Abstract
Tidal marshes rank among Earth’s vulnerable ecosystems, which will retreat if future rates of relative sea-level rise (RSLR) exceed marshes’ ability to accrete vertically. Here, we assess the limits to marsh vulnerability by analyzing >780 Holocene reconstructions of tidal marsh evolution in Great Britain. These reconstructions include 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 that marshes are nine times more likely to retreat than expand when RSLR rates are ≥7.1 mm/yr. Coupling estimated probabilities of marsh retreat with projections of future RSLR suggests a major risk of tidal marsh loss in the twenty-first 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.
Highlights
Tidal marshes rank among Earth’s vulnerable ecosystems, which will retreat if future rates of relative sea-level rise (RSLR) exceed marshes’ ability to accrete vertically
Sea-level index points in regions of Scotland, close to the center of ice loading, record a non-monotonic pattern, showing deglacial RSL fall during the early Holocene (−7.7 to −0.7 mm/yr), before a rise throughout the mid-Holocene (0.0–6.0 mm/yr) to create a highstand, which was followed by RSL fall to present (−1.7 to 0.0 mm/yr)
In middle Great Britain (NE and NW England), at regions closer to the margins of the last glacial maximum ice limit, there is a transition from sites with a small or minor mid-Holocene highstand to sites where RSL is below present throughout the Holocene
Summary
Tidal marshes rank among Earth’s vulnerable ecosystems, which will retreat if future rates of relative sea-level rise (RSLR) exceed marshes’ ability to accrete vertically. Sea-level tendency[25] describes the increase or decrease in marine influence recorded by an index point, as indicated by a change in tidal marsh sediment stratigraphy or a transgressive or regressive contact[25].
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