1. www.pol.ac.uk Monitoring and Predicting Long Term Global Sea and Land Level Changes Philip L. Woodworth Permanent Service for Mean Sea Level Proudman Oceanographic Laboratory psmsl@pol.ac.uk

2. 2 Contents

3. 3 National Sea Level Networks The UK ‘A’ Class Network is an example of a national tide gauge network which contributes data to the Permanent Service for Mean Sea Level (PSMSL) international data bank - see http://www.pol.ac.uk/psmsl/

4. 4 PSMSL Data Coverage Map shows the global coverage of PSMSL data

5. 5 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

6. 6 UK MSL change UK mean sea level (MSL) is rising, consistent with the global picture Plot shows MSL "relative" (to the land) as measured by tide gauges Corrected for local land movements, the "absolute" MSL trend has been about +1mm/y = +10cm over past century Compare to IPCC prediction of +47cm in 21st century

7. 7 Two Problems with Present ‘Global’ Sea Level Data Set (1) ‘Sea Level’ measurements are relative to land level. SOLUTION  Measure Land Levels using new geodetic techniques such as GPS and Absolute Gravity (2) 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  IOC GLOSS programme to densify the existing tide gauge network, and programmes of satellite altimetry to measure sea levels from space

8. 8 UK GPS Network Map shows current network of UK GPS receivers which monitor vertical and horizontal land movements. (Operated by the University of Nottingham in collaboration with DEFRA and POL.)

9. 9 Land movements In addition to GPS measurements etc. we can use geological data and geodynamic models to estimate vertical land movements (Shennan, 1989 estimates shown here, in mm/yr) Land subsidence/uplift can result from: –post-glacial rebound –water extraction –sediment compaction –earthquakes etc. Not all of these processes can be modelled.

10. 10 Space and deep ocean data Satellite Altimetry Deep ocean sea level recorders

11. 11 Why Then Do We Need Tide Gauges Now in the “Age of Altimetry”? Acoustic Gauge in Australia Principle of continuity, relative low cost of gauges (altimetry has been operational during only the last decade) Long records for secular trend/acceleration studies need to be continued (e.g. for input to IPCC) Higher frequency sampling important in straits and other areas High latitude regions of ice coverage cannot be monitored by altimetry Altimeter calibration Coastal applications

12. 12 Questions and Answers Q. Has global sea level risen during the 20th century ? A. Yes. By 10-20 cm. (There are many references - see the IPCC Third Assessment Report for a review) After all this monitoring there must be many scientific questions and answers:

13. 13 Questions and Answers Q. Do we understand why it has risen? A. Yes. (More or less)

14. 14 Why has sea level risen? Main driver has been the 0.6 º global temperature change during the past century, but there have been many contributors to the sea level change (numbers are approximate in cm): Thermal expansion (5), Glaciers/ice caps (3), Greenland (0.5), Antarctica (-1), Ice sheets ongoing since last glacial max. (2.5), Permafrost (0.3), Sediment deposition (0.3), Terrestrial storage (-3.5) ……. (See IPCC TAR for a review)

15. 15 Questions and Answers Q. Is the rate of rise increasing ? A. No. From 20th century data alone. A. Yes. From 18th-20th century data.

16. 16 Increasing rate of rise 6 of the longest sea level records from Northern Europe showing a small acceleration of sea level change into the 20th century

17. 17 Questions and answers Q. How much might sea level rise in the 21st century? A. 9-88 cm with central value 48 cm from 35 emission scenarios and 7 AOGCMs (IPCC Third Report). Predictions relatively insensitive to emission scenarios over next few decades. (N.B. rises will not be the same in all parts of the world because of the readjustment of the ocean circulation to climate changes.)

18. 18 Long Term Changes in Sea Level 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

19. 19 Questions and Answers Q. How important will the 21st century changes be? A. MSL changes coupled to changing meteorology, surges, tides, waves (water depth changes) ==> Extreme level analyses.

20. 20 Extreme Level Analysis This shows the ‘return period’ of levels being exceeded at Lowestoft (a typical east coast port) at present (solid line) and with a 50 cm rise in MSL (dotted line). The 50 cm causes return periods (for a given level above ODN) to be reduced by about a factor of 10.

21. 21 Coastal areas at risk Areas below 1000- year return period level By 2100: the 1 in 1000 year flood level (shown here in red) may become a 1 in 100 year level

22. 22 Questions and Answers Q. Will sea level continue rising beyond the 21st century? A. Yes, for 100s of years (the ‘Sea Level Commitment’) as the lower levels of the ocean warm.

23. 23 ‘Sea Level Commitment’ Scenarios of long term sea level change beyond the 21st century (from IPCC). Rises of 1-4 m might be expected over several 100 years.

24. 24 SUMMARY: MEASUREMENTS  TIDE GAUGES The PSMSL data set is the basis of understanding that sea level has risen during the past 100 years.  NEW GEODETIC TECHNIQUES Developments in new geodetic techniques (GPS, DORIS, Absolute Gravity) are progressing for monitoring vertical land movements. This will eventually provide estimates of ‘absolute’ sea level change.

25. 25 SUMMARY CONTINUED EARTH OBSERVATION TECHNIQUES (1) Satellite radar altimetry has the potential to provide truly- global sea level change estimates, rather than at coastlines as for tide gauges. However, gauges continue to be required for continuity, local coastal use and for altimeter calibration. (2)Space gravity will provide precise measurements of the geoid and of temporal gravity variations  a range of applications to sea level science. Challenge is to combine the several measurement techniques into one global monitoring system.

26. 26 SUMMARY: SEA LEVEL TRENDS  In 20 th century, global sea level rose by 10-20 cm, with a slow acceleration between the 19 th and 20 th centuries.  In 21 st century, the rise could be of order 50 cm (IPCC TAR). (However, note the importance of interannual variability and of the role of extremes)

27. 27 SUMMARY: UK CONTRIBUTIONS TO THIS FIELD  UK (POL/NERC) hosts the PSMSL global sea level data bank. Funds monitoring in UK and South Atlantic, Gibraltar etc.  BAS (NERC) expertise in glaciology  Hadley Centre (Met Office) is a world centre for modelling sea level change due to climate change  Tyndall Centre (NERC/hosted by UEA) for collaborative studies of impacts of climate change  DEFRA plays major role in supporting UK sea/land level monitoring and science  POL will lead the formation of a UK National Tidal & Sea Level Facility in 2002