Measuring Long Term Sea Level Change

1. Measuring Long Term Sea Level Change Philip L. Woodworth Proudman Oceanographic Laboratory, Liverpool plw@pol.ac.uk

2. Contents • Methods for Measuring Sea and Land Level Changes • Problems with the Data Sets • What We Know about How Sea and Land Levels have Changed and will Change

3. Long Term Sea Level Changes • We know from geologists that sea level has changed over many1000s of years largely as a result of the exchanges of water between the ocean and ice caps • So we should not be too surprised if sea level is still changing

5. 20K BP

6. 15K BP

7. 12K BP

8. 9K BP

9. 7K BP

10. Long Term Sea Level Changes • For this talk our ‘long term’ is the last 200 years (since the invention of the ‘tide gauge’ or ‘sea level recorder’) and the next 100 years

11. Local Hero William Hutchinson measured the heights and times of high waters at the Old Dock gates Liverpool 1764-1793 These were the first systematic tidal measurements in the UK

12. Classical Float Gauge (from about 1832)

13. UK Float Gauge at Holyhead Float gauges are still important and can be made into digital gauges with the use of encoders

14. Countries Share Data through International Data Banks such as • The Permanent Service for Mean Sea Level • on behalf of the International Council for Science • which is based at Proudman Oceanographic Laboratory in Liverpool

17. Sea Level Changes in Last 100 Years • Past 100 years • Most PSMSL records show evidence for rising sea levels during the past century • IPCC Third Assessment Report concluded that there has been a global rise of approximately 10-20 cm during the past 100 years

18. There are only medium-length African records Mombasa Zanzibar Port Louis Simons Bay Takoradi Alexandria

19. There are Two Main Problems with the Present ‘Global’ Sea Level Data Set • ‘Sea Level’ measurements are relative to land level. SOLUTIONMeasure Land Levels using new geodetic techniques such as Global Positioning System (GPS) and Absolute Gravity (AG)

20. Land Movements can be: • Slow and monotonic e.g. Glacial Isostatic Adjustment (Post Glacial Rebound) • or • Fast and irregular e.g. tectonics in Chile, Alaska etc. • Geodynamic models of the solid Earth exist only for GIA and even they are not perfect.

21. GIA/PGR Earthquake Ground-water pumping Harbour development/ Sedimentation For comparison: Hot spot (normal?)

22. GIA models for VLM corrections Peltier ICE-4G GIA model N.B. There is nowhere on Earth which does not experience some effect of GIA

23. There are Two Main Problems with the Present ‘Global’ Sea Level Data Set • ‘Sea Level’ measurements are relative to land level. SOLUTIONMeasure Land Levels using new geodetic techniques such as GPS and Absolute Gravity

24. Two Main Geodetic Tools for Measuring Land Level Changes GPS Absolute Gravity

25. GPS Networks (UK) • The current network • of UK GPS receivers • which monitor vertical • and horizontal land • movements. • (Operated by the • University of • Nottingham with POL.)

26. There are Two Main Problems with the Present ‘Global’ Sea Level Data Set • Uneven geographical distribution: the PSMSL data set is under-represented in Africa, Antarctica etc. and there are no long term records from the deep ocean. SOLUTIONSThe GLOSS programme to densify the existing tide gauge network, and programmes of satellite altimetry to measure sea levels from space. Also some deep ocean measurements.

27. The GLOSS Programme GLOSS Core Network (GCN) with approx. 300 stations

28. Complementary Tools for Measuring Sea Level Changes Tide Gauges Satellite Altimetry Sea Floor Systems

29. We can use the PSMSL data set (in spite of its recognised deficiencies) to ask a number of questions about Past and Possible Future Sea Level Changes

30. Questions and Answers Q. Has global sea level always been changing ? A. Yes. Sea level has changed by over 120 m during the last 20,000 years, sometimes changing at a rate 10 * faster than the presently-observed rate of change.

31. Questions and Answers • Q. Has global sea level risen during the 20th • century ? • Yes. By 10-20 cm. • There are many references, see the IPCC Third Assessment Report for a review (the Fourth Assessment Report is in preparation)

32. 20th Century Sea Level Rise Estimates Region, VLM Rate ± s.e. (mm/yr) Gornitz and Lebedeff (1987) Global, Geological 1.2 + 0.3 Peltier and Tushingham (1989, 1991) Global, ICE-3G/M1 2.4 + 0.9c Trupin and Wahr (1990) Global, ICE-3G/M1 1.7 + 0.13 Nakiboglu and Lambeck (1991) Global decomposition 1.2 + 0.4 Shennan and Woodworth (1992) NW Europe, Geological 1.0 + 0.15 Gornitz (1995)d NA E Coast, Geological 1.5 + 0.7c Mitrovica and Davis (1995), Davis and Mitrovica (1996) Far field, PGR Model 1.4 + 0.4c Davis and Mitrovica (1996) NA E Coast, PGR Model 1.5 + 0.3c Peltier (1996) NA E Coast, ICE-4G/M2 1.9 + 0.6c Peltier and Jiang (1997) NA E Coast, Geological 2.0 + 0.6c Peltier and Jiang (1997) Global, ICE-4G/M2 1.8 + 0.6c Douglas (1997)d Global ICE-3G/M1 1.8 + 0.1 Lambeck et al. (1998) Fennoscandia, PGR Model 1.1 + 0.2 Woodworth et al. (1999) UK & N Sea, Geological 1.0 + 0.2 N.B. All these analyses use the same PSMSL data set

33. Questions and Answers Q. Do we understand why it has risen? A. Yes. (Or at least ‘more or less’)

34. Why has sea level risen? Main driver has been the 0.6 ºC global temperature change during the past century but there have been many contributors to the sea level change. See IPCC TAR for a review (4AR will be published in February 2006).

36. Melting Low Latitude Glaciers (Alaska etc.)

37. 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

38. 20thCentury Sea Level Rise: is the 1.5 mm/yr observed rate too high? OR are climate-related contributions (0.7 mm/yr) underestimated? The enigma... (Walter Munk, 2001)

39. Questions and Answers • Q. Is the rate of rise increasing ? • Yes. On basis of study of long records spanning 18th to 20th centuries. • A. Not clear. On basis of 20th century tide gauge data alone. • Yes. On basis of altimeter and tide gauge from the 1990s.

40. Sea level change contains an acceleration of sea level rise from the 19th to the 20th centuries probably due to climate change

41. Questions and Answers • Acceleration in the1990s? • Look at altimetry • Look at tide gauge records. • Look at IPCC-type models. • Answer: • Yes

42. Rate of sea level rise (Jan. 1993-Dec.2004) 3 mm/yr Jason-1 Topex/Poseidon

43. Sea level trends (from Anny Cazenave)

44. from 177 PSMSL tide gauge records in 13 regions (Holgate and Woodworth, 2004)

46. Church et al. (2004) Church et al., 2004

47. Questions and answers • Q. How much might sea level rise in the 21st century? • 9-88 cm with central value of 48 cm based on 35 emission scenarios and 7 AOGCMs (IPCC Third Report).

48. Changes in Sea Level in the Next 100 Years • Next 100 years • a rise between 9 and 88 cm • a central value of 48 cm • a rate of approx. 2.2 - 4.4 • times that of the past • 100 years (IPCC TAR) Projected sea level rise, IPCC 2001

49. Questions and answers • Q. Will the sea level rise in the 21st century be the same everywhere? • No. Rises will not be the same in all parts of the world because of the readjustment of the ocean circulation to climate change, and also due the different magnitude of local vertical land movements.