Summary of changes and trends

• Sea level records from Liverpool, Newlyn, Portsmouth and Dover show local short-term variations in amplitude and phase of tidal constituents but no long-term trends.
• There is no evidence of a trend in sea level surges at Liverpool since 1768, Newlyn since the 1920s, or Portsmouth and Dover since the 1960s.
• Global mean sea level (MSL) has risen by about 120 meters since the last ice age around 20,000 years ago and by 1.0 to 2.0 mm per year during the 20th twentieth century.
• After adjusting for land movements, ‘absolute’ sea level around the UK coast has increased by about 1mm per year during the 20th Century.
• ‘Relative’ MSL, due to the combined effect of absolute MSL changes and land movements, is increasing around most of the UK coast but remains constant or is even decreasing along some northern coasts.
• UK MSL shows an increase in the rate of rise towards the second half of the 19th Century. However, sea level is now rising on average less fast than over a base period of 1921-1990; i.e. there has been a decrease in the rate of rise in the 20th Century.
• Trends in UK extreme sea levels match MSL trends closely.

1. Introduction

Sea levels are a combination of tidal level, surge level, mean sea level and waves and their interaction. Any change in mean sea level affects sea level directly but also modifies tide, surge and wave propagation and dissipation by changing the water depth. Increased depth gives longer tidal wavelength and hence the tidal pattern is shifted, resulting in an increase or decrease in tidal levels. The generation and dissipation of surges partly depends on water depth because the wind-stress effect increases in importance as the depth decreases. Increased depth in coastal waters leads to greater wave energy transmitted to the shoreline.

As the most serious coastal flooding events at the UK coast are caused by a combination of high tides, surges and waves, any overall long-term increases in tidal level, surge or waves will increase the frequency of flooding along a coastline, especially if a rise in mean-sea level provides a higher “base-line” for them. Also, rising sea level will reduce beach width and increase coastal erosion, particularly through the effect of increasing wave energy.

A rise in sea level can cause the loss of salt marsh and mudflats, thus having an effect on ecosystems, particularly on intertidal habitats. Also, the impact of sea level rise on a changing wave climate, and hence on water turbidity, will have an impact in the near shore leading to biological effects.

1.1 Tidal levels

These are the regular motions of the sea generated by astronomical forcing due to the varying gravitational attraction of the Moon and the Sun. UK waters respond strongly to tidal forcing at the Atlantic Ocean boundary and the presence of the British Isles creates a series of more or less separate basins in which the tidal wave, incident from the deep ocean, is reflected and amplified to varying degrees. The general response in UK waters is to amplify the semi-diurnal (two tides a day) component of the tide and particularly strong responses occur in the Irish Sea and the Bristol Channel.

Semidiurnal lunar tides increase and decrease in range over an 18.6 year period because of changes in the lunar declination cycle. When the declinations are small the semidiurnal tides are bigger. The most recent maximum in semidiurnal tides was in 1997, with a subsequent fall to 2006. The theoretical modulations are 3.7% about the mean, but in practice because of shallow water effects, around the UK the modulations vary locally around 2%.

Click here to see a figure of the long-term cycles in the range of tidal sea level variations at Newlyn.

Click here to see a figure of the tidal level characteristics at UK coastal sites (Source: STEMgis).

Click on the image to see an animation of tidal levels propagating around the UK coast during a specific “storm surge” event, from 5pm on 8th - 4am on 11th November 1998
[GIF animation, 453KB.].

Click here to see the same animation in AVI [2.0MB].

Courtesy of Colin Bell, POL.

1.2 Surge levels

These are caused by changes in atmospheric pressure and associated wind stress, and can result in water levels above (“positive surge”) or below (“negative surge”) those of the normal tide. The wind-stress effect depends upon water depth and increases in importance as the depth decreases whereas the pressure effect is independent of depth. “Storm surges” are generated by major meteorological disturbances, and can result in sea level changes of up to several metres lasting a few hours to days, depending upon the storm duration, water depth and the extent of the storm.

1.3 Mean sea level (MSL)

This is defined as the height of the sea averaged over a period of time, such as a month or year, long enough that fluctuations caused by waves and tides are largely removed. Daily, monthly, seasonal and annual variations of MSL include contributions from tides and surges and are due to changes in atmospheric pressure, wind stress, density and/or water circulation. Around the UK, MSL changes about 10 cm seasonally, and a maximum in late summer.

