Professor David Smith, University of Oxford

In most parts of the world, coastal flooding from the sea is the product of rapid and high magnitude events superimposed upon long-term, or secular, changes. In the British Isles, the high magnitude events may be storm surges or tsunami. The secular events are one the one hand land movements caused by isostatic changes following the last glaciation, and on the other, sea surface movements caused by climate change, the combination of land and sea surface changes resulting in an apparent, or relative change in sea level at the coast.

Since the Fourth Biennial Flood Report was produced, we have learnt more about both coastal flood events and secular sea level changes in Shetland, and although our knowledge is still generalised, we are closer to understanding the threats posed to economy and society in Shetland.

The only study to have outlined changes in the level of the sea surface offshore NW Europe since the last glaciation was that of Morner. He based his graph on a projection of shoreline altitudes in Scandinavia. The main features of the graph are the fluctuations evident and the rapid rise in the sea surface up to about 6000 radiocarbon years BP. In Shetland, secular sea level changes will have included a component of land movement due to the effect of ice loading and subsequent unloading. At the maximum of the last glaciation, Shetland was covered by an ice sheet which recent information indicates probably extended westward to the edge of the continental shelf in the Faeroe-Shetland channel. This will undoubtedly have resulted in isostatic effects, as may the nearby Scandinavian ice sheet. There would have been crustal uplift in the area as and after the ice melted, but this uplift would have varied according to the original thickness of the ice, thus greater over Mainland than over the outlying islands. Taking account of this, even with the complications of both the proximity of the Scandinavian ice sheet and water loading on the continental shelf as the ice retreated, it is highly likely that crustal movements varied across Shetland, so that rates and patterns of relative sea level change in the past will have varied according to location.

Future sea surface changes around Shetland are likely to reflect global climate change. In the IPCC Third Assessment, a range of scenarios is given, estimating global average sea level rise of between 9 and 88 cm from 1990 to 2100. The best estimate figure was 49 cm. It should be recognised that this relates to generalised global changes: in different areas of the world, including Shetland, the actual changes will be different. In addition, if the land is sinking – and this has not been proven – the relative change will be greater. Recent research is suggesting that the rate of rise in global average sea surface levels will increase after about 2040, and in this context the estimate given in the Fourth Biennial Flood Report of up to 50cm over the 2004 level by 2100 could be on the conservative side. Further, it should be noted that modelling recently undertaken indicates that as sea level rises, the tidal range in estuaries will be greater than the rise by up to 25%. Overall therefore, whatever sea level rise scenario is proposed, the consequences for Shetland’s indented coastline with its long firths should not be underestimated, despite the uncertainties.