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Understanding Sea Level Rise Measurements

Explore the complexities in measuring sea level rise due to factors like steric vs. eustatic changes, observations from tide gauges and satellite altimetry, and historical data uncovering the last 6000 years. Discover the impact of global phenomena on sea level rise rates.

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Understanding Sea Level Rise Measurements

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  1. Sea Level Rise: Can we explain what we measure? Anny Cazenave LEGOS-GRGS/CNES Toulouse, France WOCE and BEYOND Nov. 2002

  2. - STERIC sea level: change in Density - EUSTATIC sea level: change in Ocean Mass Temperature Salinity Water mass exchange with continents and ice caps

  3. 20th Century Sea Level Rise, IPCC, 2001 -1.0 0.0 1.0 2.0 mm/year Thermal expansion Glaciers Greenland (present) Antarctica (present) Ice sheets (long term) Permafrost Sedimentary deposits Continental waters TOTAL OBSERVATIONS -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 mm/year

  4. SUMMARY OF RECENT RESULTS  During the 1990s, sea level has been rising by ~ 3 mm/yr evidence for a recent acceleration ??? Steric and eustatic sea level change recognized as highly non uniform overestimate of the 20th century sea level rise by tide gauges ??? New rates for glaciers and ice sheets recent melting Land water mass contribution to sea level rise can now be estimated

  5. Observations of Present-Day Sea Level Rise

  6. Sea Level Rise during the 1990s from Satellite Altimetry

  7. TOPEX/POSEIDON

  8. SEA LEVEL TRENDS measured by TOPEX/POSEIDON (1993-2001) Trends in mm/year

  9. Advantages high-precision high spatio-temporal resolution global coverage absolute measurements Drawbacks time series still short (10 years) no coverage of high-latitude oceans (T/P & Jason) Satellite Altimetry for Global Mean Sea Level

  10. Sea Level Rise during the 20th century from Tide Gauges

  11. WORLDWIDE TIDE GAUGE NETWORK -2001-

  12. Tide gauges used by B. Douglas (1997)

  13. SAN FRANCISCO BREST NEW YORK STOCKHOLM

  14. Main processes causing vertical land motions • Tectonic deformations • Seismic activity • Volcanic activity • Viscous deformation of the Earth’s mantle (Post Glacial Rebound) • Land subsidence due to sediments compaction, underground water extraction, etc. • Surface loading due to air and water mass redistribution among atmosphere, oceans, continental water reservoirs and ice caps

  15. Deformation of the Earth’s mantle under ice sheet loading and POST GLACIAL REBOUND

  16. TIDE GAUGE-BASEDSEA LEVEL RISE FOR THE 20th CENTURYGlobal • 1.5 +/- 0.5 mm/yr (IPCC, 2001) • 1.71 +/- 0.55 mm/yr (Douglas, 2001; 27 sites) • 1.84 +/- 0.35 mm/yr (Peltier, 2001; 27 sites) • 1.5 +/- 0.4 mm/yr (Tamaseia et al., 2001; 23 sites)

  17. Advantages historical information (back to 1900) useful for calibrating onboard altimetry systems Drawbacks heterogeneous and limited coverage gaps in data records land motion contamination data sensitive to local perturbations Tide Gauges for Global Mean Sea Level

  18. Sea level change during the last 6000 years 20th century -6000 yearspresent From Lambeck, 2002

  19. CONSTRAINTS FROM ASTRONOMICAL/GEOPHYSICAL OBSERVABLES

  20. Astronomical/Geophysical Observables Non-tidal Earth ’s rotation acceleration or length of day -LOD- deceleration: -0.6 ms/cy for the past 2500 years Secular decrease of the Earth ’s dynamical flattening : -3x10-11/yr for the past 25 years Secular movement of the Earth ’s rotation  pole: 10 m/cy towards Greenland over the past 100 years

  21. Length of Day (LoD) variations for the past 2500 years LoD J2 From Wahr and Davis, 2002 year -500 2000

  22. Secular decrease of the Earth ’s flattening (J2) From Cox and Chao, 2002

  23. Length of Day (LoD) variations for the past 2500 years LoD J2 From Wahr and Davis, 2002 year -500 2000

  24. Secular Decrease of the Earth’s Flattening PGR + present-day melting A+G A observation Post Glacial Rebound From Sabadini and Vermeersen, 2002

