A Service of The Greening Earth Society   

Testing the Waters A Report on Sea Levels

John L. Daly
Greening Earth Society Science Advisor


A serious problem confronts any researcher who looks into the question of tides and sea levels, especially in a search for that elusive concept known as "mean sea level," (MSL) or "Zero Point of the Sea" (as Captain Sir James Clark Ross, the 19th century British Antarctic explorer, called it. [36]

Not only is it difficult to determine true MSL of any one location, it is even more difficult to detect any changes in that level over time. Imagine attempting to measure mean sea level on a Hawaiian surfing beach. Although sea level and tides work over longer timescales, the essence of the problem is much the same. Study of sea level has now taken on more urgency because of the predictions of sea level rise that might result from any global warming as a consequence of an increased atmospheric concentration of carbon dioxide (CO2). [23]

Climate modelers and the Intergovernmental Panel on Climate Change (IPCC) predict that one of the consequences of global warming will be rising sea levels due to thermal expansion of the ocean water mass and the melting of non-polar glaciers [21] [23]. They claim the oceans already have risen 18 cm (7 inches) during the 20th century, an annual rate of 1.8 mm per year. They further predict that the oceans will rise approximately a further 50 cm (19½ inches) during the 21st century [45], an accelerated annual rate of 5 mm per year [25].

Such predictions have taken on hysterical proportions when policy institutions such as the US Environmental Protection Agency (EPA) [44] readily adopt a "worse than worst case" scenario. EPA claims that a 1 meter sea level rise will inundate 7,000 square miles of dry land, 50–80% of U.S. wetlands, and cost the U.S. between $270- and $475-billion alone. With a final flourish, they claim their estimates "are almost certainly too low."

In support of their assertions about past sea level rise, the IPCC cites records from tide gauges, of which one of the oldest is from Brest, in France (Fig.1). [35]

Figure1: Annual MSL at Brest, France [34]

Here we have a sea level in 1805 of approximately 6,900 mm rising to about 7,100 mm by the late 1990s, a rise of 200 mm (nearly 8 inches) or more than a millimeter per year.

The future rise in sea level, according to the IPCC [23] [25], will be caused by two factors. First, the waters of the oceans will expand in volume due to global warming, just as metal expands when heated. Second, warming will result in melting of many non-polar glaciers, further adding water that will drain into the oceans. Looking beyond the 21st Century, there are hints of a still greater sea level rise of several meters if the great ice sheets of Antarctica and/or Greenland begin melting and flow into the sea.

Another school of thought – one first postulated by British meteorologist, Sir George Simpson in 1938 [40] and confirmed by several studies since [30] [41] [31] [26] – suggests that a warming ocean will result in more evaporation and thus more cloudiness. The "Simpson Effect" (as it has been called [13]) would increase precipitation over the polar regions in the form of snowfall and thereby build up the ice mass. Thus the "frozen water" would be permanently lost from the oceans. Yet more moisture will precipitate over internal land catchments (e.g., the Caspian Sea and East Africa’s Lake Victoria), again removing water from the ocean mass. These processes not only are believed to be sufficient to offset the rise in sea level predicted from ocean warming, but could result in falling sea level of up to 7 mm per year. [30]

Prof S. Fred Singer not only demonstrates that warming does not raise sea levels, but also observes an inverse relationship between global temperature and sea levels. This is mainly due to the increased removal of water from the oceans to the ice caps, thereby canceling the effect of any thermal expansion of the oceans upon sea level during periods of climatic warming. [41]

IPCC predictions of sea level rise presuppose there will be a significant human-induced global warming (a questionable assumption given the lack of warming to date as measured from satellites and the lack of polar warming which would be a signature unique to a greenhouse-induced warming). "Significant" is defined here to mean a warming of 1°C or more. Current trends would suggest an "insignificant" warming of only a few tenths of a degree over the next 100 years.


Time and Tide

We all are familiar with tides, the twice-daily rise and fall of the level of the sea as the fluid mass of the oceans adjusts to the changing gravity interactions between the earth, moon, and sun. There are "spring tides" and "neap tides," the tendency for the tides to be greater during a full moon and a new moon than they are during a half moon. Less obvious are other tidal variations: numerous subtle "harmonics" superimposed on the basic tidal pattern and the observed changes in general sea level that ebb and flow over many years.

The moon exerts the primary influence on our tides. As the earth rotates, the moon's gravity pulls at the ocean, causing it to rise in a wide wave riding in the wake of the moon’s relative passage across the sky. The sun also raises a similar (though smaller) tide, being 46 percent of the moon's. When the sun, moon, and earth are aligned, the combined sun/moon gravity raises a larger tide than usual – a spring tide. When the moon is at right angles to the line between the earth and sun, during a half-moon, the two gravity forces counteract one another, resulting in a smaller than usual tide – a neap tide. Every 28 days (the time taken for one lunar orbit) there are two periods of spring tide and two of neap tide.

But the moon's orbit is not circular; it is elliptical. As a consequence, the effect of the moon's gravity is not uniform throughout the 28 days. The earth's orbit around the sun also is elliptical, which causes the effect of the sun's gravity to vary throughout the year. Finally, the moon does not orbit the earth on the same plane as earth's Equator; it orbits at an angle to it. This results in the moon sometimes passing directly over regions of the Northern Hemisphere, while at other times it is over the Southern Hemisphere. Since the tide follows the moon, the tides will be higher in those regions where the moon's orbital path periodically takes it "high" in the sky, and lower in those regions where the moon's orbit periodically takes it "lower" in the sky.

There are many more sun/earth/moon orbital variables (harmonics) to be considered. These are supplemented by strictly earthbound variables such as coastline and seabed topography, ocean current, and the meteorological effects of wind and pressure. Strong winds can push tides higher against some coasts. These are known as "king tides." High atmospheric pressure can depress the level of the sea by about 1 cm per millibar (the unit of measurement of barometric pressure) while low pressure "pulls it up" a similar amount.

