Sea Level Changes At Holyhead
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.
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 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).
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.
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|
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.