Unravelling climatic influences on late Holocene sea-level variability

Predictions of future coastal change in response to 'Greenhouse Warming' have been based on a historical, secular trend in sea-level rise, with an estimated total rise of up to 0.88 m, and an average estimate of 0.4 ± 0.1 m over the next 100 years (Houghton et al., 2001). However, our knowledge of past decadal to centennial sea-level change and its influence on coastal evolution over the last few hundred to 1000 years is scant. This chapter outlines the climatic and non-climatic factors influencing relative sea-level (RSL) change, and discusses the advancements in Late Holocene studies that are enabling a multi-decadal to centennial examination of coastal evolution and the influence of sea-level changes over the 2000 years. High-resolution proxy sea-level data are being interpreted from fixed biological indicators, such as intertidal coral microatolls and encrusting tubeworms and barnacles together with saltmarsh and chenier sedimentary sequences. Three case studies on the resolution of last millennial sea-level and climate changes are presented for the Southwest Pacific Ocean, South Atlantic coast of South Africa, and the northeast Atlantic Ocean coastline of USA. A fourth case study is presented on the application of multicentennial coral δ¹⁸O derived sea-surface temperature (SST) time-series to temporally extending RSL trends from tide-gauge measurements. Broadly, the recent studies described above depict some agreement in regional sea-level history between oceanic regions; however, each region displays a characteristic signal. There is considerable evidence from both the northern and southern hemispheres that sea-level was at least 0.2-0.5 m lower than late 20th century sealevel during the period AD 1400-1850 (including the Little Ice Age). Estimates of higher sea-level during the prior period AD 700-1300 are less regionally consistent in both hemispheres, and in the southern hemisphere, may principally reflect the influence of the post-glacial global isostatic adjustment processes, rather than climatic factors alone. This general hypothesis requires rigorous examination using developing highresolution palaeo-methods in both coastal evolution and palaeoceanographic studies.