While this list of variables is not exhaustive, it is sufficient to demonstrate the complexity of ascertaining changes in mean sea level. At any given moment, some variables act to reinforce the tide while others dampen it. It takes 18.6 years for every permutation and combination of the astronomical variables to play themselves out at any given location [18]. Only then can a researcher calculate long-term MSL with some degree of confidence and precision.

A global "Zero Point of the Sea" is an elusive concept.


Mean Sea Level – "Zero Point of the Sea"

Mean Sea Level is defined as "the mean level of the water surface over a specific long series of measurements. No matter how long a period of data is averaged, the ideal true mean sea level is unattainable because changes are taking place over long and short time scales." [7] Where sea level is measured at only one location, it is sometimes referred to as "Relative Sea Level" (RSL) because the measurement specifies the height of the sea in relation to local landmarks. If the land is rising or sinking, it would manifest itself as apparent rises and falls in sea level, even if the actual level of the ocean remains unchanged. Before the advent of satellites, it was impossible to establish a truly global MSL.

As a descriptive term, Mean Sea Level often is used to mean either the MSL at a single location (meaning it is really RSL) or over an entire region. Global MSL ultimately is determined by the quantity of water in the oceans, the temperature of that water, the volume of water stored in the Antarctic and Greenland ice sheets, the volume of water stored as ice in non-polar glaciers, and the quantity of water stored in natural and man-made inland catchments, lakes, and reservoirs.

The standard instrument used to determine MSL and tidal extremes is a tide gauge. A tide gauge measures the height of the sea at regular intervals in order to record the passage of high and low tides, and the harmonics unique to the location where the tide gauge is situated. Most gauges are installed in populated areas throughout the Northern Hemisphere, particularly in Europe and North America. There are relatively few tide gauges accompanied by long records in the Southern Hemisphere.

Figure 2: Annual mean sea level at Aberdeen, Scotland, UK [35]

Figure 2 shows one of the oldest tide gauge records in the world, that of Aberdeen on the east coast of Scotland. There are a few European gauge records that are older (e.g. Brest, France and Amsterdam, Holland), but their records are broken and contain gaps of several years that make height comparisons between periods of data somewhat problematic. Aberdeen is interesting in that its record has been continuous since 1862.

The peak level at Aberdeen was around 1950 at 7087 mm and the lowest point around 1889 at 6887 mm. The overall sea level rise since 1862 is around 70 mm (less than 3 inches), or an averaged annual rate of around +0.5 mm/yr, which is half the rate recorded at Brest and less than one third the global rate claimed by the IPCC.

Can the Aberdeen record be taken at face value? Does it really reflect trends in global sea level? The short answer is: We don't know.

Tide gauges, like the instruments used to collect surface temperature data, are subject to several local errors that can distort the data. Just as temperature data is affected by urban heat islands, tide gauges located at major cities or ports also are subject to urbanization – mainly the tendency of large cities to subside due to the weight of the structures and changes in the underground water table. The larger the city, the greater is the tendency toward subsidence. This is a creeping effect that, over time, will manifest itself at a tide gauge as a rise in relative sea level. Cities located on alluvial, low-lying coasts are the most affected.

For example, Adelaide in South Australia is showing a strong sea level rise that is not evident at nearby smaller ports (Port Pirie, Port Lincoln, Victor Harbour). The Adelaide anomaly subsequently has been found to be caused by the long-term withdrawal of groundwater beginning with European settlement. It has given rise to localized urban subsidence [3]. A similarly example of subsidence is Bangkok, Thailand, where the sea has risen a meter in the last 30 years. But the sea has not really risen; the land is sinking [38].

A second kind of subsidence error arises from the fact that most tide gauges are mounted on man-made structures, e.g. piers and docks. Over decades, the structures undergo subsidence unless they are built upon bedrock.

In the Netherlands, the tide gauge record from Amsterdam, cited in the IPCC draft report [25], is the longest in the world. It extends back to 1700. Another Dutch record from Hoek Van Holland dates from 1865. Both show a distinctive long-term sea level rise since the mid 19th century. While this also may be due to urban subsidence, one factor makes all Dutch tide gauge data questionable for use in global sea level studies. The problem has its origin in the saying, "God made the world, but the Dutch made Holland" which acknowledges the extensive reclamation of land from the sea characteristic of centuries of Dutch history.

Two key variables used to determine tide height are seabed topography and coastal topography. The Dutch extensively have changed both. Over several centuries, the Dutch have reclaimed land from the sea, altering their coastline and permanently altering tidal flow patterns. [38]

During the 20th century, the Dutch reclaimed much of the large inland gulf known as the Zuider Zee (Figure 3) and dammed off the rest from the North Sea. Before these coastline changes were made, the area was a large tidal sink. Tidal water flowing up and down the English Channel and North Sea previously could drain in and out of the Zuider Zee. Now it is unable to do so, thus raising local sea level outside the Ijsselmeer Dam, particularly during high tide. The surrounding sea is shallow (typically only 25–30 meters) and makes the entire region tidally sensitive to coastal terra-forming of the kind undertaken by the Dutch.

Figure 3: Map of Holland, showing the former Zuider Zee, now landlocked by the Ijsselmeer Dam.
(Based on map from Microsoft™ Encarta96 World Atlas©)

Although the Dutch are among the most strident advocates of the Kyoto Protocol due to their sensitivity to the prospect of sea level rise, their programs of land reclamation over several centuries may have contributed significantly to the very problem they now perceive as threatening their national interest.

We have a very different record from Stockholm, Sweden.

Figure 4: The raw sea level record from Stockholm [35].

Clearly, sea level at Stockholm is falling – by about 40 cm (15½ inches) over 110 years – despite the possibility of city subsidence. But here is how the IPCC portrays Stockholm in the latest Third Assessment Report draft.