MSL changes measured by coastal tide gauges contain contributions both from real changes in ocean level and from vertical movements of the land upon which the gauges are situated. Therefore MSL ‘relative’ to the land also depends on local (e.g. sediment compaction or ground water extraction) or regional land movements (e.g. as a result of post-glacial ‘isostatic’ adjustment). Thus ‘absolute’ sea level has had any land movements removed from the ‘relative’ MSL signal. Long term, ‘secular’, changes in absolute MSL are mainly caused by changes in water volume, e.g. an increase caused by the melting of grounded ice or the thermal expansion of seawater due to heating.

Although small in global terms, the ice sheet that covered much of the British Isles was large enough for post-glacial isostatic adjustment processes to produce contrasting relative MSL changes at different locations. Maximum relative land uplift, approximately 1.6 mm/ yr, occurs in central and western Scotland and maximum subsidence, approximately 1.2 mm/yr, in southwest England. Sediment consolidation, arising from compaction as the sediment accumulates and from land drainage, increases the subsidence in areas with thick sequences of Holocene sediments, with an average effect equivalent to an extra approximately 0.2mm/yr land subsidence, but more in parts of southeast England, approximately 0.5 – 1.1mm/yr (Shennan and Horton, 2002).

Click on the image to see a map of land uplift or subsidence in Great Britain, from the radiocarbon dating of microfossils contained in sedimentary deposits.
Courtesy of Ian Shennan, Durham University.

1.4 Measuring and monitoring sea level

Descriptions of the monitoring networks that regularly measure sea level are given in Chapter 1, including details of how to access near real-time data.

Click here for a list of monitoring networks and data sets.

2. Trends in tides and surges

Sea level records from Newlyn, Portsmouth and Dover show local short-term variations in amplitude and phase of tidal constituents but no long-term trends (Araújo at al., 2002). Tidal elevation records in the lower estuary of the River Mersey show almost no changes to the predominant tidal constituents over a sixty-three year period (Lane, 2004).

Araújo et al., (2002) showed that there is no evidence of a change in surge levels at Newlyn since the 1920s, Portsmouth since the 1960s or Dover since the 1960s; and that there was no correlation between the Newlyn surge levels and the NAO. Analysis of surge statistics from the Liverpool tide gauge data has shown that there were no long term changes over the period 1768 to 1999 (Woodworth and Blackman, 2002).

Click here to see sea level trends from the UK National Tide Gauge Network.
Link to http://www.pol.ac.uk/ntslf/trends.php

Figure 1 shows the data from the five longest UK MSL records at Aberdeen, North Shields, Sheerness, Newlyn and Liverpool. The records from Aberdeen and Liverpool are composites from more than one gauge at each site, whereas the others are from gauges where there is a full benchmark datum history, and therefore in the Revised Local Reference (RLR) subset of the Permanent Service for Mean Sea Level (PSMSL). All five stations show a positive trend (i.e. an increase) in MSL, relative to the land, as do the majority of the other shorter records (Woodworth et al., 1999).

Various shortcomings in tide gauge data have meant that there is as yet no consistent estimate of the rate of change of MSL for Northern Ireland (OST, 2004b). These shortcomings include physical changes that have occurred to the Belfast harbour gauge such as relocation and harbour development and the shortness of the records of the recently installed gauges at Portrush and Bangor. Therefore the record of MSL at Malin Head (Republic of Ireland) is used by the Ordnance Survey as their geodetic gauge for Northern Ireland. Observations suggest there has been an almost zero change in relative sea level since records were started in 1958, but a large recent dip in the record is almost certainly instrumental (Woodworth et al., 1999).

Due to post-glacial recovery, the land in Scotland and northern England is uplifting but in southeast England it is submerging. After adjusting for these natural land movements, ‘absolute’ sea level around the UK coast has increased by about 1mm per year, or 10cm for the 20th Century. This trend is consistent with, but at the lower end of, the range of uncertainty of 10-20cm of the estimates of global change by the IPCC (IPCC, 2001). The combined effect of absolute MSL changes and land movements mean that relative MSL in the UK is mainly increasing but remains constant or even decreases along some northern coasts. In UK waters, sea level changes show a small ‘sea level acceleration’ component, which is consistent with being the result of an acceleration towards the second half of the 19th Century (Woodworth, 1999), as well as considerable inter-annual and inter-decadal variability.

There is coherent variability in sea level changes around the British Isles (BI) (Woodworth et al., 1999). Therefore their “BI sea level index” can be used as a guide to the 'average state' of MSL in UK waters, see Figure 2. The index is computed from the five longest UK MSL records at Aberdeen, North Shields, Sheerness, Newlyn and Liverpool. Each record has been detrended using the trend computed over a base period of 1921-1990; thereby, in principle, removing low-frequency geological and climate-change contributions. Therefore the index shows averaged interannual variability after long term trends have been removed.

