April 12, 2012
Amidst the latest controversy surrounding Envisat and their unexplained retroactive changes to their satellite sea level database (which, yes you’ve guessed it, have increased sea level rise in the last few years), we should not lose sight of tide gauges, which have been monitoring sea levels for a century or more.
Tide gauges, while not being subject to the calibration issues that satellite measurements face, have one major drawback. Many coastal locations round the world are subject to isostatic changes. Since the end of the Ice Age, land previously covered by glaciers has ben slowly rebounding, while others have been sinking. In the UK the effect can be seen on this map.
Rates of Isostatic Rebound
in Great Britain (in mm/yr)
An added complication comes when silting and erosion affect coastal areas. However, although these factors can affect absolute sea level changes, they don’t affect relative sea level changes, at least not over short time scales as their effect is a very long term process.
In this series, I will be looking at a cross section of UK sites and examining tide gauge records to see if there are any trends in the rate of sea level rise. We start at North Shields, which is situated on the Tyne in the north east of England. On the map above, it fits perfectly into the “green” zone, where isostatic change is pretty much zero.
Figure 1 shows the annual mean sea levels there between 1896 and 2009. (There is no data for 2010 or 2011). (Full data is available from PMSML – http://www.psmsl.org/data/obtaining/stations/95.php )
Since 1896, the level has increased from 6793mm to 7007mm, an increase of 214mm or 188mm per century or 7.4 inches. This figure is, of course, pretty much in line with global estimates over the last century, so we seem to have picked a fairly representative site!
But has the rate of increase been increasing in recent decades? The evidence from Figure 1 would suggest no. There is a small blip upwards between 1992 and 2002 following a big drop in 1991, but this has already been reversed and the 10 year average line since 1970 is bang on the long term trend.
But we don’t have to rely on eyeballing this graph. We can go one better than that and look at the year on year changes in Figure 2.
The 10 year average shows quite clearly that the rate of change has not increased since the start of the record. Figure 3 shows the 10 year average line in much closer focus.
Sea level rises over the last 10 years are below the long term mean (red line).
DEFRA are forecasting a sea level rise of 13mm / year by the end of the century. Perhaps someone should tell the North Sea.