Figure 5: The IPCC corrected sea level record from Stockholm (scale in mm) [25].

Figure 5 is a "detrended" record of the sort that corrects for a phenomenon that places all European and North American tide gauge data in doubt. The phenomenon is known as "Post Glacial Rebound" (PGR). During the last Ice Age, the region in which Stockholm was located was buried under several kilometers of ice. The Ice Age ended about 10,500 years ago with a rapid melting of the ice sheets over Europe and North America. Their melting resulted in sea level rise. And, with the ice gone, the plasticity of the mantle below the solid crust of the earth began to force the crust upward because the dead weight of the ice no longer was present. This process has gone on since the last Ice Age, is happening now, and will continue to do so well into the future.

Figure 6: The North Atlantic Basin, showing the areas most affected by PGR (Based on map from Microsoft Encarta96 World Atlas©)

PGR is underway all over Europe, North America, and east Asia (see Figure 6). [34] These are the continents most affected by the enormous ice sheets. Those regions weighed down by ice (those bounded by the continuous red line) continue to uplift like Sweden. This, in turn, causes peripheral regions around the margins of the former ice masses (bounded by the dashed red line) to subside as the continental crusts adjust and rebalance to the weight redistribution.

The North Atlantic actually is a huge basin semi-enclosed by continental landmasses. A 1400 nautical mile gap between west Africa and South America connects it to the world's other oceans. This may explain partly why long-term tide gauge records from within that basin are not always consistent with records outside. It also is unfortunate that all the really old tide gauge records (those that extend back into the 19th Century) come from Europe – the center of the PGR zone. Later American records are affected similarly.

There is a clear example of PGR from the steady rise in relative sea level at Trieste, Italy. Trieste is located at the northern end of the Adriatic Sea where there are hardly any tides to complicate sea level measurements.

Figure 7: Monthly mean sea level at Trieste, Italy. 1905-1999 (year marks not shown but can be inferred)

The Trieste record mirrors that of Venice, across the Gulf of Venice. Because northern Italy is on the margins of where the great ice sheets were located during the last Ice Age, the steady and largely linear rise in sea level from 1905 (amounting to about 15 cm or nearly 6 inches) clearly is unrelated to transient climate events. Rather, it has an underlying geological cause, namely PGR. This long-term rise pre-dates 1905. Compare paintings by 18th Century artists Francesco Guardi and Canaletto with modern photographs of Venice’s famous Rialto Bridge (Figure 8). The comparison reveals a sea level much lower than the 15 cm represented in Trieste’s 20th Century tide gauge record.

Figure 8: Rialto Bridge, Venice around 1770 (Canaletto) compared with today.

Sea level has been rising in Venice for hundreds of years, since well before greenhouse gases became an issue. This local sea level rise clearly is of geologic, not climatic, origin. Urban subsidence also is a major contributing factor in Venice.

It is not just the absence of the weight of the ice that causes uplift and subsidence in the affected regions. The melting of the ice raised global sea levels by over 120 meters (393 feet) [25]. This changed, in turn, geological stresses on regions far away from the ice because of the added weight of the water on the continental shelves. This causes a slow readjustment and rebalancing of continental crusts everywhere as they compensate for the added weight of water.

Most tide gauge records that extend back into the 19th Century are of generally poor quality. They have extensive breaks in data, as is shown in IPCC charts [25]. These records also originate in places affected by PGR and show sea level falling in places like Sweden while rising a similar amount (3.5 mm/yr) in areas like the Chesapeake Bay on the US Atlantic coast [15]. In these cases, it is the vertical movement of the land itself that is causing the apparent (or relative) change in sea level.

In other parts of the world many tide gauges are installed in tectonically active areas, such as North America’s Pacific coast [19], Japan, and New Zealand [15]. Here too, one cannot attribute apparent sea level changes solely to the sea itself because the land upon which the tide gauge is mounted is geologically unstable. Indeed, one should regard tide gauge data from anywhere along the geologically active parts of the Pacific Rim (the so-called Ring of Fire) as profoundly compromised by tectonic activity. Likewise, tide gauges along any other coastline affected by tectonic activity are equally compromised.

One could disregard all tide gauge data from geologically unsuitable locations. But if we do, we end up with hardly any data at all [38]. So we should at least recognize that much of the data being analyzed is of comparably poor quality and is badly distributed, geographically.

To make sense out of the mass of contradictory sea level data, a series of de-glaciation models have been developed using calculations of "earth mantle viscosity" to determine how the mantle and overlying continental and seabed crusts would react to the melting of the great ice sheets at the end of the last Ice Age.


The "ICE-3G" Model

As in the case of Stockholm with its sharply falling sea level, IPCC scientists made a massive correction to the data, turning an RSL fall into an MSL rise. This outcome results from adjusting the observed data with correction factors derived from the ICE-3G model developed by Peltier and Tushingham in 1991 [34]. Their model purports to describe crustal movements of the continents and seabed in the wake of the demise of the great ice sheets. The model depends on calculations about the plasticity of the earth's mantle upon which the crustal land masses "float."

ICE-3G is the model most used for correcting tide gauge data against PGR [15]. It's creators were among the first scientists to link global sea level rise and the Greenhouse Effect, in 1989 claiming that sea levels were rising at a rate in excess of 1 mm/yr [33].

An impression has been conveyed to the public, media, and policymakers that the sea level rise of 18 cm in the past century is an observed quantity and thus open to little dispute. What is not widely appreciated is that this quantity is largely the product of modeling and not a product of observation. It is therefore very much open to dispute, especially when observed sea level data from many parts of the world fail to live up to the IPCC claims.

The ICE-3G model is a global, theoretical treatment used to describe how the ice masses melted and disintegrated at the end of the last Ice Age, and the crust’s consequent readjustments under pressure from the lithosphere below. The model has some inherent weaknesses.