The inter-annual changes in the index (Figure 2) are related to changes in local meteorological forcing (storm surges) and to oceanographic changes in shelf and nearby deep ocean circulation. The index shows a dip in the early 1990s that is as deep as the ‘mid-1970s dip’ that exists in all UK records. There are also dips around 1920, 1940 and 1962. The generally negative values in the latter part of the index indicate that sea level is now rising on average less fast than over a base period of 1921-1990; i.e. that there has been an overall deceleration, rather than an acceleration, in twentieth century MSL in UK waters. The inter-decadal variability in the index is reminiscent of that of other oceanographic and meteorological parameters in the North Atlantic, such as surface temperature, salinity, wind stress and storms (Woodworth et al, 1999).

4. Mean sea level and the North Atlantic Oscillation

Wakelin et al. (2003) have shown that winter (December to March)-mean values of monthly MSL and the NAO Index (Jones et al., 1997) are significantly correlated over much of the northwest European shelf. There is a clear spatial pattern in the correlation, with strongly positive (> 0.8) values in the northeast and strongly negative (< -0.7) in the south. This is consistent with a positive NAO Index corresponding to anomalously low (high) atmospheric pressure in the north (south) leading to a hydrostatic increase (decrease) in the sea level due to direct pressure changes (the inverse barometer effect). The sensitivity of the sea level to the NAO is strongest in the southern North Sea, where most of the sensitivity is present also in the non-hydrostatic component of sea level, i.e. that due to changing wind stress. The rest of the North Sea has correlation > 0.3, while around the north and east coasts of Scotland the correlation exceeds 0.6. For most of the rest of the shelf, the correlations are below the level of significance.

Wakelin et al. (2003) also showed that the relationship varies with time, with sea level for 1909 – 1954 showing a lower correlation with the NAO compared with 1955 – 2000. Also, there was an increase in correlation between the periods 1959 - 1979 and 1980 - 2000 for the North Sea. Sea level pressure anomalies related to the NAO were located further eastwards during the latter period (Hilmer and Jung, 2000), thus increasing the associated westerly winds and wind-induced sea level.

5. References

Araújo, I., D.T. Pugh and M. Collins (2002). Trends in components of Sea Level around the English Channel. Proceedings of Littoral 2000, Porto, Portugal, September 2002, 1-8.

Dixon, M.J. and J.A. Tawn, (1992). Trends in UK extreme sea-levels: a spatial approach. Geophysical Journal International, 111: 607-616.

Hilmer, M. and T. Jung (2000). Evidence for a recent change in the link between the North Atlantic Oscillation and Artic sea ice export. Geophysical Research Letters, 27(7): 989 – 992.

IPCC (2001). Climate Change 2001, the scientific basis. J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden and D. Xiaosu, Editors, Cambridge Univ. Press.

Jones, P.D., T. Jónsson and D. Wheeler (1997). Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South-West Iceland. International Journal of Climatology, 17: 1433-1450.

Lane, A. (2004) Bathymetric evolution of the Mersey Estuary, UK, 1906-1997: causes and effects. Estuarine, Coastal and Shelf Science, in Press.

Law, F.M., F. Farquharson, A. Brampton, M. Dale and R.A. Flather (2003). Environmental change indicators (including those related to climate change) relevant to flood management and coastal defence. Defra/EA R & D technical report FD2311-TR.

OST (2004b). Aspects of the Foresight project specific to Northern Ireland.. Prepared by Stuart Suter and John Chatterton. Foresight Flood and Coastal Defence Project - Phase 2 Overview Report, Chapter 2, Appendix I.

Pugh (2004). Changing Sea Levels. Cambridge University Press.

Shennan, I. and B. Horton (2002). Holocene land- and sea-level changes in Great Britain. Journal of Quaternary Science: 17, 511-526. doi: 10.1002/jqs.710.

Wakelin, S.L., P.L. Woodworth, R.A. Flather and J.A. Williams (2003). Sea-level dependence on the NAO over the NW European Continental Shelf. Geophysical Research Letters, 30(7): 1403, doi: 10.1029/2003GL017041.

Woodworth, P.L., M.N. Tsimplis, R.A. Flather and I. Shennan (1999). A review of the trends observed in British Isles mean sea level data measured by tide gauges. Geophysical Journal International, 136: 651-670.

Woodworth, P.L. and Blackman, D.L. (2002). Changes in extreme high waters at Liverpool since 1768. International Journal of Climatology, 22: 697-714.

Woodworth, P.L. (1999). A study of changes in high levels and tides at Liverpool during the last two hundred and thirty years with some historical background. Proudman Oceanographic Laboratory Report No. 56, 62pp & figs.