| Summary: | Rates of regional and global sea-level rise during the 20th century were faster than in any century over the last 3000 years. Sea-level rise accelerated between ~1850 and ~1950, before greenhouse gases became the dominant forcing agent, which suggests, in part, a natural origin. The acceleration appears to have been more rapid in the Southern Hemisphere, which, according to geophysical theory, could point at a contribution from Northern Hemisphere land-based ice melt. More high-resolution relative sea level (RSL) reconstructions from the Southern Hemisphere are needed to test this hypothesis, and to complement a limited dataset of proxy and tide-gauge records. This study establishes three new RSL records for southeastern Australia (covering ~1830 – 2018) from analyses of salt-marsh sediments. New training sets of contemporary salt-marsh foraminifera were used for transfer-function analyses to derive palaeo sea-level estimates. High-resolution chronologies were established via Accelerator Mass Spectrometry radiocarbon, radiogenic lead, stable lead isotope ratios and pollen analyses. The new records demonstrate that, when corrected for glacio-isostatic adjustment, sea level has risen by ~0.2 – 0.3 m since ~1830 in southeastern Australia. Rates of sea-level rise were especially high over the first half of the 20th century, with maximum average rates of 4.0 (-0.4 – 7.1 95 % confidence range) mm yr-1, but there is regional variability between sites. A modelled sea-level budget indicates that the acceleration was initially driven by the barystatic component (including gravity, rotation and deformation), but subsequently amplified and driven by sterodynamic sea-level change. An analysis of the sea-level fingerprints of the barystatic component to 20th century global sea-level rise points at a significant input from the Greenland (17 %) and Antarctic Ice Sheets (11 %) as well as glaciers in Alaska (14 %), the Russian Arctic (10 %), western Canada and the US (9 %), south Asia and southern Andes (8 % each).
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