  • It assumes that when the great ice sheets melted there was no change in the oceans’ surface area. This is wrong of course, because vast areas of the continental margins were flooded as sea level rose. The model’s creators acknowledge this, but believe the sea level effect to have been small – something very much in dispute.
  • ICE-3G assumes that the mantle and crust were in equilibrium (general balance) before the "big melt" began. This is an unrealistic assumption because the Ice Age was a very dynamic period, geologically.
  • The model was calibrated against 192 RSL sites from around the world. However, 169 are in the North Atlantic Basin (including the Arctic). It only lists 18 as from the Pacific Ocean. Most of those were along North America’s Pacific Coast. As noted earlier, this is a tectonically active area. Only five sites cover the entire South Atlantic, Indian, and Southern Oceans – nearly two-thirds of the world's oceanic area.
  • ICE-3G uses "fudge factors." Where there is a mismatch between the observed RSL site and the model’s result, the modeled ice load either is increased in thickness or its melting is delayed. This is reminiscent of a similar procedure used in some of the General Circulation Models used to study climate change where the sun’s energy output either is turned up or turned down to stabilize the model at a desired temperature.
  • ICE-3G predicts a sea level rise of 115 meters during the de-glaciation. But physical evidence from most other studies places this figure at 120 to 130 meters [34]. Peltier & Tushingham attribute the discrepancy to "missing ice" in regions assumed to have been ice-free but might, in fact, have been glaciated.

What should be clear is that the ICE-3G global de-glaciation model is really a North Atlantic/Arctic model. Its accuracy must deteriorate therefore as distance increases from the location of the ancient ice sheets, especially for those regions outside the North Atlantic Basin. For the rest of the world's oceans and crust, this model may have little relevance, especially given the paucity of RSL sites from the Pacific and Southern Hemisphere oceans against which it can be calibrated.

Even within the North Atlantic Basin where ICE-3G could be expected to exhibit its greatest accuracy, a 1996 study of the southeastern seaboard of the U.S. by Davis and Mitrovica [14] finds that the model overestimates sea level rise in that region due to incorrect calculation of lower mantle viscosity. From Key West to Cape Hatteras, their recalculation of mantle viscosity results in a reduced estimate of sea level rise of between 2.28 – 1.45 mm per year.

As for the world’s four other great oceans, it is not only the model’s theoretical elegance that should be judged, but more important, how its application to the sea level problem accords with observed reality. The IPCC estimate of +1.8 mm/yr sea level rise in the 20th Century is critically dependent upon the processing of tide gauge data using this model.

Finally, it must be stressed that ICE-3G does not and cannot correct for tectonics such as exist along the Ring of Fire in the Pacific Ocean. It does not and cannot correct for local urban subsidence such as exists in Adelaide, Venice, and Bangkok. It does not and cannot correct for subsidence of the man-structures upon which tide gauges are mounted. With or without the ICE-3G model, all of these local errors still exist and make global estimates of sea level change very difficult to validate.

Whatever degree of confidence is placed in this model, to use it in determining past global sea level changes means that the IPCC estimate of +18 cm sea level rise over the last 100 years cannot be regarded as an observed value. Rather it is a largely modeled value subject to a high margin of error due to local distortions.


The IPCC Claims ...

Using historical tide gauge data, Atmosphere-Ocean General Circulation Models (AOGCMs that couple oceans and atmosphere) and the ICE-3G model, IPCC scientists have interpreted the past, present and future of sea level to make the following basic claims [25]:

  • Sea level already has risen between 10 and 25 cm (4–10 inches) during the 20th Century, with a preferred value of 18 cm (7 inches), or an historical rise of 1.8 mm/yr.
  • A less-than-average rise will occur in the Southern Ocean. How much less is not specified.
  • Sea level changed by only 30–50 cm over timescales of several centuries during the previous 5,000 years.
  • A sea level rise of 21–92 cm is projected over the next hundred years, with a preferred value of around 50 cm (representing an acceleration in sea level rise to 5 mm/yr).

The predicted 21st Century sea level rise is made up partly as follows:

Contributing factor

Lowest estimate (cm)

   Highest estimate (cm)

Thermal expansion of the ocean
Non-polar glacier melt
Antarctica ice accumulation/melt
Greenland ice accumulation/melt
Melting of permafrost

   - 8

- 2


Most of the claims are qualified by caveats that relate to changes in water storage on land that may reduce the sea level rise by 10 cm. If the figures appear to be inconsistent with the broader estimates, they reflect confusion within the IPCC itself as it wrestles with multiple scenarios, contradictory model results, and caveats about unknown quantities, e.g. water storage on land.

There are other factors that contribute significantly to sea level change. They include human land use [37] and the effect of natural internal catchments that keep water from returning to the ocean. But the contributing factors listed above are considered to be the major ones. Human activities such as irrigation, land reclamation, flood levees, dam building, and reservoirs were estimated by Sahagian et al to have contributed at least one third to the "observed sea level rise" during the last hundred years and will continue to do so at a rate they estimate to be 0.54 mm per year [37].

The negative "lowest estimate value" shown for Antarctica indicates that the IPCC now acknowledges the possibility that the Simpson Effect (which involves increased precipitation over the polar regions) may result in a negative contribution to sea level change in the 21st Century.

Environmental activists frequently point toward the West Antarctic Ice Sheet as a source of possible catastrophic sea level rise. Its collapse would raise sea levels by about 5 meters (16 feet). But, according to the IPCC and numerous other research bodies (including the Australian Antarctic Division), this possibility is off the climatic agenda for at least a thousand years and probably as long as 7,000 years, even with global warming. As to the chance of it collapsing due to purely natural reasons, this is rated by the IPCC at only 100,000:1 [25].

The IPCC has made it very clear that, in their view, past and future changes in sea level are mainly driven by the state of the climate. We now must examine the credibility of these IPCC claims, beginning with their claim that sea levels already have risen 18 cm during the 20th Century.


