1. Global sea level change: What drives the decadal variability? Analysis of near global altimetry data suggests a rise of 2.9 mm/yr (3.3 mm/yr after GIA correction) over the past decade On the face of it, this is a large, recent acceleration Global sea level is believed to have risen 1-2 mm/yr over the past 100 years − Sparse dataset from tide gauges Simon Holgate, Permanent Service for Mean Sea Level (simonh@pol.ac.uk)

2. Questions to be answered.... Has there been an acceleration in sea level rise over the past century? How great is the variability in rates of sea level rise? − Is a decadal rate of 3 mm/yr that unusual? How do Spanish tide gauges fit this picture?

3. Spatial variability in altimetry trends 1993-2003 (From Cazenave and Nerem, 2004)

4. Intergovernmental Panel on Climate Change The IPCC AR4 gives 1.8±0.5 mm/yr for global mean 20 th century sea level rise Largest component of sea level rise is thought to be thermal expansion (thermosteric rise) - Huge uncertainties in thermal expansion, melting rates, groundwater run-off and impoundment - Large gaps in datasets

5. IPCC AR4 rates of sea level rise components 1961-2003 (mm/yr)ObservedModelled Thermal expansion0.42±0.120.5±0.2 Glaciers and ice caps0.50±0.180.5±0.2 Greenland – 20th century effects 0.05±0.12 * 0.14±0.41 * Antarctica – 20th century effects Estimated from tide gauges1.8±0.5 Total 1.1±0.51.2±0.5 Difference (TG - contributions)0.7±0.7

6. IPCC TAR estimated rates of SLR components (mm/yr)MinimumCentral valueMaximum Thermal expansion0.30.50.7 Glaciers and ice caps0.20.3 0.4 Greenland – 20th century effects0.00.050.1 Antarctica – 20th century effects– 0.2– 0.10.0 Ice sheets – adjustment since LGM0.00.250.5 Permafrost0.000.0250.05 Sediment deposition0.000.0250.05 Terrestrial storage – 1.1– 0.350.4 Total – 0.80.72.2 Estimated from observations1.01.52.0

7. Datasets Tide gauge records selected from the Revised Local Reference dataset of the Permanent Service for Mean Sea Level (PSMSL) - Records have been corrected for local datum changes and checked for obvious problems such as abrupt changes due to earthquakes or trends due to local factors (e.g. Subsidence due to water extraction)

8. The PSMSL Dataset The PSMSL dataset is global but has many gaps in time and space − Contains over 53 000 station years of data from about 1800 stations − Only ~180 stations are of sufficient quality at 50 years

9. Methods Not possible to link RLR records together − No accurate geodetic surface available 20 th century acceleration investigated using Peltier's ICE-4G model of Glacial Isostatic Adjustment − Assumes GIA is only geological process leading to vertical land movement Sea level data corrected for inverse barometer effects using HadSLP2 reanalysis

10. Regional distribution of tide gauge records for 20 th C study Nine long station records selected Successive, over-lapping 10 years means calculated

11. Variability in decadal rates over the 20 th century Highest rate in the past 100 years is for the decade centred on 1979, second highest rate is 1938 Lowest rate is 1963

12. Total sea level rise has been ~170 mm over the past 100 years − Mean rate is 1.74  0.16 mm/yr Sea level rise has decelerated − 1904-1953: 2.03  0.35 mm/yr, 1954-2003: 1.45  0.34 mm/yr Integrated sea level rise 1904-2003

13. Spanish tide gauge records There are 27 time series in the PSMSL from Spain 12 time series are on the Atlantic coast Only 4 Atlantic series are longer than 20 years

14. Long Atlantic tide gauge records The 4 longest records have similarities but also major differences ‘Coruña I’ has some problems in the early record

15. Decadal Rates of Change Decadal variability is quite large Similarities between stations are weak The relationship with the global mean is also poor

16. Wind and Pressure Monthly Contributions Multivariate regression using static pressures and wind fields greatly reduces monthly variability >50% reduction in SD La CoruñaSantander

17. Wind and Pressure Decadal Contributions On decadal timescales, variance reduction is small Implication that variability derives from open ocean La CoruñaSantander

18. Model Variability POLCOMS model 12km resolution Domain: 40 o N-65 o N, 19 o W- 13 o E 40 year run: 1960-2000 Danish Met reanalysis wind forcing Single year deep ocean boundary condition No decadal variability! “Stations” much more coherent than real data

