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 2

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 world wide change in sea-level is called

EUSTATIC SEA-LEVEL

change 10

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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 17

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Summary of spatial-temporal scale of processes contributing to M ean S ea L evel (C) Melting of ICE (D) Plate Tectonics 100 m 10 m 1 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 ) (A) Exchange of water with continents (Groundwater, Lakes, etc.)

(B) Temperature expansion

NOTE: A,B,C D   change in volume of water 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) 

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Differential incorporation of 

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and 

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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) 

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Differential incorporation of 

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and 

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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) 27

WOCE Sea Level Stations as of February 2000

(from Tidal Gauges) WOCE is the World Ocean Circulation Experiment  http://woce.nodc.noaa.gov/wdiu/ 28

What can we say about these tide gauges record?

Arica Iquique Coquimbo 6/23/2001 6/24/2001

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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) 34

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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 (19 th 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 41

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Other processes contributing to M ean S ea L evel on shorter Timescales and on local spatial scales.

100 m 10 m Tsunami (Tidal waves) 1 m 1 cm Vortices (Eddies) Wind generated waves Waves by ships Freshwater floods Planetary Waves Ocean Current Tides Atmospheric Pressure 1 min 1 hour 1 day TIME 30 days El NiÑo Ocean-Atmosphere interactions Decadal changes in Ocean Circulation 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) 44

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. 52

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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