Sunday 24 September 2017

Sea Level Rise (or Not) in Ireland ?

Climate change has caused the sea levels around Ireland to rise by almost 7cm since the early 1990s. This is due to the rising temperature of the planet, greenhouse gas emissions (GHGs) and the melting of glaciers. And the next five to 15 years is a crucial time period within which to act if we want to halt any permanent changes to the planet.  According to the Environmental Protection Agency (EPA), climate change is most obvious in our changing sea levels.  “Observed climate change impacts are most evident in the global temperature record, sea-level rise, loss of glaciers and ice sheets and changes in the nature and intensification of precipitation events,” the EPA report states.“  Since 1993, average sea level has risen around Ireland by just over 3cm per decade,” the report reads.  The agency said that Ireland is particularly “vulnerable” to climate change because we are an island country with major cities located close to coastal areas - Irish Examiner.

There are only two sets of sea level data available in Ireland that I can find - Malin Head and Dublin.

Malin Head shows a fall in sea level from 1958 to the early 2000s. This comes from PSMSL.ORG.

Malin Head

The part in red has been flagged with possible errors. But clearly, choosing to measure the trend from the early 1990s will show a small rise in sea level. But measuring from the beginning of the data i.e 1958 would show a fall in sea level.

So its interesting that the EPA have chosen 1993 to begin their sea level trend from.
 Since 1993, average sea level has risen around Ireland by just over 3cm per decade - EPA
After 2002, the Office of Public Works took over operation of Malin Head. Again, no upward trend can be seen in the data. In fact the data begins at 3.1m in 2004 and ends at just below 3.1m in 2011.

Malin Head 2004-2011

It's not clear how the sea level fell from approx 7m to 3m between the 2001 and 2004 graphs when the OPW took over operation. I presume that they changed the local reference height.

The other data available comes from Dublin. This data goes back to 1938 and goes up to 2009.

Here there was a very small rise in sea level :

The mean sea level trend is 0.07 millimeters/year with a 95% confidence
interval of +/- 0.42 mm/yr based on monthly mean sea level data from
1938 to 2009 which is equivalent to a change of 0.02 feet in 100 years.

So in 71 years there has been a sea level rise of 4.97mm - or about half a centimeter

This is a long way from the 7cm rise since the early 1990s as reported by the EPA and Irish Examiner.

This then leaves just the satellite data. This shows a rise of 7cm between 1993 and 2016 and a further 1cm in 2017. However, it's important to note that the satellites need to be calibrated using the tide gauges :

Sea level can also be retrieved with measurements from space-borne sensors. These sources indicate a sea-level rise around Ireland of approximately 2–3 mm per year since the early 1990s [till 2012]. However, satellite measurements need to be calibrated carefully, the most common method being the use of tide gauges. 


This raises the question - how can the satellite data and tide gauge data vary by so much ? Compare it with the most reliable tide gauge data - Malin Head from 2004 to 2011*. Malin Head is also perfect considering the satellite data is for the North Atlantic.

Comparison of Malin Head Sea Level with North Atlantic Satellite Data

Satellite 2004-2011

The satellite shows an overall increase of approx 1.5cm. 

Malin Head shows no discernible increase at all. Notice how Malin head shows variances of 0.1, 0.2 and 0.3 of a metre within the same year. That means the sea was rising and falling by as much as 30 centimeters in some years ! That is a rise of almost one foot ! 

By comparison, the largest variance within any given year in the satellite data is just 2 centimeters. 

I do not understand why the satellite does not pick up the seasonal sea level variations of one foot in a year which incidentally are confirmed by sea level data at Aberdeen in Scotland. This makes me suspicious of the satellite data. But if the satellite is correct, then how is Malin Head not showing any increase at all ? This would mean the reference point on the land would be shifting at the same rate (highly unlikely ?). 

All in all, the case is not closed on sea level rise in Ireland. It may well be the case that in the future scientists will make an announcement about problems with satellite sea data similar to the announcement they made last week about their temperature models.

Also, is 24 years a long enough period to draw any conclusion ?

*According to PSMSL for Dublin - After examining the historical files it was discovered that data for Dublin up to 2001 was supplied as Mean Tide Data and not Mean Sea Level data. Because of this the data has been flagged within the database.