  25. Viscosity of the Earth’s mantle Long-wavelength geoid Sketch of the Earth’s interior

  26. Sketch of the Earth’s interior Upper mantle Lower mantle Core Viscosity profiles from global circulation models

  27. Secular decreases of Length of Day and Earth’s flattening are not strong constraints on present-day eustatic sea level rise

  28. DO WE UNDERSTAND OBSERVATIONS OF PRESENT-DAY SEA LEVEL RISE?

  29. Sea Level Rise during the 1990s

  30. Topex/Poseidon (1993-1998) SEA LEVEL TRENDS FOR 1993-1998 (mm/yr) Upper map : Observed (Topex/Poseidon) Bottom map: Computed (thermal expansion) Thermal expansion (1993-1998) - -27 mm/yr +27 From Cabanes et al., 2001

  31. World ocean temperature data base S. Levitus, C. Stephens, J.I. Antonov & T.P. Boyer NOAA Atlas NESDIS 40, 2000 • Global gridded time series of ocean temperature data at different depths: • 0-500 m : yearly means for 1945-1998 • 0-3000 m : 5-year means for 1945-1995

  32. Topex/Poseidon sea level--- Thermal expansion---- Résidual---- From Cabanes et al., 2001

  33. Sea level rise measured by Topex/Poseidon is mainly caused by thermal expansion (warming) of ocean waters

  34. Sea Level Rise during the 20th century

  35. 20th Century Sea Level Rise, IPCC, 2001 -1.0 0.0 1.0 2.0 mm/year Thermal expansion Glaciers Greenland (present) Antarctica (present) Ice sheets (long term) Permafrost Sedimentary deposits Continental waters TOTAL OBSERVATIONS -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 mm/year

  36. Past Century Sea Level Rise: is the observed 1.5 mm/yr rate too high? OR are climate-related contributions underestimated?

  37. Steric sea level trends Steric contribution (thermal expansion) to sea level rise for 1955-1995 Global mean steric sea level 0.5 +/- 0.05 mm/yr From Cabanes et al., 2001

  38. STERIC SEA LEVEL TRENDS (1955-1995)

  39. Steric sea level trends map and tide gauge sites used by Douglas (1997) 1.3 +/- 0.1 mm/yr 1.4 +/- 0.1 mm/yr 0.5 +/- 0.05 mm/yr 0.5 +/- 0.05 mm/yr From Cabanes et al., 2001

  40. SEA LEVEL RISE (1955-1995)  Steric average at the 25 tide gauge sites: 1.3 +/- 0.1 mm/yr  Observed by tide gauges (25 sites) : 1.6 +/- 0.15 mm/yr Thermal expansion (global mean) : 0.5 +/- 0.05 mm/yr Salinity effect (global mean) : 0.05 +/- 0.02 mm/yr

  41. Map of steric sea level trends and GLOSS tide gauge network Steric sea level: - global mean (in blue) -subsampled at GLOSS sites (in red)

  42. The inhomogeneous distribution of historical tide gauges used to estimate the 20th century sea level rise cannot capture the regional thermosteric variability Tide gauge-based estimates of 20th century sea level rise too high???

  43. EUSTATIC SEA LEVEL RISE (ocean mass change)

  44. Eustatic contribution from land water storage (change in the global water cycle)

  45. 20th Century Sea Level Rise, IPCC, 2001 -1.0 0.0 1.0 2.0 mm/year Thermal expansion Glaciers Greenland (present) Antarctica (present) Ice sheets (long term) Permafrost Sedimentary deposits Continental waters TOTAL OBSERVATIONS -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 mm/year

  46. Atmosphere Precipitation : Precipitation Snow Pack Evaporation Evaporation Transpiration Lakes Rivers Soil Moisture Oceans Ground Waters

  47. Water mass balance : dW/dt = P - E - R W: Water stored in soils P : Precipitation E : Evapotranspiration R: Runoff GLOBAL HYDROLOGICAL MODELS 

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