20th Century Sea Levels

Recent studies of tide gauge data suggest that there has been a sea level rise in the last hundred years of between 10 and 25 cm with a preferred value of 18 cm. But this finding requires application of the ICE-3G model correcting the data for the effects of PGR in order to establish a rise of that magnitude. Expressed as an annual rate, the average rate of increase is 1 to 2.5 mm/yr, with a preferred value of 1.8 mm/yr.

Once we depart from the Northern Hemisphere’s tide gauges (particularly those within the North Atlantic Basin) and from those compromised by local tectonic activity, a very different picture emerges about sea level. The following graphs either are directly from the Permanent Service for Mean Sea Level (PSMSL) [35], or drafted from data from the same source.

Here is the monthly MSL from Montevideo, Uruguay, with a 12-month smoothing added.

Figure 9: Monthly and smoothed relative MSL from Montevideo, Uruguay.

The record has several short breaks. With the exception of an anomalous peak in sea levels during the early 1980s, there appears to be little change at all in sea level at Montevideo.

Let’s look across the South Atlantic east of Montevideo and examine the sea level record at East London, South Africa.

Figure 10: Monthly MSL from East London, South Africa, 1967–1998.

This record also is broken, but other South African tide gauges are even worse. This problem is common all over the world. At East London we see a peak variation in sea level of around 40 cm (15.7 inches) during the early 1970s although there is no significant long-term trend.

Let’s cross the Indian Ocean, and examine the record for Vishakhapatnam on the east coast of India.

Figure 11: Monthly MSL from Vishakhapatnam, India.

Overall, this is a good quality record. It has only a few, brief breaks. There is little overall change over a 57-year record. That is sufficient time to have resulted in a sea level rise of over 10 cm if the IPCC’s claim is correct.

Moving southeast, we can examine records in Australia. Here is a short-run sea level series from Newcastle, New South Wales.

Figure 12: Monthly MSL from Newcastle, NSW, Australia, 1972–1986 [34].

In this record, the time span is only 14 years. This is less than the 18.6 years required for a full cycle of astronomical variables to have played out. Nevertheless, there has been an overall fall in sea level during the time of the available record.

Looking south toward Tasmania (an island state of Australia, 200 miles south of the mainland and deep in southern latitudes at 42°S), we can examine records for Spring Bay (Fig.13).

Figure 13: Monthly MSL from Spring Bay, Tasmania, Australia, 1992–1998 (scale in cm).

Spring Bay enjoys use of a state-of-art tide acoustic gauge mounted in a rural location on coastal bedrock, thus avoiding urban subsidence problems. This site faces directly onto the Southern Ocean and has a very small tidal range of about 70 cm between high and low tide. Although the record is short, again no long-term change is evident, as yet.

Looking toward the western Pacific Ocean, we find Nauru (Fig.14).

Figure 14: Monthly MSL from Nauru, western Pacific Ocean.

Nauru clearly demonstrates the impact of the El Niño Southern Oscillation on sea level in the western Pacific. During the two major El Niño events of recent years (1982-83 and 1997-98), there is a sharp, but temporary, fall in sea level. Statistically, this would amount to a long-term fall in sea level over the period of record. But it is clear from the graph that sea level on either side of the El Niño events is largely unchanged. For this reason, it is prudent to exclude anomalous El Niño effects from statistical estimates of long-term sea level.

From these examples – places known to be tectonically stable – there is little evidence of long-term sea level rise. We already have established that the partially enclosed North Atlantic Basin is severely affected by PGR. Once outside that basin, the effect of PGR seems to diminish significantly. This raises questions as to how much confidence should be placed in the ICE-3G model as it concerns regions well removed from the North Atlantic.

The Australian National Tidal Facility (NTF) at Flinders University in Adelaide published a `Mean Sea Level Survey' in 1998 to establish sea level trends around the Australian coast from tide gauges having more than 23 years of hourly data in their archive [29]. This survey is particularly relevant for global application because Australia is tectonically stable and is affected much less by PGR than are Europe, Asia, and North America. Because nearly two-thirds of the world's total oceanic area is in the Southern Hemisphere, Australia is especially suited to monitor Southern Hemisphere trends. It probably best represents true MSL, globally. Also, Australia’s coast adjoins the Indian, Pacific, and Southern Oceans. This fact renders its data indicative of sea levels in three oceans, not just one.

The NTF identifies the following tide gauges as meeting their long-term criteria. NTF calculates the annual sea level change for each location in millimeters per year. Sea level rises are shown in red, falls in blue. The stations run counter-clockwise around the Australian continent, beginning with Darwin in the Northern Territory. Compare these results against the IPCC’s claimed rate of +1.8 mm/yr.


Years of data  

Est. trend
(mm per year)

Darwin, NT
Wyndham, WA
Port Hedland, WA
Carnarvon, WA
Geraldton, WA
Fremantle, WA
Bunbury, WA
Albany, WA
Esperance, WA
Thevenard, SA
Port Lincoln, SA
Port Pirie, SA
Port Adelaide - Inner, SA
Port Adelaide - Outer, SA
Victor Harbour, SA
Hobart, TAS
George Town, TAS
Williamstown, VIC
Geelong, VIC
Point Lonsdale, VIC
Fort Denison, NSW
Newcastle, NSW
Brisbane, QLD
Bundaberg, QLD
Mackay, QLD
Townsville, QLD
Cairns, QLD




Eleven of the 27 stations record a fall in sea level, while the mean rate of sea level rise for all stations combined is a mere 0.3 mm per year. The average record length is 36.4 years. This rise is one-sixth that anticipated by the IPCC. Nor is there an obvious geographical pattern of fall versus rise. Both are distributed along all parts of the coast.

