Roger Higgs featured at the Geological Society of America annual meeting in Denver (2016) a few weeks ago, and presented a paper that claimed there was historic evidence for a 5m rise in global sea levels in the Late Roman/early Saxon era. Higgs, who lives in Bude, famous for its surfers on the north Cornish coast, runs his own company and is a busy man, as they say. At face value this idea would seem to support the climate change position as the inference is that the Roman Warm period was responsible for the rapid and steep rise in sea levels (at least around Britain). However, he is quite explicit. The sea level rise was due to eustatic processes. As such there are distinct similarities with the SIS article on the same subject by Steve Mitchell a few years back (and the views of Dick Gagel following on concerning the Dunkirk Transgression event dated to the same period).
Mitchell, like Higgs, visualised a pattern in sea level oscillations that could be traced through a large part of the Holocene, and both think we are on the verge of another loop upwards in the 21st century. These kind of facts from the past are not usually dwelt on to a great degree by climate scientists but presumably some or all of them are aware of the predicted trend in sea levels otherwise they would not predict such a massive upward spike in the upcoming 100 years, even though there is currently little evidence of such a trend.
Higgs also claims that sea levels fell back in the Mid and Late Saxon period and began to rise again in the pre-Norman era, citing evidence of Norman castles and towns with a sea dock which were subsequently stranded above today's sea level high. Mitchell, in one of his articles, mentioned Harlech Castle, and Higgs cites Bodiam and Pevensey castles in SE England. However, Mitchell was primarily concerned with the Late Roman period and the connection between the archaeological layer known as 'dark earth' (usually at the lowest part of Roman towns such as Leicester, London etc) and the rise in sea levels/the general water table (as well as the tidal reach of rivers such as the Thames and Ouse). However, the nearest research to Higgs comes from geographer Basil Cracknell. He accumulated considerable data during his career and wrote some 250 articles in a variety of learned and popular outlets and gathered the evidence up in book form after he retired. This is 'Outrageous Waves: Global Warming and Coastal Change in Britain through two thousand years' – published by Phillimore Books of Chichester in 2005. Like Higgs, he has a rapid sea level rise in Late Roman and the Medieval period, and declines in between. However, he also included the Little Medieval Warm period (the Tudor era) that led into the Little Ice Age in the 17th century. Apart from that the timescale mirrors that of Higgs – see https://gsa.confex.com/gsa/2016AM/webprogram/Paper278985.html
Cracknell suggested the warm periods led to the rise in sea levels as the one seems to closely follow the other. Of course, the slant of the book towards the fashionable global warming meme at the time of publication was probably due to Phillimore as they probably envisaged more sales with a populist message. It's not everyone that is interested in historical sea and river levels, a fairly limited reader base one might imagine. Higgs puts the warming phases into focus, using the work of Ljungquist on solar activity highs. However, he is interpreting historical data too – and it is a theory. Perhaps the sun was more active at those times – but this is not necessarily so. The same effect could have occurred if the orbit of the earth periodically encountered wide streams of dust in space – such as the Taurid complex. In that situation the sun's energy would be dimmed by an opaque sky, dust trapped for long periods in the upper atmosphere. Volcanic activity has a similar effect. Hence, we can theorise the sun was more active during the warm periods or the sun was less active (than normal) during the succeeding cool periods. Same effect. The point still needs to be addressed – why did sea levels rise? Where did all the water come from? One is therefore forced to think in terms of eustasy – which puts the question into touch in that the causes of eustatic sea level rise are also debatable. Into this melee we have Sir Harry Godwin who coined the phrase 'the Romano British Transgression' event (back in the 1940s) which ran contemporary with the Dutch 'Dunkirk Transgression' event of the same period (see Dick Gagel in SIS). Godwin was influential in his time, his research taking place in the Somerset Levels (on the western side of Britain) and the Fen Lands (on the eastern side of England). Mitchell included evidence from much further afield, from Scotland to all points south. JNL Myres, writing some decades ago on the post-Roman period and the A/S invasion, actually brought in the transgression event as one of the causes of the migrations. He had clearly read Godwin's work and went on to say the land went down and the sea level went up – classic eustastic behaviour. That was the only geological reference of the period I came across in Myres book and it is a fact that archaeologists and historians in general rarely mention changes around the coastline of Britain when writing up their research – which I always find disappointing. A bit of cross discipline analysis would be nice – especially now that we have marine archaeology.
Getting back to Higgs (and the sea level curve quoted by Mitchell) we may ask if the current warming phase (post Little Ice Age) will result in sea level rise in the coming decades (as forecast by the CAGW enthusiasts). Or will we have to wait for another cooling phase?
PS … I often find it useful to look at the bibliographies. Higgs is not disappointing as he references Rhodes Fairbridge, Godwin, Bond (of Bond event fame), Ahmed (recent article), the IPCC, Kemp, Lamb, and various others. Cracknell, whose sources are generally older than those of Higgs, also quotes HH Lamb and Godwin. I recommend reading the Higgs link – even printing it out and saving it for future reference.