19. Summary Has sea level rise accelerated over the past century? Not significantly, but possibly over the past 130 yrs. Has there been a recent acceleration in the rate of sea level rise? The 1990s rate is high but not significantly different from other 20 th C decadal rates Decadal variability in Atlantic sea level records is much larger than the global mean Variance reduction using static pressures and winds is very effective at monthly time-scales Atmospheric forcing appears to have little influence at decadal time-scales. Variability probably comes from the deep ocean Coastal sea level and extremes are what count for humans

22. Global sea level curve based on EOF reconstruction EOF based reconstruction of sea level by Church and White (2006) 20 th century sea level rise is 1.7 mm/yr Significant acceleration of 0.013 mm/yr 2 over the last 130 years

23. Comparison of 20 th century rates from EOF and tide gauges Comparison of Church and White’s EOF reconstruction of rates of sea level rise with those directly obtained from tide gauge records

24. Church and White shows slight acceleration in second half of 20 th century while others decelerate – acceleration/deceleration highly dependent on period chosen Comparison of reconstructions of 20 th Century sea level change

25. The longest sea level records (all European) show an apparent acceleration over the longer term − recovery from the Little Ice Age? (From Woodworth, 1999) Comparison with other long sea level records

26. Relationship of sea level to SST and heat content ∉ Relationship of sea level to atmospheric forcing through heat content?

27. Relationship of rate of sea level change to global temperatures Rahmstorf has found a statistical relationship between global temperatures and the rate of sea level change (Science Express, 2006)

28. Comparison of projections of sea level rise to 2100 Comparison of mean projected sea level rise for climate change scenarios with those calculated by Rahmstorf’s statistical method

29. Sea level variability and volcanism Church et al (2005) found a significant link between global ocean heat content and volcanism They argue that volcanism has slowed global sea level rise since 1960

30. Regional variation in sea level rise over past 55 years ∉ Neighbouring regions tend to be the most coherent ∉ Shows the large scale of sea level variability ∉ Some regions are anti-correlated e.g. E and W Atlantic ∉ Sea level rise in the 1990s was most marked in Australasia, Scandinavia and N. Europe

31. Variability in rates of sea level rise over the past 55 years ∉ Highest rate in the past 55 years is for the decade centred on 1995, followed by 1979. ∉ Lowest rate is centred on 1987 ∉ High rates of SLR are not significantly correlated with ENSO

32. Regional distribution of tide gauges for past half century study In total 177 station records suitable for analysis over past 55 years Successive, over-lapping 10 years means calculated – Averaged into 13 regional means

33. Global comparison of tide gauge and altimetry rates ∉ Comparison of the tide gauge and altimeter derived sea level trends 1993-2002 ∉ Colours inside the circles indicate the tide gauge trends

34. Coastal sea level rise from altimetry, 1993-2002  The area weighted mean sea level rise from coastal altimetry is 3.7 mm/yr - Closer to the tide gauge value of 4 mm/yr than to the open ocean value of 2.8 mm/yr  Implication that coastal sea level rose faster than the open ocean over the past decade

35. Relationship of sea level to SST and heat content ∉ Relationship of sea level to atmospheric forcing through heat content?

36. Comparison of 20 th century rates from EOF and tide gauges Comparison of Church and White’s (2006) EOF reconstruction of rates of sea level rise with those obtained from tide gauge records

37. Total sea level rise has been ~170 mm over the past 100 years − Mean rate is 1.74  0.16 mm/yr Sea level rise has deccelerated − 1904-1953: 2.03  0.35 mm/yr, 1954-2003: 1.45  0.34 mm/yr Integrated sea level rise 1904-2003

38. RHS shows reconstruction using a varying number of gauges through time produced using Singular Spectrum Analysis - Multi-Taper Method (Jevrejeva et al, 2006) − Mean rate: LHS 1.74  0.16 mm/yr; RHS Sea level rise has deccelerated − 1904-1953: 2.03  0.35 mm/yr, 1954-2003: 1.45  0.34 mm/yr Comparison of reconstructions of 20 th Century sea level change

39. Decadal rates of sea level rise in the 20 th century. Simon Holgate, Permanent Service for Mean Sea Level, Proudman Oceanographic Laboratory, Liverpool, UK (simonh@pol.ac.uk) www.pol.ac.uk/psms l

40. Motivation 2 Cabanes et al suggest that thermosteric rise may account for most sea level rise observed over the past half century Implication that sea level rise due to all other sources is negligible Solid line: sea level rise from “Douglas set” of 25 tide gauges Dotted line: themosteric rise from “Douglas set” Dashed line: global thermosteric rise From Cabanes et al, 2001

