on the shore of the Hudson Bay in
  Canada. The postglacial forebulge collapse peaks
  at the Chesapeake Bay and shows up strongly in
  the plot (Hampton Roads, Virginia, is located at
  the entrance to the bay). The rates of sea-level
  rise in the Mid-Atlantic region are as much as
  twice the global value of about 2 mm/y.

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• Modeling the glacial isostatic adjustment yields
  present-day rates of relative sea-level (RSL)
  rise for the whole globe (central panel).
  Surrounding the panel are plots of RSL records at
  eight geographical locations. In each plot, the
  records are determined by carbon-14 dating of
  individual geological indicators of past sea
  level and are compared with the RSL history
  predicted by the model.

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Two scales of sea level change

• 2. Local, or relative, sea level change
• (b) Regional tectonic forces
• Subduction along an active margin causes
  general uplift of the continental crust (Oregon
  coast)
  
• The continental crust along passive margins
  subside, raising local sea level (Atlantic coast
  of the U.S.)

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What is the current rate of GSL rise?

• Published values range from 1.0 to 2.4 mm/y.
• If it is 1 mm/y, then global warming and thermal
  expansion of the oceans is the cause and it
  resulted from 0.6 C increase in global
  temperatures during the last 100 years.
• However, if value is closer to 2 mm/y, then
  global climate models are incorrect and more
  significant contributions are required from
  Greenland and Anartica.