seaxe left Anglo Saxon History

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High Tide changes in the last 2000 years

Table of Contents

A bit more about Roman Conditions
I have used our page Was the high tide in Saxon times 5 metres higher ? - Pevensey to explain our findings, however:-

The University of Durham Archaeological Department has suggested a similar high tide rise in the Wash in late Roman times based on sediment analysis (early Saxon). ‘and indeed calculations for Spalding based on estimates of highest tides suggest a maximum rise of 4.22m, echoed by 4.52m on the Fen Causeway between Peterborough and Denver in Roman times.’

World temperature, evidence of change

Craig Loehle - Non Tree ring evidence
Craig Leohle carried out research in 2007 to try to show the relative temperature changes over time based on a number of factors, these exclude tree ring data which is effected by other factors than the mean temperature.

A Temperature reconstruction with 95% confidence intervals shown, from Loehle and McCullough (published in Energy and Environment, 2008): Correction to: A 2000 year global temperature record based on non-tree ring proxies. As this reconstruction is based soley on proxy data, the recent, measured global average temperature was not added to the end of the record as was done by Mann. The last data point represents a 29 year average centered around 1935.

The center line shows the most likely temperature changes in June, July and August based on the Northern Hemisphere data.

Alaskan sedimentary core(midge data) evidence
This chart shows Global Temperature data estimated from historic midge data from Moose Lake in Alaska for the last 3000 years, the vertical blue line shows 0AD as our other data on the next pages starts at this point.

This data is from ‘Six millennia of summer temperature variation based on midge analysis of lake sediments from Alaska’ carried out by Benjamin F. Clegg, Gina H. Clarke, Melissa L. Chipman, Michael Chou, Ian R. Walker, Willy Tinnere and Feng Sheng Hu

We have overlayed it with a couple of the South of Englands most severe storms which effected the coastline plus the ‘Roman Warming’ , the ‘Medieval Warm Period’ and the ‘Little Ice Age’.

The vertical line shows 0AD or 2000 years back in time.

Central Greenland Temperature Reconstructions
This data is from ‘Kobashi and Alley GISP2 Central Greenland Temperature Reconstructions’ and re-analysed by Andy May.

In 2000, Richard Alley released an ice core temperature reconstruction for Central Greenland using Oxygen isotope ratios. This was subsequently repeated by Kobashi using Argon and Nitrogen isotopic ratios from air bubbles in the ice to estimate paleo-temperatures. The overall effect is shown on the following chart.

The vertical line shows 0AD or 2000 years back in time.

Average chart data I will use to imply high tide levels
All the above data sets implies a range of global temperature changes as that shown in the chart to the right, which shows an average line, plus a potential variant range based on data and calibration uncertainties.

I will now use this average line to show an implied high tide levels based on this data, then try to see if this fits in with historic observations.

Historic High Tide level records
There are a number of pieces of historic evidence implying high tide level/temperature changes from Roman times, and I will try to show these in relation to the average temperature and its effect on water being frozen in the polar ice sheets.

  • Early Saxon island names(AD470) 4-5mtrs higher than at present - High tide levels Pevensey
  • Late Roman(AD400) high tide levels in the Wash of 4.5 to 4.7mtrs higher than at present - Durham University
  • The Jesus, 1,000 tons, built at Smallhythe, Kent, entered service AD1417 – high tide level had to be 3 mtrs higher than today to allow the vessel to sail to the sea.
  • Romney Marsh Innings(Marsh reclamation) starts after AD1300 as high tide level drops.
  • Smallhythe stops shipbuilding in late 1400’s as the river silts up (it is more likely that dropping high tide levels made the river too small.)
  • Ostia Antica was Rome’s sea going port in both AD409 and AD849 (only needs 3mtr to be sea going – both fit the shape)

Other items that imply Roman High Tide Levels in the UK
There are a number of other pieces of historic information implying aprox 4 metres higher tide level in Roman times, which can be seen in Ivan D Margary 'Roman Roads in Britain' book.

Assuming the high tide level was 4+ metres higher in Roman times then:
  • Roman Road M51 that stops at Dunhall in Somerset
    - this would be on the coast so probably a Romn Harbour
  • Roman Road M27 stops at Burg le Marsh in Lincolnshire
    - this would also be on the coast again probably a port.
  • Ermine Street goes to the west of the Wash near Peterborough
    - avoiding the coastline.
  • Ermine Street goes through Water Newton
    - which would potentially be on Ermine Street and the coast of the wash.
  • Peterborough was originally known as Medeshamstede
    - which means the meadow village on the river with a landing place.
  • Further implied evidence from Hastings area Domesday records
    - Showing salt houses from Domesday and their current locations.
  • Another piece of implied evidence from Norfolk Domesday records
    - Showing salt houses from Domesday and their current locations.
  • And more from Norfolk Domesday records
    - Showing the areas where the sea would have been if you zoom in towards the east of the county.

