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Sea Level Changes At Holyhead

April 30, 2012

By Paul Homewood




This will complete the series of tidal gauge trends from around the UK, following on from North Shields and Newlyn. (No other stations have records going back far enough on the PSMSL website).

First a word of caution. PSMSL warn about the accuracy of the data upto 1960, which was far noisier then other nearby stations. This is evident on our first graph in Figure 1.



Figure 1


Since 1970, the rate of increase has been fairly constant, with the 10 year running average close to the long term trend. During this period, 1970-2008, total increase has been 56mm, or a rate of 147mm per century, about 6 inches. Again we can check the isostatic effect, which shows that this part of the coast is not really affected, neither rising nor sinking significantly. (Holyhead is on Anglesea, at the North West tip of Wales).



Rates of Isostatic Rebound in Great Britain (in mm/yr)

Figure 2 shows the annual changes, and again, the noisiness of the data up to 1970 is noticeable.


Figure 2


Figure 3 shows the 10 year running average in much closer detail. At the final reading in 2008, this running average is –0.3mm. (In other words, sea level in 2008 is actually 3mm lower than 1998).



Figure 3

The long term trend shown by the linear regression suggests a very slight increase. This is, however, heavily influenced by the drops in sea level recorded in the 1940’s and 1950’s. Figure 4 illustrates very clearly that since 1970, the rate of increase in sea level has been slowing down and is running at as little as 2mm per year.



Figure 4

So what have we learned?

The following table gives the sea level increase between 1979 and 2008 for the three stations.


  Sea Level Increase – mm Increase per Decade – mm Isostatic Effect – mm Sea Level Change Excl Isostatic Effect
Newlyn 81 27 11 16
Holyhead 40 13 0 13
North Shields 62 21 0 21


Excluding the effect of isostacy, in other words looking at what the sea is doing and not the land, the changes at each of the three sites, over the last 30 years,  are entirely consistent with the generally accepted view that sea levels globally have increased by about 200mm during the last century. This is important, because it means that, in the UK at least, that rate of change has not increased in the last 30 years.

Furthermore there is no evidence at all that this rate of change is currently increasing.

  1. TinyCO2 permalink
    April 30, 2012 9:46 pm

    And all while the North Atlantic is in its warm phase which might have an effect.

    Nice work!

  2. May 1, 2012 4:18 am

    The Holyhead data prior to 1960 certainly looks inconsistent. Natural phenomena just don’t change like that without some major reason.

    The “horizontal” plots (Figures 2 and 3) of the changes in the annual and 10-year average levels are certainly easier for many people to understand. However, this only works if the data is complete, which is not generally the case. The annual level changes require data in two adjoining years, while the 10-year averages require 11 consecutive years of data. Missing data can always be estimated but I am not keen on doing this. In my opinion it is better to use all available data, expressed either as the annual (mean) value or this value divided by the long-term mean.

    There is also sea level data available at Aberdeen since 1862 which shows the long-term trend of sea levels in the UK.

    • May 1, 2012 9:45 am

      Thanks, Brian. I steered clear of Aberdeen as it was made up of two separate sites, but I might have another look.

      There are a few gaps in the data for Holyhead, mainly in the 1970’s, though Newlyn was complete. I have infilled these with estimates. Although this is not ideal, the 10 year averages should not be significantly affected either way.

  3. May 2, 2012 6:45 pm

    Beware of PMSL from now on. They’ve adjusted several Oz stations upwards by as much as 260mm with no notification in the notes at the bottom of the pages, apart from an RLR baseline change (which the NTC would have taken into account already). I emailed them, but with no reply.

    I noticed a problem with their online plots some time ago. The data points were shifted one month to the right, so that e.g. December 2009 was above the 2010 tick mark. I don’t use the plots, I “do it myself”, but knew al lot of people do. I checked a few blogs and saw the same problem going back at least a year (the bloggers had copied the graphic, so it’d been happening for some time).

    I got no reply, but about 5 weeks later, all the plots were aligned correctly.

  4. Andyj permalink
    May 19, 2012 7:47 pm

    Not far away in Roman times, the town of Chester was a major and thriving sea port.
    In fact within the memory of living man there were house with jettys against the high tide in the Wirral.

    Not any more. It’s all high and dry.

    This is all down to geology. So if they monitor the sea to parts of a millimetre. Who’s watching the land?

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