English Coastal Archaeological Evidence of a Fifth-Century (Dark Ages) 4-Meter Sea-Level Rise in 70 Years, Portending a Similar Rise Imminently

Higgs, R., 2026. English coastal archaeological evidence of a fifth-century (Dark Ages) 4-meter sea-level rise in 70 years, portending a similar rise imminently. Journal of Coastal Research, 42(1), 165–195. Charlotte (North Carolina), ISSN 0749-0208. Published English archaeological literature is reviewed here from a geological (sedimentological) perspective. Roman-built Londinium's estuary-side wall (AD ∼270), four south-coast forts (AD ∼300) and a seaside palace (AD ∼100), all carefully excavated, tightly dated (tree rings, coins, pottery), and meticulously catalogued by archaeologists over many decades, yield evidence indicating an ∼4-m sea-level rise in only ∼70 years, spanning AD ∼430 to 500 (early Dark Ages), following AD 410 Roman abandonment of Britain. (A comparably fast 2–3 m rise within 100 years is known for the Marine Isotope Stage 5e interglacial, before our current Holocene interglacial.) The evidence includes excavated stumps, up to 2 m tall, of Londinium's Thames estuary-side defensive wall with its entire waterside face eroded, implying that the high spring-tide level rose 3 m+ after AD 300, constrained to pre-AD 500 by other archaeological evidence. The rise equates to the geologically based global Rottnest transgression (loosely carbon-dated AD ∼350–950) of the pioneering 1961 Fairbridge Curve of Holocene oscillating sea level; it may also account for late fifth-century mass migration of Anglo-Saxons to SE Britain. The Rottnest transgression can be explained only by Antarctic ice-cliff collapse, probably reflecting a known Arctic warm interlude (possibly sun-driven) in AD ∼400, the corresponding exceptionally warmed Arctic sea-surface water reaching Antarctica ∼30 years later by conveyor-belt ocean circulation. Arctic warmth since 2005, anthropogenically boosted, exceeds the AD ∼400 peak, portending another large, rapid sea-level rise imminently.