But there's more. As was explained earlier, Adelaide is a prime example of local sea level rise due to urban subsidence [3]. Its two stations (listed above) are the only ones where a sea level rise is greater than the IPCC estimate. The NTF survey points out the Adelaide anomaly and directly attributes it to local subsidence, not sea level rise. NTF reaches its conclusion on grounds that the neighboring stations at Port Lincoln, Port Pirie, and Victor Harbour show only a rise of +0.3 mm/yr between them. If one excludes Adelaide from the list, the average sea level rise for the other 25 stations is only 0.16 mm/yr, or less than one-tenth the IPCC estimate.

If this were not convincing enough, there is one further piece of evidence from Australia that demonstrates how the IPCC – and the ICE-3G model that underpins its predictions – is in error concerning the magnitude of 20th Century sea level rise.


The ‘Isle of the Dead’


Figure 15: The ‘Isle of the Dead' with Point Puer in the immediate foreground.
(Photo John L. Daly, Aug 29th 1999, late afternoon)

" ‘Dead Men's Isle’ is a picturesquely sorrowful spot - so soothing in its melancholy, so placid in its solitude."

: David Burn, visitor to the ‘Isle of the Dead’ in 1842

The Isle of the Dead is not mentioned in any reports of the IPCC. However there is intensive research focused by several institutions on this sleepy little isle at the bottom of the world in Tasmania [24]. Australia's Commonwealth Science and Industry Research Organization (CSIRO Marine Research Division) is being assisted by the head of the Inter-Agency Committee on Marine Science & Technology, Dr. David Pugh, who is based at the University of Southampton, UK.

The Isle of the Dead is over two acres in size and situated within the harbor of Port Arthur in southeastern Tasmania . A large and undeveloped harbor, it opens out directly to the Southern Ocean. The isle itself is a graveyard (the origin of its eerie name) containing the graves of some 2,000 British convicts and free persons who died at the nearby convict colony of Port Arthur between 1832 and 1870. Port Arthur is now a heritage historic site and is visited by thousands of tourists every year who view the convict buildings and ruins, and enjoy the popular night-time "ghost tours."

To understand the intense scientific interest in the isle, one must travel back in time.


In September 1840, Britain’s renowned Antarctic explorer, Captain Sir James Clark Ross, sailed from Hobart Town (the capital of Van Diemen's Land, the former name for Tasmania) on a 6-month voyage of discovery and exploration to the Antarctic. He had two expedition ships, Erebus and Terror.

His voyage was highly successful. He returned to Hobart Town in April 1841 to refit and resupply his ships and to spend the southern winter in temperate latitudes. Upon his arrival, he was disappointed to learn that a golden scientific opportunity had been missed. During his absence, a Baron Von Humboldt had proposed to British Colonial Secretary, Lord Minto, that mean sea level marks should be struck on newly discovered coasts and islands. News of Humboldt’s proposal had arrived too late for Ross to act upon it during his voyage to the Antarctic. As he later related this in his book (published in 1847): [36]

The fixing of solid and well secured marks for the purpose of showing the mean level of the ocean at a given epoch, was suggested by Baron von Humboldt, in a letter to Lord Minto, subsequent to the sailing of the expedition, and of which I did not receive any account until our return from the Antarctic seas, which is the reason of my not having established a similar mark on the rocks of Kerguelen Island, or some part of the shores of Victoria Land [in Antarctica].

Despite the missed opportunity, Humboldt's idea still appealed to Ross and to Van Diemen's Land’s Governor Sir John Franklin, himself a naval man. Consequently, both made a point of visiting Port Arthur, 40 miles from Hobart Town. There they met Thomas Lempriere, a senior official of the convict colony. Lempriere was a methodical observer and recorder of meteorological, tidal, and astronomical data. Here is Ross’ account of his visit to Port Arthur:

My principal object in visiting Port Arthur was to afford a comparison of our standard barometer with that which had been employed for several years by Mr. Lempriere, the Deputy Assistant Commissary General, in accordance with my instructions, and also to establish a permanent mark at the zero point, or general mean level of the sea as determined by the tidal observations which Mr. Lempriere had conducted with perseverance and exactness for some time: by which means any secular variation in the relative level of the land and sea, which is known to occur on some coasts, might at any future period be detected, and its amount determined.

The point chosen for this purpose was the perpendicular cliff of the small islet off Point Puer, which, being near to the tide register, rendered the operation more simple and exact. The Governor, whom I had accompanied on an official visit to the settlement, gave directions to afford Mr. Lempriere every assistance of labourers he required, to have the mark cut deeply in the rock in the exact spot which his tidal observations indicated as the mean level of the ocean."

Ross further explained why he chose Port Arthur for a mean sea level mark instead of the Derwent estuary closer to Hobart Town, where Erebus and Terror were moored.

The tides in the Derwent were too irregular, being influenced greatly by the prevalence of winds outside and the freshets from the interior, so that we could not ascertain with the required degree of exactness the point of mean level.

The "permanent mark" at the "zero point, or general mean level of the sea" that Ross wrote about has proved to be more permanent than even he bargained for. It is still there, in perfect condition.

Figure 16: The Ross-Lempriere sea level benchmark on the Isle of the Dead.
(photo by John L. Daly at mean tide, Aug 29, 1999. Benchmark is 50 cm across.)

In these photos, the line and arrow mark is a standard British Ordnance Survey Benchmark. It is 50 cm (nearly 20 inches) across. It stands (in the photo) about 35 cm above the water level. These photos were deliberately taken at the time of mean or half-tide on that day. They illustrate the enigma that is the Isle of the Dead. How is it that a benchmark struck at the "zero point" or the "mean level of the sea" as so explicitly described by Ross, is now 35 cm above today’s mean level? Has the sea level dropped?

(For a more detailed discussion about the fascinating origin of this benchmark, see the Appendix.)

