Sea Level Shenanigans

  • Date: 25/02/10

Last year the Met Office, the Natural Environmental Research Council and the Royal Society released a joint pre-Copenhagen Conference statement that included as one of its five main scientific points:

“There is increasing evidence of continued and accelerating sea-level rises around the world.”

At around the same time the Royal Society also said in a press statement touching on sea level changes that, “…estimates generally larger than those previously projected including evidence of continued and accelerating sea-level change around the world.”

However, a closer look at the data supporting this statement reveals that it is difficult to justify. What is the evidence that sea levels are rising and, indeed, accelerating?

Measuring sea level change is difficult and up to 1992 relied on Tidal Gauges. The ones used in the UK were at Newlyn in Cornwall and in Liverpool. There are many factors that have to be accounted for. Tidal Gauges in geologically active regions of the world can be influenced by sea level changes induced by tectonic movements. The post-Ice Age geostatic rebound whereby land rises when the layer of ice is removed can also be important in some regions.

Given these, and other influences, it was possible to produce a graph of sea level changes since about 1880. Figure 1. Click on the figure to enlarge.

Recent_Sea_Level_Rise

Clearly, there is significant variability in sea level over the annual and decadal timescales as well as the obvious long-term trend. However, between 1900 and 2000 sea level rose by 18.5 cm or an average of 1.8 mm per year.

The three-year average line shows the trend is linear but there is evidence of a change of gradient about 1910. Although the errors for the data are somewhat larger, by a factor of four, for the pre-1900 data, a modest statistical case can be made for a break of slope around 1910 from 0.9 mm per year to 1.8 mm per year.

The satellite era of sea level measurement began in with the launch of Topex/Poseidon in 1992 and continued with the launch of Jason-2 in 2008.

From the start there was a problem reconciling the data sets from the Tidal Gauges and the satellite measurements. The satellite data indicates a much larger rate of sea level increase. Figure 2. Click on the figure to enlarge.

alt_gmsl_seas_rem

Satellite data yields about 3.2 mm per year or about double that of the Tidal Gauges.

There could be many reasons for this. Either the Tidal or the satellite data could be subject to a bias, or they could be measuring different things (the satellites sending back global data are restricted to latitudes no greater than 66 degrees for example). Given the complex steps required to calibrate satellite data and convert it into sea level readings there is much scope for unrecognised errors. It would be too much of a coincidence to postulate that the rate of increase in sea level had doubled at the same time that a new way was used to measure it! Whatever the situation, the two data sets show continuing sea level rise but the rate of that rise is incompatible between them and nobody has provided a convincing answer why this should be so.

This means that the satellite data do not provide evidence that the rate of sea level rise has been accelerating. Additionally, there is no evidence of any change in the rate of sea level rise over the period of the 1992 – 2010 satellite data.

But does the long-term Tidal Gauge data show an acceleration in sea level rise?

Bruce Douglas of the University of Maryland looked at this data set and concludes that sea level rose at 1.8 mm per year between 1880 – 1980. The data was based on 24 measurement sites with the average data set length being 83 years. Douglas detected no acceleration at all between 1880 – 1980.

The claim for the late 20th century acceleration in sea level rise comes from Church and White (2006) who say, “Multi-century sea level records and climate models indicate an acceleration of sea level rise but NO 20th century acceleration has been detected.

So they set out to look for one. (I shall return to multi-century sea level records later).

Church and White looked at post-1950 Tidal Gauge data that they say shows a larger rise after 1993 but no significant acceleration which is a confusing statement.

Then they go back to 1870 (when the data was rather poor) and analyse the 1870 – 2004 period yielding a 1.7 mm per year average rise. Over this period they say they found an increase in the rate of sea level rise of 0.013 +/- 0.006 mm per year squared.

The important point is that this acceleration only shows up when the uncertain data back to 1870 is included in the analysis and it reflects more the slight change in gradient seen about 1910 than it does any recent changes.

Church and White fit a quadratic curve to the Tidal Gauge data (1870 – 2004) and look for deviations from this curve. However, since it is more likely that the data is better represented by two straight lines with a break at around 1910 this means that at the ends of the data there will be larger deviations from the gently changing slope of the quadratic curve producing a false result. They also add the satellite data to Tidal Gauge data but include a “offset” of 150 mm to smooth it in.

Holgate, writing in Geophysical Research Letters in 2007, looked at nine long sea level records from around the world and concluded that the high recent variability suggested by some in sea level changes was nothing unusual. In the earlier part of the 20th century he found a rise of 2.03 +/- 0.35 mm per year (1904 – 1953) and for the latter part he found 1.4 +/- 0.34 mm per year (1954 – 2003).

To my mind the evidence presented for acceleration in seal level changes is unconvincing. Even so it has become part of the literature of climate change.

The Forth Assessment Report for the IPCC has the following to say about sea level changes. It states that the sea level rise was 1.8 mm per year between 1961 – 2003 and that the rate was faster between 1993 – 2003 (ie the satellite era).

Both those statements are economical with the truth. Yes, the sea level rise was 1.8 mm per year between 1961 – 2003 but it was also 1.8 mm per year between 1910 – 1961. It has not changed. It also glossed over the Tidal Gauge/Satellite mismatch in saying the rate of sea level change was faster (3.1 mm per year) between 1993 – 2003. This is comparing apples and oranges and not saying so. The IPCC says later in the document that it is unclear if the 3.1 mm figure was due to a natural decadal variation or a “return to a long term trend” is unclear. However, they have no valid scientific justification for making such a claim or for saying there has been an acceleration the rate of sea level change.

The IPCC concludes that there is “high confidence” that the rate of sea level rise increased from the 19th to the 20th century. So it did, but this is far from the whole case. It increased around 1910 and not during the purported anthropogenic climate change era. This is another misleading IPCC claim.

The IPCC is not alone in using partial data to illustrate man’s influence on the environemt. In his submission to the fothcoming Parliamentary Science and Technology Inquiry in the leaked emails from the University of East Anglia, the Memorandum submitted by Professor John Beddington, Government Chief Scientific Adviser says, in support of man’s influence on the climate that, “global sea level has increased by about 10 cm in the last 50 years.” But it did the same in the previous 50 years when man’s climatic influence is deemed minimal so it is in fact meaningless and taken out of context.

There are many fascinating questions concerning sea level changes. Historically some scientists argue that from observations of ancient ports that sea level remained unchanged between the 1st Century AD and around 1800 AD at which point it started its current rise due to factors unconnected with mankind. Twenty thousand years ago it was 140 metres below where it is today. It then began a slow rise (with occasional dramatic pulses)  until about eight thousand years ago when it has changed relatively little. Today’s changes in sea level are tiny when seen in terms of the changes seen during the last glaciation.

In conclusion, the case for an anthropogenic influence or for a recent acceleration in the rate of sea level rise is unjustifiable given the current data.

Feedback: David.Whitehouse@thegwpf.com

 

 

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