Meeresspiegel änderungen und Batimetrie aus Satellitenbeobachtungen Teil 2

1. Meeresspiegeländerungen und Batimetrie aus SatellitenbeobachtungenTeil 2 Monographischer Kurs mit Übungen am Rechner. FU Berlin, Januar 2003 Carla Braitenberg Dipartimento Scienze della Terra, Università di Trieste, Via Weiss 1, 34100 Trieste Berg@units.it Tel +39-040-5582258 fax +39-040-575519

2. Sea level changes inferred from tide gauges • With tide gauges the direct measurement of sea level is made. • Measurement is local, influenced by several factors: • Air pressure, temperature, sea tide (can reach 20 m locally), marine currents • Vertical crustal movements due to isostasy, tectonics, anthropogenic factors, sediment compaction • Database of tide gauges: Permanent Service for Mean Sea Level (PSMSL) • Generally correlation of tide gauges situated in different towns is very good at seasonal and longer time periods: see figure of San Diego and San Francisco 1905-1990

3. Comparison of sea level in San Diego and San Francisco (800 km distance) San Diego and San Francisco normalized and detrended relative sea levels (mm/yr). Sampling: 1 month. (Douglas et al., 2001)

4. Sea level changes inferred from tide gauges • Problems encountered in estimating global sea level change: • Distribution of tide gauges (PSMSL) with time series > 60 yrs is inhomogeneous: England, Upper Adriatic, N-America (E and W coast), China, Japan, Australia

5. Locations of best RLR stations Locations of RLR from PSMSL data base that exceed 20 yrs (red), 40 yrs (green) and 60 yrs (blue) in length.

6. Problem in averaging • For global averaging a homogeneous coverage would be important. In the averaging process a basin with greater number of gauges enters with greater weight: creates a bias towards its variations. • Solution: replace with one representative station or with averaged equivalent station. • Annual trends of different stations show great scatter. • See Histogram of trends derived from PSMSL stations longer than 20 yrs.

7. Histogram of Sea level trends Histogram of sea level trends of the individual PSMSL stations

8. Contour plot of 20-th century European sea-level trends (Douglas, 2001)

9. Observations on Hawaiian Islands Annual mean sea level measured by tide gauges at Honolulu (Oahu Isl.) and Hilo on Hawaii Isl., over 1950 to present (Nerem and Mitchum, 2001). Difference could be due to different ages of islands (Oahu older, and maybe has reached equilibrium)

10. Annual mean sea level time series measured by tide gauges at Manila (14°31’N,120°59’E), Luzon (Philippines) and Bermuda (32°20’N,64°50’W). Bermuda is a high quality record; Manila is affected by probably anthropogenic subsidence (Nerem and Mitchum, 2001). Two stations at low latitudes

11. Tectonic influence on tide gauge records

12. PSMSL stations • The tide gauge records are from the stations Palermo and Messina (PSMSL). Palermo record: RLR data, Messina: metric data. From PSMSL: • RLR datum: approximately 7000mm below mean sea level, with this arbitrary choice made many years ago in order to avoid negative numbers in the resulting RLR monthly and annual mean values. The detailed relationships at each site between RLR datum, benchmark heights, tide gauge zero etc. are not normally required by analysts of the dataset, although they can be made available on request. • Metric data: raw data file

13. Messina 1908 earthquake • Event: December 28, 1908, I0=XI MKS-64 • Epicentral distances: • Messina:0-5km • Palermo: 200 km • Earthquake was accompanied by rapid subsidence of Messina coast by tens of cm (max 70 cm). • In the years (5-8yrs) before the EQ extensive crustal uplift was present (2cm/yr) Reference to event: Bottari et al., 1992

14. Global sea level rise from tide gauges • Requirements: • Correction for Glacial isostatic adjustment available • Long enough record: interannual and longer sea level variations make long-term trend difficult to calculate (at least > 60-70 yrs) • Free of vertical tectonic crustal movement • Long term trends are insensitive to small changes in record length • There are 175 RLR stations with records > 50 yrs • Only 24 used for global rise estimation.

15. Global sea level rise from tide gauges Sea level trends corrected for glacial isostatic adjustment for different lengths of record. Scatter also for long records due to geophysical and other reasons (Douglas, 2001).

16. 20-th Century Sea level trends from records > 70 yrs, not corrected for GIA (table 3.2) Douglas, 2001

17. Isostatic corrected sea level trends. Isostatic adjustment averaged over interval -.5 to +.5 kyr BP, column -.5 Peltier, 2001)

18. Global mean sea level trend (Douglas, 2001) • The 24 stations give an average trend of 1.91  0.75 mm/yr (standard deviation)

19. Global mean sea level trend with GIA correction • To each station the GIA correction according to Peltier (2001) is applied, leading to an average trend of 1.91  0.47 mm/yr • Finally the 24 stations are associated to 11 groups. Each group contains 1-6 stations, and the average trend is calculated. The average of the group-trends gives: • Tide gauge records not corrected for GIA: 1.71  0.55 mm/yr • Tide gauge records corrected for GIA: 1.84  0.35 mm/yr

20. Sea level trends grouped

21. Mean sea level trends from tide-gauges 1931-2001 • 27 stations with > 70 yrs data sets • observed trend: 1.91 ± 0.75 mm/yr • isostatically corr.: 1.91 ± 0.47 mm/yr • 11 groups • observed trend: 1.71 ± 0.55 mm/yr • isostatically corr.: 1.84 ± 0.35 mm/yr (Douglas, 2001; Peltier, 2001)