RECONSTRUCTING QUATERNARY SEA LEVELS – DATA OR MODEL ESTIMATES?

Since the mid-1970s, sea-level change science underwent a rapid expansion, shown by the number of participants at conferences and the quantity of papers published. With an internationalisation of the science we see methodological structures that transcend different schools of thought and others that remain within distinct groups. I will highlight the former. Many of the key advances come when we integrate modelling studies with field data. But it helps even more when we also view our field-derived data as model estimates with quantified uncertainties.

If we adopt a generalised model, such as expressing schematically the change in relative sea level at a specific location as the sum of a time-dependent eustatic sea-level function derived from the model of ice history; the total isostatic effect of the glacial rebound process including the ice (glacio-isostatic) and water (hydro-isostatic) load contributions and the redistribution of ocean mass; the tectonic effect; and the total effect of local processes within the coastal system; we can use this model to adopt a rigorous hypothesis-testing framework. This contrasts to an earlier dominance of inductive approaches.

A case study of relative sea-level changes in Great Britain and Ireland shows how we can integrate the results of quantitative modelling of glacial isostatic adjustment with field-based reconstructions of relative sea-level change since the Last Glacial Maximum to provide a range of stakeholder-relevant information. At the national scale, for many users of sea-level research, the net effect of all components is the most useful measure of Late Holocene relative sea-level change as the background trend of anthropogenic-induced sea-level change. This shows an ellipse of relative uplift, ~1.4 mm per year, around the deglaciated mountains of Scotland. The pattern of three foci of relative subsidence, ~1 mm per year, results from the additional interactions of the Holocene meltwater load on the Atlantic basin and the continental shelf and the signal due to far-field ice sheets. Stakeholder needs, their local to national-scale remits, and responsibilities must determine how they use the baseline rates of relative sea-level change. At a local scale we must separate processes such as sediment compaction, which may be larger than the isostatic component.