PREDICTING 21ST CENTURY TIDAL MARSH VULNERABILITY TO SEA-LEVEL RISE USING HOLOCENE RELATIVE SEA-LEVEL DATA

Tidal marshes are widely recognized as one of the most vulnerable ecosystems to relative sea-level rise, because they occupy a narrow elevation range, where marsh plants collapse and convert to tidal mud flat or open water if inundated excessively. But regional and global model simulations of the future ability of marshes to maintain their elevation with respect to the tidal frame are uncertain. The compilation of empirical data for tidal marsh vulnerability is, therefore, essential to address disparities across these simulation studies. A hitherto unexplored source of empirical data are Holocene records of tidal marsh evolution.

Here, the limits to marsh adaptability are revealed through the analysis of over 400 reconstructions of tidal marsh collapse and conversion to tidal mud flat or open water from 54 regions in Great Britain during the Holocene. The ice sheet that covered much of northern Great Britain at the Last Glacial Maximum was large enough for glacial isostatic processes to produce contrasting local relative sea-level histories during the Holocene that ranged between -7 and 14 mm/yr. Holocene records indicate a 90% probability of tidal marsh collapse at sites with relative sea-level rise exceeding 7.3 mm/yr (95% CI: 6.6-8.6 mm/yr). Although most modern tidal marshes in Great Britain have not yet reached these sea-level rise limits, our empirical data suggest widespread concern over their ability to survive rates of sea-level rise in the 21st century under high emission scenarios.