Paper No. 295-8
Presentation Time: 3:15 PM
NEGLECTED BRITISH HISTORIC EVIDENCE FOR A 5M MEDIEVAL GLOBAL SEA-LEVEL RISE: PORTENT OF THE MODERN RISE
England and Wales excel for studying 2-0ka eustasy, being stable (c.1mm/y GIA subsidence) and rich in published archaeological evidence for sea-level (SL) change, e.g. buried Roman structures reveal a c.3m Roman fall preceded a c.5m pre-Norman rise, while many Norman towns and castles have a seadock stranded a few meters above today's high-tide line. SL high- and lowstand values deduced here correlate (and notably scale) to warm and cold peaks on Ljungqvist's (2010) Northern Hemisphere temperature curve: Roman Warm Period (peak at c.160CE), highstand of +5m (rel. to present mean SL); Dark Ages Cold Period (DAP; c.530) +2m lowstand; Medieval Warm Period (MWP; c.950) +7m high; Little Ice Age (LIA; c.1690) -30cm low. Similar meter-scale interglacial SL oscillations are likewise known from the previous interglacial, MIS5e. Scarcity of reported geological evidence for the proposed MWP 7m SL high may reflect (1) brevity (<20y?) precluding reef- or bench formation, (2) coalescence with MIS5e raised-shore features, and (3) proneness of raised intertidal encrusting fauna (barnacles etc.) to dissolution by rain. Contrary consensus that 2-0ka SL change is minor (<30cm) is largely based on saltmarsh core dates that point to continuous 2-0ka peat growth. However, a kink in all core age-depth graphs (linear trend of C14-dated samples is <50% as steep as post-C14 trend) suggests C14-age exaggeration by an unknown estuarine reservoir effect (brackish saprobes fix CO2 released by older, deeper plant decay?); projecting the post-C14 line gives core-bottom ages <0.5ka. The modern SL rise, which, based on tide-gauge records, began c.1780 (lagging c.90y behind start of modern warming, i.e. LIA peak), is likely locked into eventually exceeding the 5m Medieval Rise, as Ljungqvist shows (1) the modern warming already exceeds DAP-MWP warming, and (2) mean temperature since c.1950 arguably exceeds the MWP acme. However, to date, c.325y after modern warming began, the Modern Rise is just 30cm, so to surpass the Medieval Rise of 5m in c.420y, a further 4.7m (min.) is predicted in the next c.100y, implying fast acceleration from today's 2.5mm/y, to >5cm/y by 2050, then probably >10cm/y for decades, driven by polar ice-sheet surge once the buttressing ice shelves melt. Cutting mankind's emissions will not reduce the imminent natural 5m (min.) SL rise.