Of course, mean tide on the day the photo was taken may not be the long-term MSL. However, CSIRO has been researching the benchmark since 1995. They have installed a state-of-the-art acoustic tide gauge at the Port Arthur jetty a mile away, set up a network of GPS (Global Positioning System) buoys around the harbor, and involved other institutions in their effort. Their unpublished conclusion is that the benchmark indeed is 35 cm above current mean sea level [12]. And they cannot explain it in a manner consistent with the Ross account.

Southeastern Tasmania is believed to be uplifting ever so slightly due to PGR, although there is no tectonic activity in the region. CSIRO installed GPS receivers and GPS marine buoys in the Port Arthur area to test for the PGR effect. It takes several years using GPS positioning to determine an uplift rate that is accurate within millimeters. However, CSIRO has made a preliminary, though unpublished, uplift estimate of 0.61 mm/yr (± 0.22 mm/yr). Over the full period of 159 years since the benchmark was struck, this uplift rate would result in a relative sea level fall of 6.2–13.2 cm (approximately 2½–5 inches), with a mid range value of 9.7 cm. This is only a fraction of the 35 cm to be accounted for. However, local geological shoreline evidence indicates an uplift much less than this (around 0.19 mm/yr), giving a total uplift since 1841 of only 3 cm [6]. The geological figure is probably the more accurate because it represents actual past uplift, whereas the (albeit preliminary) GPS result can only represent a current rate of uplift.

In 1888, a Capt. Shortt made a scientific survey and assessment of the benchmark in an effort to determine its exact origin and meaning. He searched for information in archives at Hobart and Port Arthur, and reported his findings in a short paper published by the Royal Society in Hobart [39].

A small tablet was found above the benchmark (the tablet "disappeared" around 1913 [28]) and it provided Shortt with the date the benchmark was struck: July 1, 1841. That date would be at a point in the lunar month when the age of the moon was twelve days. In order to measure sea level under conditions similar to those which existed then, Shortt made his calculation of MSL and the benchmark height when the age of the moon also was twelve days. He concluded (in 1888) that the benchmark was 34 cm above mean sea level. This is only a centimeter less than the CSIRO estimate of 35 cm, 112 years later. As Shortt was familiar with Ross’ account, he was perplexed as to why a mean sea level benchmark would be 34 cm above MSL 47 years later.

In 1985, Bruce Hamon, a scientist from Sydney, also studied the benchmark. He concluded that it was 36 cm above MSL [22]. He examined tide data from nearby Hobart using its tide constants to establish the current point in the 18.6-year cycle, so we can have some confidence in his estimate.

Hamon's is only the second (and last) paper in the scientific literature about the Ross-Lempriere benchmark. Since then, nothing has been published – not even interim research results from CSIRO. All that has appeared in the public domain are a few media releases, none of which impart the important and astounding information that a mean sea level benchmark struck in 1841, more than 150 years later spends most of its time above the waterline!

Since the benchmark has been observed to be the same height above relative MSL using three sets of good observations 112 years apart, sea level clearly has not changed at Port Arthur in all that time. Being tectonically stable and subject only to minor PGR, land uplift hardly provides an adequate explanation for the lack of sea level rise since 1888 and the possibility of a sea level drop between 1841 and 1888.

The benchmark powerfully confirms what the Australian Mean Sea Level Survey [29] tells us, namely, the rate of sea level rise over much of the 20th Century only has been 0.16 mm/yr, less than one tenth of the IPCC's estimate of 1.8 mm/yr. This survey implies a sea level rise of only 1.6 cm for the entire century, consistent with observations and measurements of the Ross-Lempriere benchmark since Capt. Shortt first observed it in 1888.

The joint "Co-ordinating Lead Author" of Chapter 11 (sea levels) of the draft IPCC Third Assessment Report [25] is Dr John Church. He heads the CSIRO Marine Research Division in Hobart, Tasmania. His organization is deeply involved in researching the benchmark as shown by their press release of 1998. In spite of this, there is no discussion in the IPCC draft, as co-authored by Church, of the benchmark or its implications for historical sea levels. The draft cites old tide records from PGR-ridden Europe and even complains about the lack of data from the Southern Hemisphere. But there is nothing about a 159-year-old sea level benchmark in the data-sparse Southern Hemisphere that predates most other records and is located only an hour's drive from Hobart.

The most comprehensive information about the benchmark and the historical events and personalities surrounding it comes from this author's website, from which further details can be obtained at the linked references given [10] [11] [12].

There is also a discussion about the origins of the benchmark in the Appendix


Sea Level ‘Secession’ for the Southern Oceans

The Australian Mean Sea Level Survey of 1998 [29] may be one reason why the IPCC has made the somewhat strange suggestion in its draft Third Assessment Report that sea level rise in the Southern Ocean will be less than that in the Northern Hemisphere. This conclusion stems from the output of Ocean-Atmosphere General Circulation Models (AOGCM), some of which predict sharp differential trends in sea levels between regions during the 21st Century.

From the results of the Australian Sea Level Survey – and the evidence of the Ross-Lempriere benchmark that supports it – one can see that the IPCC's 1.8 mm/yr claim for past global sea level rise simply is wrong. The source of the error quite probably is the ICE-3G model when it is used to adjust tide gauge data outside the North Atlantic Basin, and to the fact that too many global inferences are drawn from North Atlantic data.

Rather than directly confront this issue, the IPCC and modelers instead have chosen to "quarantine" the Australian survey by their suggestion that the Southern Oceans can somehow go their own way when it comes to sea level rise. Given that the Southern Hemisphere holds nearly two-thirds of the world's oceans, this clearly is not a tenable position over the long term.

Their suggestion adds a new dimension to the term "Down Under."


The previous 5,000 years

Another claim by the IPCC concerns sea levels having been largely stable since the end of the last Ice Age, changing less than 30–50 cm over time scales of several centuries during the previous 5,000 years [25]. This suggests that sea levels prior to industrialization varied only slightly over thousands of years. But were sea levels really that stable? Not according to scientists Prof. Peter Flood, Dr. Robert Baker and Dr. Bob Haworth of the University of New England in New South Wales, Australia.

