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Sea level changes from Roman to Modern Times

Disclaimer
Please Note: I will only be explaining sea level rises and historic temperatures with relation to Historic events, I am not trying to explain either for or against Global Warming.

I will try to explain what I believe is the relationship between historic temperatures and the corresponding sea levels implied by this data.

This will include the calculations I have used to show the correlation between the two.

A bit more about Roman Conditions
I have used the Saxon place names as our measure of sea level rises, however:-

The University of Durham Archaeological Department has suggested a similar sea level 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.’
https://www.dur.ac.uk/east-lincs-history

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 sea 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 sea level based on this data, then try to see if this fits in with historic observations.

Historic Sea level records
There are a number of pieces of historic evidence implying sea 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 - Author
  • Late Roman(AD400) sea 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 – sea 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 sea level drops.
  • Smallhythe stops shipbuilding in late 1400’s as the river silts up (it is more likely that dropping sea 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)

Data extraction and Sea Level calculation
Assumptions I have used to carry out these calculations to try to match the Sea Level to the temperature variations :-
  1. The Year 1900 is used as the base sea 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 sea 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 sea level historic data we have is to fit the curve, then we need a factor of about 2.5 to adjust the sea level mm/year to about 1.25mm/year then the historic data fits the sea 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 sea 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 sea level was about 4 metres higher in Late Roman/Early Saxon times.

If this is the case then its likely that the sea 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 sea levels.

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

Conclusion

It would seem that the Roman Warm Period caused over a few hundred years a rise in sea 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.



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Author Simon M - Last updated - 2021-09-07 16:59:58
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