PREDICTING TIDAL MARSH SURVIVAL OR SUBMERGENCE TO FUTURE SEA-LEVEL RISE USING PALEO SEA-LEVEL DATA

Tidal marshes are vulnerable to relative sea-level rise (RSLR), because they occupy a narrow elevation range, where marshes retreat and convert to tidal flat, tidal lagoon or open water if inundated excessively. But regional and global models differ in their simulations of the future ability of marshes to maintain their elevation with respect to the tidal frame. Some landscape models predict up to an 80% decrease in global tidal marsh area by 2100, with substantial marsh loss even when RSLR rates are less than 8 mm/yr. By contrast, other simulation studies suggest that, through biophysical feedbacks and inland marsh migration, marsh resilience to retreat is possible at RSLR rates in excess of 10 mm/yr. The compilation of empirical data for tidal marsh vulnerability is essential to addressing disparities across these simulation studies.

Here we assess the limits to marsh vulnerability by analyzing >780 Holocene reconstructions of tidal marsh evolution in Great Britain, which includes both transgressive (tidal marsh retreat) and regressive (tidal marsh expansion) contacts. The probability of a marsh retreat was conditional upon Holocene rates of RSLR, which varied between -7.7 and 15.2 mm/yr. Holocene records indicate marshes are nine times more likely to retreat than expand when RSLR rates are ≥7.1 mm/yr. Coupling probabilities of marsh retreat with projections of future RSLR suggests a major risk of tidal marsh loss in the 21^st century. All of Great Britain has a >80% probability of a marsh retreat under Representative Concentration Pathway (RCP) 8.5 by 2100, with areas of southern and eastern England achieving this probability by 2040.