Transcript Document

Sea-Level changes
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Learning Objectives
The shape of the planet: difference between Geoid and Ellipsoid
The concept of Mean Sea Level
Article: http://www.esri.com/news/arcuser/0703/geoid1of3.html
Processes that control the Mean Sea Level and its changes
Sea level changes over millions of year
Measuring sea level (tide gauges, altimetry,
18)
Sea level rise in the last century and global warming
Article: http://yosemite.epa.gov/oar/globalwarming.nsf/content/ClimateTrendsSeaLevel.html
http://www.agu.org/revgeophys/dougla01/dougla01.html
Economic implications of Sea level changes
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What does it mean to be at an altitude
of 4000 m?
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What does it mean to be at an altitude
of 4000 m?
It mean that I am 4000 m above the
Mean Sea Level (MSL)
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Model of the shape of the Earth
geoid: The equipotential surface of the Earth's gravity field which best fits, in a
least squares sense, global mean sea level (MSL)
http://www.esri.com/news/arcuser/0703/geoid1of3.html
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The height of the Earth surface
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by definition:
Mean Sea Level = 0 m = equilibrium level
 Changes in volume of water
 Changes in shape and volume of ocean basins
Changes are measured as relative changes to a
reference datum
This reference datum can be a fixed one (e.g.
distance from the center of the earth) or local
(coastline).
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A change in volume of seawater in one ocean
will affect the level in all others. Any such worldwide change in sea-level is called EUSTATIC
SEA-LEVEL change
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Sea Level Change
?
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Other effects of plate tectonics
e.g. Upper Cretaceous (90 Ma) MSL > 300 m
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 MSL (meters)
Summary of spatial-temporal scale of
processes contributing to Mean Sea Level
(C) Melting of ICE
(D) Plate Tectonics
100 m
• Thickness and area of continental crust
• Thermal state of crust
• Load on oceanic by land mass and sediments
(can generate localized changes in MSL,
e.g. subsidence of North Sea )
10 m
1m
(A) Exchange of water with continents (Groundwater, Lakes, etc.)
(B) Temperature expansion
NOTE:
A,B,C  change in volume of water
D  change in shape of container
1 cm
1 day 100
1000
100 Ka
TIME (years)
10 Ma
100 Ma
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Other processes complicating the study of
mean sea level (ice or sediment loads)
The concept of Post Glacial Rebound (PGR) !!!
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The subsidence of the Northern Sea
(associated with the loads of sediments)
Rate of change in Sea Level
mm/year
Scandinavia
Northern Sea
Great Britain
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Measuring sea level changes in time:
(in past times)
18
Differential incorporation of 18 and 16 isotopes of oxygen into the calcium
carbonate in the skeleton of marine organisms. (Read hand out!)
Radio-carbon dating of continental
margins
(in modern times)
tide gauges
Located at coastal stations, they measure the relative change in sea level.
altimetry
Satellites in orbit around the planet use radar altimetry to measure the
height of the sea level (accuracy of 2 cm).
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NOTE: Sea Level changes occur on a
variety of timescales
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Sea Level Change
(variations on different timescales)
Higher frequency
variability
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Sea Level Change
(variations on different timescales)
 Broad range of variability on different timescales
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Sea Level Change in the last 100 yr
(from Tidal Gauges)
Res. Curve = Obs. – Tides - Atmos. Press.
=
Observations
of Sea Level
Atmospheric
Pressure
+
Tides
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Measuring sea level changes in time:
(in past times)
18
Differential incorporation of 18 and 16 isotopes of oxygen into the calcium
carbonate in the skeleton of marine organisms. (Read hand out!)
Radio-carbon dating of continental
margins
(in modern times)
tide gauges
Located at coastal stations, they measure the relative change in sea level.
altimetry
Satellites in orbit around the planet use radar altimetry to measure the
height of the sea level (accuracy of 2 cm).
temperature of the ocean
Thermal expansion of ocean  changes in height (= Steric Sea Level)
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WOCE Sea Level Stations as of February 2000 (from Tidal Gauges)
WOCE is the World Ocean Circulation Experiment  http://woce.nodc.noaa.gov/wdiu/
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What can we say about these
tide gauges record?