Data extraction and High Tide Level calculation
Assumptions I have used to carry out these calculations to try to match the High Tide Level to the temperature variations :-
  1. The Year 1900 is used as the base high tide level as the raw data only goes up to 1935
  2. The base average temperature is taken as 13.8 degrees Celcius
  3. Each individual 100 year data should be added/subtracted from the previous one
  4. Negative temperature variations increase the water held in the Polar Regions
  5. Positive temperature variations decrease the water held in the Polar Regions
  6. Water held in the Polar Regions decreases or increases at a constant rate dependent on the temperature variation from 13.8 degrees Celcius
  7. Each variant of 0.1°C changes the high tide level by 0.5mm century
As this chart was based on starting point of 0.5mm/year per 0.1°C equivalent to 50cm per century we put in a factor adjuster so this could be changed if we found the curve didn't fit.

So if all the high tide level historic data we have is to fit the curve, then we need a factor of about 2.5 to adjust the high tide level mm/year to about 1.25mm/year then the historic data fits the high tide levels in most places.

Please NOTE:-
We are currently working with only a small data set of historic data so this is likely to be improved in the future.

If we plot the high tide levels over time we get the chart to the left, which implies some interesting information about the historic coastlines of the world.

So from our plot it would appear that the high tide level was about 4 metres higher in Late Roman/Early Saxon times.

If this is the case then its likely that the high tide levels were even higher in Battle of Hastings time at nearly 5 metres higher than in 1900, so well over 4 metres higher than the current high tide levels.

This does not bode well for those coastal cities around the world if the average temperature of the world is increasing.

Historic Items that seem to imply the high tide level profile
Some other interesting historic facts that seem to match the estimated high tide level profile are shown below:
  • Late Roman high tide levels of 4.5 to 4.7mtrs Durham University AD400 (4.1 mtr calc)
  • Early Saxon island names 4 to 5mtrs Author AD470 (4.4 mtr calc)
  • Ostia Antica was Rome’s sea going port in both AD409 (4.1 mtr calc) and AD849 (3.5 mtr calc only needs 3mtr to be seagoing)
  • AD900 ish the Vikings used the Wash as a major route to take Cambridge (3.9 mtr calc)
  • Northeye, an island on the Pevensey marshes is settled between AD1086 and AD1287 (4 mtr calc when its recorded as a limb of the cinque port)
  • Domesday book AD1086 shows that Ashburnham now 6 miles inland had 30 salt houses (5.0 mtr calc)
  • Post AD1300 small scale reclamation of the Wash started by local monks (3.9 mtr calc)
  • Romney Marsh Innings starts after AD1300 as high tide level drops. (3.9 mtr calc)
  • The Jesus, 1,000 tons, built at Smallhythe, Kent, entered service AD1417 (4.1 mtr calc – high tide level had to be 3 mtrs higher than today to allow the vessel to sail to the sea.
  • AD1546 (3 mtr calc) - Henry VIII built a 300 ton warship the ‘Great Gallyon’ at Smallhythe
  • Smallhythe stops shipbuilding in late 1500’s (2.6 mtr calc) as the river silts up (it is more likely that dropping high tide levels made the river too small.)
  • AD1650 (1.8 mtr calc) large scale drainage of the Wash starts to take place the land sinks as the peat dries out.
  • Maps from the mid 1700’s (1.2 mtr calc) show the river Rother was navigable(to sea vessels) to a point near Bodiam bridge, and also the Brede was navigable to Brede Bridge.

Early map of the isle of Thanet
Thomas Elmham's Historia Monasterii S Augustini Cantuariensi The Isle of Thanet is an area in the far East of modern Kent it is not an island at this moment in time.

The name Thanet is derived from the Saxon 'þegn æg' pronounced 'thane ay' which means servant or mercenary island, over time after the original saxon meaning was lost then 'the isle of ' was added to the name.

The image on the left is a modern reconstruction of a map from Thomas Elmham's Historia Monasterii S Augustini Cantuariensis and shows that Thanet was an island in the early 1400's.

Many thanks to for this research.

To enable Thanet to be an island such as the illustration shows would require the sea to be at least 3 - 4 metres higher than today.


It would seem that the Roman Warm Period caused over a few hundred years a rise in high tide levels of about 4.5 metres, which implies that the Medieval Warm Period caused at least the same rise in 1066 and possibly more, and modern day is still lower than these rises so we have more land above water than in the past, and of course more to loose as the world heats up.

Please Note: we have used the phrase 'high tide' as opposed to 'sea levels' as the latter appears to be a very emotive phrase due to the current climate discussions. The two phrases may be the equivalent, but we have no evidence of this.

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Author Simon M - Last updated - 2024-03-14 15:40:53
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