Climate Change 01 (1) in its “Summary for Policymakers”  has a headline “Global average sea level has risen.” This is followed by the statement “Tide gauge data show that global average  sea level rose  between  0.1 and 0.2 metres during the 20th century”. But how representative are tide-gauge data?

Chapter 11 - “Changes in Sea Level” (2)  has 41 pages  of speculations,  estimates, forecasts and models. Only just over three pages (3) are devoted to “Mean Sea Level Changes over the Past 100 to 200 Years”.

The level of the sea is a highly variable quantity; affected by the earth’s orbit, by the moon, by all aspects of the climate. When it is measured from a land-based facility, the measurement is affected by changes in the level of the land.

There are many land-based  locations around the world where the level of the sea is measured, and often automatically recorded. These measurements have been collected  and made available to the public on the Permanent Service for Mean Sea Level (PSMSL) website (4). Currently there are some 1850 records listed. They vary considerably in their length. They also vary in the quality of equipment and the degree of supervision.

The measurement stations are most often near  large cities in the Northern Hemisphere. They, therefore, cannot be considered as providing a representative sample of the world’s oceans. Any averages derived from them cannot be regarded as evidence of a globally averaged  change in sea level, but only of the particular sites of measurement.  Climate Change 01  (5) puts it this way:

“The sea level records contain significant interannual and decadal variability and long records are required in order to estimate reliable secular rates that will be representative of the last century. In addition, sea level change is spatially variable because of land movements and of changes in the ocean circulation. Therefore a good geographic distribution of observations is required. Neither requirement is satisfied with the current tide gauge network.”

Corrections to land measurements such as the “Post Glacial Rebound” (the continued recovery from the last ice age)  and plate movements have to be made by the use of models that are often based on limited geographical regions, and are prone to large uncertainties. Again, Climate Change 01  (5)

“However, this procedure may underestimate the real current eustatic change because the observed geological data may themselves contain a long-term component of eustatic sea level rise.”

Corrections due to urban development are even more difficult to make The level of all cities falls as ground water is removed and heavy buildings are erected. Tide gauge equipment may itself tend to subside after years of battering by the sea.

The need for corrections means that the quoted figures for sea level change bear only a remote relationship to the actual measurements. They are the result of processing by models and of multiple corrections (3, 6).

The researches of Mörner (7) of the International Association of Quaternary Research, which are critical of the IPCC methods, are largely  ignored by them.

In Chapter 3 it is pointed out that many remote weather stations show no evidence of an overall  temperature rise over the past century. Similarly, there are many of the more remote, and comparatively stable tide measuring stations which have not registered a rise. Also, many records showing a rise show a sudden jump rather than a smooth increase, suggesting a relationship with a particular event, such as the erection of tall buildings, or an airport, or a change of instruments.

As an example, many sea level records for the more remote Pacific islands show no sign of a rise over the past twenty years. Figure 5.1  for Funafuti, the capital of Tuvulu (4) shows that there has been no detectable change in sea level since 1978. Yet there are strident claims that Tuvulu is in danger of being swallowed up by the ocean. The New Zealand Government has agreed to special immigration concessions as a result of this false belief

Figure 5.1  Mean Sea Level record for Funafuti, Tuvulu, from 1977 to 1999 (4)

A recent National Geographic documentary called “Drowning Paradise”  (8) devotes a whole hour to this forthcoming tragedy, for which there is no evidence.

Other Pacific islands showing no detectable change in sea level (4) are ;

Many others show a stable period followed by a sudden jump, most likely due to hotel or airport construction, or a hurricane. Most of them also show no mean temperature increase over the period. The El Niño events of 1983 and 1998 show low readings.

Since 1993 mean  sea level has been measured by satellite altimeter observations. The latest record is shown in Figure 5.2 (9)

On the face of it, it shows a rise in sea level over the period. Cabanes et al (10) have shown that the period from 1993 to 1998 is  compatible with the ocean temperature measurements of  Levitus et al (11), referred to in our Chapter 4. However, the later period is heavily influenced by the 1998 El Niño event;  the satellites have had various problems of calibration and correction (12)  and  five years is an insufficient time to determine a climate trend. It is interesting that Cabanes et al find that the Topex/Poseidon measurements are double those estimated from tide gauges,  direct evidence that the tide gauge information is from a distorted, biased sample.

All the calculations, estimates and forecasts of future sea level behaviour are dependent on the belief that the earth’s surface temperature is increasing. If, as is argued in Chapter 3, this is not so, all these, together with the future projections of sea level rise given in Chapter 11 of Climate Change 01  (13)  can be regarded as completely without theoretical foundation.

To summarise: there is no firm evidence for recent rises in sea level unrelated to land movements and climate events such as El Niño

Figure 5.2 Mean sea level as measured by TOPEX/POSEIDON satellites since 1993 (9)

References

1.     Climate Change 01  page 4

2.     Climate Change 01  page 639 Changes in Sea Level.

3.     Climate Change 01  paragraph 11.3.2 Mean Sea Level Changes over the Past 100 and 200 years, pages 661-664

4.     Permanent Service for Mean Sea Level (PSMSL)  http://www.pol.ac.uk/psmsl/programmes

5.     Climate Change 01  page 661

6.     Daly, J L  2000 Testing the Waters: A Report on Sea Levels, Greening Earth Society,  http://www.greeningearthsociety.org/Articles/2000/sea.htm

7.     Mòrner N A 1998 Postglacial variations in the level of the sea: implications for climate dynamics and solid earth geophysics.  Review of Geophysics 36 603-689

8.     National Geographic Society Documentary 200I “Drowning Paradise”

9.     AVISO website http://www.jason.oceanobs.com/html/portail/actu/actu-welcome-uk.php3

10.  Cabanes, C, A Casenave, C Le Provost 2001, Science 294 840-842

11.  Levitus, S, J I Antonov, T P Boyer, C Stephens 2000, Science  287  2225-2229

12.  Climate Change 01  page 663

13.  Climate Change 01  Chapter 11, Figures 11.11 and  11.12.  pages 670-671.

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