Arica
Iquique
Coquimbo
6/23/2001
6/24/2001 29
Sea Level trend based on Tidal Gauges
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altimetry
Satellites in orbit around the planet use radar altimetry to measure
the height of the sea level (accuracy of 2 cm).
http://www.ecco-group.org/animations_iter21/TP_ps21.mpeg
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Spatial pattern of sea level change
1993-2003 (from Satellite)
cm/yr
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Geographical distribution of sea level trends (in mm/yr)
computed from TOPEX/POSEIDON altimetry between January 1993 and December 1999. Yellow and red colors
correspond to sea level rise, while blue color corresponds to sea level drop.
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Sea Level Change in the last 10 yr
(from Satellites)
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An example of Temperature measurements:
Expendable Bathythermograph (XBT) Lines
XBT
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Steric sea trend changes 1955-1996
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Closer look at Sea Level Change in the last 100 yr
from Tidal Gauges and Dynamic Height (=thermal expansion of oceans)
Tidal Gauges
Satellite
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ol. 294. no. 5543, pp. 840 - 842
DOI: 10.1126/science.1063556
Reports
Sea Level Rise During Past 40 Years Determined from Satellite and in
Situ Observations
Cecile Cabanes, Anny Cazenave, Christian Le Provost
The 3.2 ± 0.2 millimeter per year global mean sea level rise observed by the
Topex/Poseidon satellite over 1993-98 is fully explained by thermal expansion of
the oceans. For the period 1955-96, sea level rise derived from tide gauge data
agrees well with thermal expansion computed at the same locations. However,
we find that subsampling the thermosteric sea level at usual tide gauge positions
leads to a thermosteric sea level rise twice as large as the "true" global mean.
As a possible consequence, the 20th century sea level rise estimated from tide
gauge records may have been overestimated.
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Reconstruction of MSL using all data sources
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The Earth's climate has warmed about 1°C (1.8°F) during the last 100 years.
(the warming follows the Little Ice Age (19th century)  1-2 mm/yr sea level rise)
thermal expansion of ocean water
reduction in volume of ice caps, ice fields,
and mountain glaciers
IMPACTS of HUMAN on SEA LEVEL:
Increase in greenhouse-gas emissions:
 Global Warming  expansion of oceans
 many of the world's mountain glaciers will disappear
 sea level rise acceleration
FUTURE:
Numerical models of the Climate System
can be used to predict future changes in Sea Level
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Other processes contributing to Mean Sea Level on
shorter Timescales and on local spatial scales.
 MSL
100 m
10 m
Tsunami (Tidal waves)
El NiÑo
Ocean-Atmosphere interactions
1m
Vortices (Eddies)
Wind generated waves
Waves by ships
Freshwater floods
1 cm
Decadal changes in
Ocean Circulation
Planetary Waves
Ocean Current
Tides
Atmospheric Pressure
1 min 1 hour
1 day
TIME
30 days
1 year
10 yr
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Spatial pattern of sea level
change 1993-2003
(from Satellite)
Steric sea trend changes
1955-1996
(from ocean temperatures)
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END of Lecture
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Review of Geomagnetic field
What is the earth magnetic field and why is it important
Explain the distribution of magnetic anomaly stripes,
seismicity, and volcanism in terms of the concept of global
plate tectonics.
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What is a magnetic field?
Magnetic fields are produced by electric currents, which can be macroscopic currents in wires, or
microscopic currents associated with electrons in atomic orbits. The magnetic field B is defined in
terms of force on moving charge in the Lorentz force law.
Units Tesla=N s / (Coulomb m) = 10,000 Gauss
F  q E  qv  B
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magnetic bar
solenoid
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Origin of the Magnetic Field
Magnetic fields are produced by the motion of electrical charges.
For example, the magnetic field of a bar magnet results from the motion of negatively
charged electrons in the magnet.
The origin of the Earth's magnetic field is not completely understood, but is thought to be
associated with electrical currents produced by the coupling of convective effects and
rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is
termed the dynamo effect.
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Why do we care about the magnetic field?
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Source of Solar Wind …
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The Earth's Magnetosphere
1) The solar wind is a stream of ionized gases that blows outward from the Sun at
about 400 km/second.
2) The Earth's magnetic field shields it from much of the solar wind. When the solar
wind encounters Earth's magnetic field it is deflected.
3) The magnetosphere represents a region of space dominated by the Earth's
magnetic field in the sense that it largely prevents the solar wind from entering.
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However, some high energy charged particles from the solar wind leak
into the magnetosphere and are the source of the charged particles
trapped in the Van Allen belts.
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Geomagnetic Polarity Reversals
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Magnetic Anomalies
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Example of Ocean Floor
Magnetic Anomalies
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End of lecture
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