Read here and here. There is an excellent and fascinating post over at Judith Curry's site regarding historical sea levels. It's part 1 of a series by Tony Brown. From one of his linked documents comes the chart on the left, and the chart on the right from a previous C3 post on sea levels. (click on images to enlarge)
Tony does an analyisis of sea levels prior to the pre-Medieval Period, pulling information from a wide assortment of historical resources. Per the documentation he reviewed, there is strong evidence that both the Roman/Byzantine and prior periods had sea levels that exceeded the current level.
"“The North Sea had a nasty little jump between 350 and 550AD, flooding the coasts of northern Europe with an extra 2 feet of water and sending its inhabitants — folk known as Angles and Saxons — fleeing (although “conquering” might be the better word) into ill-prepared Roman territories. At the start of this rise, the areas we know as the Fens were a well-settled part of Roman Britain ruled from the town of Duroliponte (Cambridge) by its native people, the Christianized Romano-Celtic Iceni. Then the sea level rose, and history’s curtain went down for two centuries.”.....At the old shore the Etang of Vendres, near the mouth of the Aude, are the ruins of a Roman Therme of the 1st or 2nd century A.D. (locally called the temple of Venus). There the walls have been washed out by waves so that they now have a deep double notch about 1.80 m above present sea level..."
"So notwithstanding the statements of the IPCC AR4 who assert a sea level status quo from ancient until modern times, there are many studies that point to a picture of relatively static sea levels after the initial Holocene rise. These then show that some 3000 years ago there was a further inundation (think Lyonesse in Cornwall) and in early Roman times levels were somewhere around current levels. Levels then rose significantly through the Roman period peaking around the 700 AD Byzantine period at levels higher than today..."
At the end of part 1 on Curry's site, Tony makes reference to how Science Daily edited out key information from a study they reported on thus leaving their readers with an impression that previous sea levels were not higher. Interesting. Confirms why today's science journalists are no longer easily trusted.