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Update to UKCIP02 sea level change estimates,
December 2005
In October 2006, Defra issued new Supplementary Guidance to support the Planning Policy Statement 25 (PPS25) ‘Development and Flood Risk’ (December 2006). The guidance is designed to provide updated climate change impacts policy guidance to operating authorities such as Environment Agency and Local Authorities, as part of the development of public funded flood and coastal management plans, programmes and projects.
The guidance contains updated sea level rise allowances which are consistent with the UKCIP02 scenarios provided below. A new guidance note has been produced by UKCIP which explains the relationship between the two sets of sea level change information.
(This page is also available as an Adobe pdf document)
In the UKCIP02 Scientific Report (Hulme et al., 2002), future changes in regional net sea level were calculated for Wales, the regions of Scotland and the administrative regions of England. The estimations of future net sea level change are based on two components: isostatic changes, which refer to adjustments in the absolute elevation of the land; and eustatic changes, which refer to variations in the absolute elevation of the sea surface caused by variations in the volume of the oceans. Together they are used to estimate net sea level change, taking into account changes in both land and sea surface level.
In the UKCIP02 Scientific Report, net regional sea level changes to 2100 were presented based on isostatic change rates from Shennan (1989) and eustatic change rates from the IPCC estimates of projected global sea level (IPCC, 2001). Subsequently, the isostatic change rates have been updated (Shennan and Horton, 2002). This technical note uses these to update the UKCIP02 estimates of future changes in net regional sea level for Great Britain. It should be emphasised that the IPCC (2001) estimates of global sea level change have not been revised.
Figure 1 shows the updated estimates of mean relative land/sea level changes (mm/yr) in Great Britain over the last 4,000 years (the Late Holocene period) from Shennan and Horton (2002). The effects of sediment consolidation are not included, since they are highly localised and can vary over relatively short stretches of coastline. These rates, combined with the Low Emissions ‘low’ IPCC estimate and High Emissions ‘high’ IPCC estimates of global sea level change (Table 1), were used to estimate net sea level change.
Figure 1. Late Holocene mean relative land/sea level changes (mm/yr) in Great Britain, positive values indicate relative land uplift or sea level fall, negative values are relative land subsidence or sea level rise. Figures in parentheses are the trends that take into account modelled changes in tidal range during the Holocene. Contours are drawn by eye as a summary sketch of the spatial pattern of change. [Source: Shennan and Horton (2002)].
Table 1. Global average sea level change (cm) relative to the 1961-1990 average, for the highest and lowest predictions from the IPCC range. [Source: UKCIP02 Scientific Report]. A larger scale version of this table is available here.
Maps showing the relative sea level change for each scenario and time slice are shown in Figure 2 (with the method used described in greater detail in Box 1). Regional averages were also calculated, based on the devolved administrations of Wales and Scotland and the administrative regions of England (Table 2). Results could not be included for Northern Ireland and Orkney & Shetland (for where no data on isostatic adjustment was available) or for the West Midlands (which, at present, contains no coastline or tidal estuaries).
Figure 2. Net sea level change for Great Britain relative to 1961-1990 for the full range of global sea level changes estimated by the IPCC, incorporating updated isostatic change data from Shennan and Horton (2002). A larger version of this map is available here.
Table 2. Updated rates of mean vertical land movement due to isostatic adjustment for the devolved administrations of Wales and Scotland and for the administrative regions of England [Source: Estimated from Shennan and Horton, 2002]. Also shown is net sea level change for Great Britain relative to 1961-1990 for the full range of global sea level changes estimated by the IPCC, incorporating the updated isostatic adjustment data. Land movement data was not available for Northern Ireland or Orkney & Shetland. A larger scale version of this table is available here.
In conclusion, the results presented by Shennan and Horton (2002) permit improved estimation of future net sea level changes for the Great Britain coastline (as compared with those results presented in the UKCIP02 Scientific Report). However isostatic change is only one component in the calculation of regional net sea level change.
The range of global sea level changes calculated by the IPCC in their Third Assessment Report (IPCC, 2001) results from both emissions uncertainty and scientific uncertainty. In other words, for each of the four emissions scenarios, the IPCC obtained a variety of projected global sea level changes using different representations of the atmosphere and ocean. The smallest (low estimate) and largest (high estimate) differ by 60 cm by the 2080s (Table 1). Furthermore, there is also uncertainty regarding regional variations in climate-induced sea level changes, occurring because the warming and expansion of ocean water is not uniform across the globe. These regional differences can vary by up to +/-50 % of the change in the global average. For sensitivity studies, the advice remains (as stated in the UKCIP02 Scientific Report) to consider changes in sea level for each scenario that are approximately +/-50 % of those shown in Table 1.
It remains the case (as in the UKCIP02 Scientific Report) that regional land movement rates were not available for Northern Ireland, meaning that net sea level change could not be calculated for the Northern Irish coast. If anyone is aware of comparable data showing regional land movement rates for Northern Ireland, please contact UKCIP so that we can incorporate them.
Footnote (August 2006)
Since the publication of this update in December 2005, Gehrels (2006) has questioned the quality and quantity of the data points used by Shennan and Horton (2002) to reconstruct past isostatic changes in South West England. In his paper Sea level rise and coastal subsidence in southwest England, Gehrels undertakes detailed analysis of the four data points located in west Cornwall and south Devon and compares these with geophysical models to conclude that the true isostatic rate for South West England is approximately –0.5 mm/yr (around half the rate of –1 mm/yr estimated by Shennan and Horton, 2002).
Notwithstanding this, UKCIP do not believe at this time that Gehrel's results necessitate a revision of our estimates of net sea level rise (as presented above in Figure 2 and Table 2). There is clearly uncertainty surrounding estimation of rates of isostatic adjustment (and a lack of high-quality geologic data needed to validate these estimates), and we do not feel it is appropriate at this time to alter our national sea level change guidance on the basis of a single, localised study. We instead await the outcome of ongoing research at Proudman Oceanographic Laboratory that aims to produce a definitive model of isostatic changes for the entire UK.
Furthermore, the estimations of future net sea level change are based on two components: isostatic changes, which refer to adjustments in the absolute elevation of the land; and eustatic changes, which refer to variations in the absolute elevation of the sea surface caused by variations in the volume of the world’s oceans. Estimates from the IPCC suggest that projected changes in sea level as a result of climate change will become a dominant factor in sea level change by the latter half of this century. Even assuming no isostatic change, an increase of global sea level of 9 to 69 cm is anticipated by 2100 (Table 1) due to eustatic factors alone.
References
Gehrels, R (2006) Sea level rise and coastal subsidence in southwest England. Report and Transactions of the Devonshire Association for the Advancement of Science 138: 25–42.
Hulme, M., Jenkins, G.J., Lu, X., Turnpenny, J.R., Mitchell, T.D., Jones, R.G., Lowe, J., Murphy, J.M., Hassell, D., Boorman,P., McDonald,R. and Hill,S. (2002) Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report, Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK. 120pp.
IPCC (2001). Climate change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. In: McCarthy, J.J., Canziani, O.F., Leary, N.A., Dokken, D.J. and White, K.S. (eds.) Cambridge University Press, Cambridge . 1032pp.
Shennan, I. (1989) Holocene crustal movements and sea level changes in Great Britain. Journal of Quaternary Science 4: 77–89.
Shennan, I. and Horton, B. (2002) Holocene land- and sea level changes in Great Britain. Journal of Quaternary Science 17: 511–526.
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