Sea Level Is Not Level: The Case for a New Approach to Predicting UK Sea-Level Rise
Gehrels, Roland, Long, Antony, Geography
Current predictions for future sea-level rise along the UK coasts are based on IPCC values for global mean sea-level change combined with information on land movements, changes in tidal range and storm surges. In this article we argue that the global IPCC sealevel values should be replaced by a suite of regional values, to take into account known processes that cause regional changes in sea-surface topography. Quantification of these processes, which include gravitational adjustments of the ocean surface and nearshore ocean-density changes, requires the use of numerical ocean and geophysical models. 'Ocean siphoning' and 'continental levering' are two additional mechanisms that are not included in IPCC assessments, but can be quantified using a modelling approach. Data of vertical land movements based on geological information alone, as presently used by UKCIP (Defra) in UK sea-level rise scenarios, are potentially unreliable as they represent, in essence, values of relative land-level (or sea-level) change. These data can be improved by including geophysical model predictions and GPS measurements in assessments of vertical land motion. A combined modelling/geological approach will produce more robust regional sea-level predictions for the UK that are of real practical value to agencies responsible for coastal defence and flood protection.
The Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report in 2007 (IPCC, 2007). The report contains predictions for global sea-level rise ranging from 0.18m to 0.59m by the year 2100 AD (Figure 1). The wide range of these predictions reflects the various greenhouse gas 'emission scenarios' that may occur this century, each depending on factors such as population growth, industrial development, and efforts to curb greenhouse gas pollution. These sea-level predictions are presented as global averages, so it is justified to raise the question 'How useful are these predictions in practice?' Take, for example, the case of a council of a coastal town in the United Kingdom that wants to ascertain whether or not their coastal defences are up to scratch for the next 100 years. The latest IPCC report includes a separate chapter on regional changes in future climate, with seven pages devoted to Europe, but, unfortunately, no such regional analysis is made for sea-level changes. The report acknowledges that sea-level rise will be variable depending on location, but does not carry this through into specific regional predictions. How useful, then, are the IPCC numbers on a local or regional scale?
In trying to provide an answer to this question, we need to understand that several factors will influence future sea-level change at the local to regional scale: a change in local mean sea level, a change in tidal range, changes in storm-surge heights and vertical land movements. The IPCC predictions only address the first of these four terms - changes in mean sea level - and only in a simplistic manner and at a global level. They do not take into account any of the other processes. It is left in the hands of national agencies to try and translate the global IPCC predictions into usable numbers for specific coastal locations. In the UK this agency is the United Kingdom Climate Impact Programme (UKCIP), a subsidiary of the Department for Environment, Food and Rural Affairs (Defra). We will highlight their current sea-level predictions towards the end of this article.
First, though, we will outline the various processes that contribute to local sea-level changes and that should be taken into account in future predictions. What we argue is that sea level is in fact far from 'level' and that future changes in sea level will vary widely from any assumed global value. Unfortunately, many of the processes responsible for such variability are essentially ignored by the IPCC, and by the many agencies involved in predicting local- to regional-scale sea-level rise scenarios. …