Late Holocene ~350–450AD solar-driven rapid global warming & sea levels

Written by Roger Higgs DPhil Oxford on April 3, 2020. Posted in Current News

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Dr Higgs provides his presentation on Late Holocene ~350–450AD solar-driven rapid global warming & sea levels and ~3-metre sea-level rise: geological, archaeological and astrophysical synthesis.

Geological and archaeological sea-level (SL) indicators from stable areas worldwide collectively indicate, between 100BC and 700AD (imprecise C14 dating), two SL falls of ~2m, separated by a ~3m SL rise that reached ~1.5m above modern SL, allowing for compaction, subsidence-uplift and probable tide-range changes.

Superbly dated (coins, pottery, dendro) archaeology in southern Britain (GIA subsidence ~1mm/yr, GPS) constrains the first fall, e.g. at Roman Londinium fort, successive quays span 50–250AD and step down 2m into the Thames estuary.

Similarly the 3m transgression and ensuing fall are constrained to ~350–450AD and ~500–650AD, e.g.: (1) coastal saltworks spanning ~200–350AD, covered by foraminifer-bearing mud (i.e. SL rise), re-habitable by the late 7th Century (royal land charters); (2) in Portchester sea-fort, a ~300AD water-well dug to 7m (over-deep relative to today’s water table) contains 2m of rubble and silt layers introduced in the 5th C (age of two jewellery items in the fill), suggesting deliberate backfilling due to salt-wedge intrusion by SL rise.

The well was re-dug in the 6th C (timber lining dendro), i.e. SL fall. Further supporting the age and severity of the rise is the 410–450AD main exodus of Anglo-Saxons to Britain, probably forced to abandon their NW European coastal-plain homelands, pinched between rising SL and hostile barbarian tribes driven west, domino-style, by Asian drought (climate change; see below).

No known documents mention the SL rise (except perhaps the ~450AD ‘Groans of the Britons’; google), possibly due to barbarians burning libraries; or maybe literate peoples constantly facing death (war, plague, drought, etc.) were unimpressed by SL rising a few centimetres a year, scarcely noticeable and certainly not life-threatening.

The ~350–450AD transgression has many local names around the world, e.g. Rottnest (Australia), Romano-British (UK), Lytham IX (UK), V (UK), Dunkirk II (Belgium), St Firmin (France), PTM-9 (Sweden), Gilbert V (Pacific), Wulfert (USA).

I propose the global name ‘Early Dark Age (EDA) Transgression’.

A similarly rapid SL rise, 2 to 3m in <50yr, is known from the previous interglacial (MIS5e). The Holocene global SL ‘Fairbridge curve’ shows the EDA- and other large (2–5m) rapid SL oscillations, separated by 500–1,500yr intervals of smaller rises and falls.

The newest curve (Fairbridge & Hillaire-Marcel, 1977) shows the EDA highstand at ~350AD, the highest of the last 2,400yr. Only glacio-eustasy explains the rapidity of ‘Fairbridge-type’ large SL oscillations; glacio-isostatic models are incompatible with the rapidity and predict only one-way rise or fall, depending on latitude (Clark, Farrell & Peltier 1978 model and its derivatives).

Within C14-dating error, Fairbridge’s Holocene highstands correlate with (1) global-warmth peaks (from proxies, e.g. tree-ring widths, ice-core oxygen isotopes) and (2) solar Grand Maxima (cosmogenic isotopes in tree rings and ice cores).

The EDA Transgression coincided with strong warming ~350–450AD, possibly the warmest of the last 2,000yr (PAGES2k 2013, 2017), quite likely warmer than now (like earlier Holocene peaks; Marcott et al. 2013); and occurred <100yr after a solar Grand Maximum spanning 275–345AD.

These correlations support Svensmark’s theory of increasing solar-magnetic output reducing cloudiness (cosmic-ray link), causing increased solar warming of the ocean (in turn warming the atmosphere). The ~100yr time-lag is currently ~80yr, based on my visual cross-correlation of post-1700AD measured global temperature (HadCRUT) and solar magnetic output, while SL (from tide gauges) is seen to lag 20yr behind temperature.

I attribute the 80yr lag to oceanic thermal inertia (slow mixing), and the 20yr lag to ocean ‘conveyor belt’ circulation (AMOC). In this model, Fairbridge-type rapid SL-rise events (2–5m within 200yr) are each due to a solar Grand Maximum ‘over-warming’ the Atlantic surface water, which then downwells in the North Atlantic and eventually upwells at Antarctica, provoking glacier-snout collapse, hence sudden SL rise (google MISI and MICI).

Another solar Grand Maximum (1937–2004) has just ended, the strongest in >2,000yr, stronger than the 275–345 one, so another metre-scale SL rise is predictable by 2100. Each Fairbridge-type rise was followed by an equally rapid 2–5m SL fall, evidently intrinsic, e.g. more snowfall, hence ice buildup, due to higher humidity caused by transgressive flooding of high-latitude coastal plains? (cf. Neumann & Hearty, 1996).

Contradicting everything above, IPCC’s 2013–14 report says the 30cm SL rise since 1800AD (tide gauges) must be due to global warming by man-made CO2, because all SL oscillations in the preceding 1,800yr were smaller, they say, citing the 2010 smoothed SL curve of co-lead-author Lambeck, who treats outliers as errors.