Late Holocene land- and sea-level changes in the British Isles: implications for future sea-level predictions

Abstract

Four decades of palaeosea-level research in the British Isles have produced a large dataset of age–altitude curves of postglacial sea-level changes. Patterns of late Holocene relative sea-level change reveal the persistent influence of the British/Irish Ice Sheet and the larger Scandinavian Ice Sheet on contemporary rates of vertical land movements. The Shennan and Horton (2002) map of late Holocene relative land movements has been used in future sea-level rise predictions by the United Kingdom Climate Impact Programme in their 2002 assessment (UKCIP02), but has been mistaken for a map of absolute land movements. In this paper, land-motion data for Britain are extracted from the Shennan and Horton (2002) relative sea-level data, and a new map of crustal land movements is presented which also includes Ireland. This procedure takes into account the regional 20th century sea-level rise (∼0.14 m) and the process of ocean syphoning ( i.e. a global fall in sea level of ∼0.3 mm/yr due to GIA induced ocean-floor lowering and re-distribution of ocean mass). The calculated land-motion rates also depend on the global late Holocene ice-equivalent sea-level change, given by the Intergovernmental Panel on Climate Change as 0.0–0.2 mm/yr. Accounting for these processes reduces the misfit between geological observations of vertical land motion and those independently derived from gravity-aligned Global Positioning System (AG GPS) measurements and shows that UKCIP02 has underestimated land subsidence in southern Britain and over-estimated land uplift in Scotland, both by 0.1–0.2 mm/yr. A best fit between GPS and geological estimates of land movements in Britain is achieved for a global long-term eustatic sea-level fall of ca 0.2 mm/yr, suggesting some global ice expansion in the late Holocene, rather than melt. If this is correct, uplift rates in Scotland would be lower and subsidence rates in southern Britain would be faster (by 0.4–0.5 mm/yr) than estimated by UKCIP02. More high-quality late Holocene relative sea-level data are required to quantify vertical land motion as accurately as possible, especially near large coastal population centres, for input into regional scenarios of future sea-level rise.