41. Overview ∉ Background to sea level measurement – Permanent Service for Mean Sea Level (PSMSL) – Tide gauges and satellite altimetry ∉ Sea level rise in the IPCC Third Assessment Report ∉ Geodetic constraints on sea level rise ∉ Beyond linear trends – Accelerations from EOF reconstructions – Volcanism and heat content – Gravitational attraction of ice sheets – Variability in global rates of sea level change ∉ Recent developments in ice sheet measurements – Altimetry and GRACE

42. Sea level measurement

43. Permanent Service for Mean Sea Level (PSMSL) Established by IUGG in 1933 and member of the Federation of Astrophysical and Geophysical Services (FAGS) Responsible for collection, analysis (including research as high level quality control), distribution of monthly and annual MSL data, provision of a wider ‘Service’ (training materials, scientific and technical advice) Funding from FAGS, IOC and NERC

44. So what are “tide gauges”? Tide gauges have changed a great deal since the first automatic gauge in Sheerness. Modern gauges use pressure, acoustics or radar - Each method has its problems

45. Altimeter data is 1 o x1 o gridded product from Leuliette et al. Correlation between tide gauges and altimeter is 0.63 Comparison of recent tide gauge and altimeter trends For the period January 1993-December 2002 – the GIA corrected tide gauges give an average rate of 4.0mm/yr – Altimetric measurements at the tide gauge sites gives 4.1 mm/yr

46. Altimetric trends in sea level From Leuliette et al. (2004)

47. Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR)

48. Estimates of contributions to global sea level rise From IPCC (2001)

49. Geodetic constraints on sea level rise

50. Geodetic measurements  Attempts to use geophysical constraints such as the rate of spin of the Earth (Length Of Day measurements) are also very uncertain  Munk (2002) combined observations of eclipses with lunar ranging and direct satellite measurements of the Earth’s inertia - Munk’s paper leaves no room for mass changes - Smoothing of eclipse data hides variability

51. Geodetic measurements 2 Polar wander appears to rule out mass melting of Greenland - Circularity problem as models of post-glacial rebound tuned with polar wander data ∉ Recent reanalysis by Mitrovica allows for 0.8 mm/yr of 20 th century ocean mass change from ice sheets & glaciers

52. Beyond linear trends

53. Global sea level curve based on EOF reconstruction EOF based reconstruction of sea level by Church and White (2006) 20 th century sea level rise is 1.7 mm/yr Significant acceleration of 0.013 mm/yr 2 (a) Monthly and yearly fits (b) departures from fit (c) decadal rates

54. Thermosteric contributions to sea level rise From Miller and Douglas (2004)

55. Gravitational effects of ice sheets The contribution of ice sheets and glaciers to rates of sea level rise varies spatially (a) Antarctica (b) Greenland (c) Glaciers (Mitrovica et al, 2001)

56. Effect of ice sheets on tide gauge measurements The effect of melting ice sheets on the rate of sea level measured by the 23 tide gauges from the “Douglas set” (Mitrovica et al, 2001)

57. GIA correction to tide gauges GIA corrected observed rates from 23 gauges in the “Douglas set” (a) uncorrected rates (b) GIA correction using ICE-4G (c) GIA correction using Davis and Mitrovica (1996)

58. Sea level variability in tide gauges

59. Why has coastal sea level risen faster? Boundary waves transmit information about sea level around the coastline - Coherent waves can be traced all around the Pacific coast of the Americas

60.  Total sea level rise has been almost 8 cm over the past 50 years  Is sea level rise going to accelerate? Integrated sea level rise 1948-2003

61. Recent developments Results from altimetry and GRACE

62. Antarctic mass loss from GRACE Velicogna and Wahr (2006) have shown a decreasing mass balance in Antarctica Largest contribution from W Antarctic ice sheet Antarctic contributes 0.4 mm/yr to global sea level (cf -0.2 to 0 mm/yr in IPCC TAR)

63. What timescale for Antarctica? Davis et al (2005) found that Antarctic expansion slowed global sea level by 0.12 mm/yr (1992-2003)

64. Contributions from Greenland ∉ Rignot and Kanagaratnam (2006) found a large acceleration in Greenland’s glaciers ∉ Ice sheet deficit has doubled in a decade ∉ Contribution to sea level rise increased from 0.23 mm/yr in 1996 to 0.57 mm/yr in 2005 (cf 0 to 0.1 mm/yr in IPCC TAR)

65. Model projections of sea level rise Mean projected sea level rise from a suite of models for climate change scenarios (IPCC, 2001)