British archaeology verifies 5th-Century rapid multi-metre sea-level rise and portends another before 2100

Worldwide geological markers of former sea-level (SL), such as wave-cut benches (raised, drowned), reveal a ~3-metre(m) SL rise, loosely carbon-dated post-50AD/pre-600. This "Rottnest Transgression" is the youngest of several m-scale rises, interspersed with m-scale falls, on Fairbridge's (1961) global-compilation Holocene-interglacial SL curve.

Copious British archaeological evidence (email me for sources), far better-dated (pottery-sherds/dendrochronology/Roman coins), confirms the Rottnest ("Romano-British marine transgression" of Godwin 1955), verifies its amplitude (~3m), and shows it spanned only ~70 years(y), ~430-500AD (early Dark Ages; Romans abandoned Britain 410AD). (An equally fast global SL rise, ~3m in decades, is proven by last-interglacial reef-facies relationships in tectonically stable Yucatan.) The Rottnest explains 5th-Century(C) mass-migration, underway by 450AD (dendro/artefacts/skeletal-DNA), of Saxon- and Angle civilians to SE Britain ('pre-subjugated' by rebellious Saxon mercenaries by 441AD), their North-Sea-coastal-plain homelands intolerably 'squeezed' between west-advancing Huns and rapid eastward shore-retreat. Among other British evidence: (1) Pevensey sea-fort (Roman-built ~290AD) straddles a promontory pointing NE into Pevensey Levels (reclaimed former tidal-flat embayment, beside English Channel). Indicating that high-spring-tide-level (HSTL) rose >2m in the 5thC, a defensive-ditch fronting the fort's SW gate contains "tidal" mud, dated early-5thC (sherds), whose top is ~1m higher than the NW-wall foundation and <0.5m higher than the SE foundation. This explains wall-collapse in both sectors (outward-toppled slabs visible on GoogleEarth), undermined by waves/currents, no later than mid-or-late 5thC (age of Early-Saxon-style sherd in sediment draping excavated wall-stump). Subsequent HSTL fall enabled William the Conqueror's 1066AD disembarkation at Pevensey fort; (2) excavated remnant stumps of Londinium's Thames-estuary-side city-wall (~270-300AD), up to 2.5m tall, show their entire outer face eroded (wall thinned ~50%), implying HSTL rose 3+m post-construction. Confirming this rise and its likely 5thC timing, across the Thames (Southwark) a peat layer containing 4thC sherds is capped by 2.8m of barren "river clay", reaching 3.2m higher than Londinium's lowest-known Thames-side wall-foundation. Proving HSTL soon fell 2+m, 1km upstream, in Lundenwic (Saxon port founded late-5thC), a building-floor dated ~700-750AD (sherds) is 1.6m lower than Londinium's highest-known wall erosion, and 1.5m below the top of the river-clay.

Such a large/fast global SL rise implies a peri-Antarctic 'MICI' ice-cliff-collapse event (Greenland lacks requisite >1km-deep grounding-line). Regarding causation, the Rottnest rise began (~430AD) only ~25y after the ~405AD warmest Arctic temperature-spike of the period 1-2000AD. This spike followed ~100y after the Sun's 310AD strongest magnetic-grand-maximum (MGM) peak of the interval 1-1885AD. The ~100y lag is attributable to ocean-thermal-inertia. The additional ~25y lag in SL response (Rottnest start) may reflect AMOC 'conveyor-belt' oceanic-circulation, specifically the time needed for ocean-surface-water, 'overwarmed' by the MGM (Svensmark effect, reduced cloudiness), to down-well in the north-Atlantic (Arctic fringe), then travel south, then up-well and encircle Antarctica, unleashing ice-collapse. The resulting iceberg-armada would cool the ocean, hence the atmosphere, causing increased global snowfall (ice build-up), intrinsically lowering SL.

Due to anthropogenic warming, the Arctic's average-surface-air-temperature exceeds, since 2005, the 405AD peak. This portends another rapid, metre-scale SL rise, beginning ~2030 (25y lag, above). Before 2100 the time-lagged effect of the Sun's even-stronger 1991 MGM peak will exacerbate warming.