Their research into semi-fossilized shellfish and calcareous coated worms in caves and walls surrounding Sydney beaches has shown that sea levels fell 1–2 meters in less than 100 years around 3,500 years ago [42] [43]. This finding challenges the assumption that sea levels have been stable since the end of the last de-glaciation 6,000 years ago. It also demonstrates that sea level and climate can change significantly within the span of a single human lifetime, that they have been changing for centuries before industrialization, and will continue to change in the future.

These scientists further point out that if sea levels were today what they were 6,000 years ago, the site of the Sydney 2000 Olympic Games would be under water!

Similar rapid oscillations in sea level took place elsewhere along the southeast Australian coast throughout the last 4,000 years, again marked by horizontal strata of fossil shellfish remains (common barnacles and tubeworms) 1–2 meters above the limit of the same species, today. These relative sea-level changes are unlikely to have been caused by local tectonic movement because at least ten sites were studied from Brisbane in Queensland to Fremantle in Western Australia, where much the same thing happened at much the same time (all verified by AMS radiocarbon dating).

From all these sites, Prof. Flood et al find that over the last 4,000 years there has been a net decline in sea level of almost 2 meters around southern and eastern Australia. They further point out that other researchers using similar indicators have found the same general picture in other tectonically stable, mid-latitude, far afield sites in Brazil, Madagascar, and New Caledonia.


Back to the Future

IPCC predictions of sea level rises for the 21st Century of between 21 and 92 cm with a preferred average of around 50 cm are based on aggregating the Atmosphere-Ocean General Circulation Models (AOGCMs) used in various countries. About the only thing these models agree on is the fact of sea level rise. Beyond that, their regional distributions contradict each other, as can be seen in Figure 17 below [25].

Figure 17: Sea level change predicted between the pre-industrial period and the late 21st century [25]

When models disagree to this extent, is there any value in averaging them in an attempt to claim the averaged value is somehow more authoritative? The ECHAM model in the top right surely must be the most bizarre, showing a simultaneous difference in sea level trends in the southeastern Pacific of over 2 meters in closely adjacent parts of the same region. The Australian CSIRO model shows a smoother global picture, while the two British HadCM models bear little resemblance to one another. The two American GFDL models are equally at odds at the regional level.

An interesting feature of the ECHAM model is its treatment of the North Atlantic Basin. It shows the North Atlantic to have a smaller sea level rise than the remainder of the world's oceans and predicts greater sea level rises in the Southern Hemisphere.

A key baseline parameter of all these models is that they follow the IPCC's "IS92a" scenario [25]. This means that built into them is the assumption that greenhouse gases, expressed as "CO2 equivalent" will increase at a rate of 1 percent per year until 2100. This is an old assumption used before 1992. Yet it is recycled for the 2000 Assessment. The assumption already has been overtaken by events.

One of the gases upon which that assumption is based is methane. Methane concentrations stopped growing in the atmosphere as of 1992 and have now largely stabilized [20]. Chlorofluorocarbons (CFC) – another CO2 equivalent greenhouse gas – now are controlled by the Montreal Protocol and no longer are a relevant factor over the long term. Other halocarbons are too insignificant to affect the climate either way. Nitrous oxides also are insignificant. As a result, only CO2 is left to achieve this one percent per year growth scenario.

But CO2 is not rising like that at all. First, its growth is linear, not exponential (as is suggested in expressing it as a percentage). Second, it has been growing for the last few decades by an average of 1.5 ppm per year. The effect on the IPCC's IS92a scenario is that instead of an effective CO2 level of nearly 1,000 ppm by the year 2100, at 1.5 ppm/yr the level would only reach approximately 500 ppm – half the estimate.

This has major implications for the AOGCMs in their sea level predictions. Because they use outdated, unrealistic, and exaggerated estimates of what the atmospheric CO2 concentration will be a hundred years from now – even without the issues about sea level rise raised in this report – the IPCC predictions should be about halved simply because of their "one percent assumption."


The Poseidon Adventure

The use of satellites equipped with altimetry instruments (particularly the Topex/Poseidon satellites) has revealed a sea level profile that hardly was guessed at before the advent of satellites [8]. Instead of finding a mean sea level applicable everywhere and modulated only by tides and weather, we find instead that the mean Zero Point of the Sea actually can be different from place to place, and differ by several feet in some instances.

Figure 19: The variable height of the sea as seen by Topex/Poseidon on cycle 273 [8]

As depicted in Figure 19, there is a difference in real time sea level by up to 1.5 meters (about 5 feet) between the western Pacific and the Southern Ocean. Even on the same latitudes, the western Pacific is 1 meter higher than the eastern Pacific. During a major El Niño, this pattern is reversed.

This is the normal background state of sea level. A similar pattern was evident from the very earliest of the TOPEX-Poseidon missions in 1992. The difference in level between the tropics and high latitudes is due to the centrifugal forces arising from earth's rotation having their greatest effect at the Equator. However, this would not prevent the southern oceans from rising in level if the tropical oceans also were rising.

Just as satellites have created a crisis within IPCC circles concerning changes in global temperature [20], so too are TOPEX-Poseidon satellites proving to be a problem for alarmist sea level predictions. The satellite altimeters measure sea level on a global basis to an accuracy of 5 cm on each pass and compare the actual surface of earth with a "geoid" represented by an integration between the satellite's orbital height and the center of the earth.

Although the single-cycle accuracy of 5 cm does not seem adequate for sea level studies where fractions of a millimeter per year are involved, this level of accuracy can be achieved statistically once there are multiple cycles with which to work. The latest data (Figure 20) is from cycle 276 where we now have an accumulated record of data over eight years to establish mean sea level change to an improving degree of accuracy. One key advantage of TOPEX-Poseidon is that sea level heights can even be measured in the open ocean, whereas tide gauges are restricted to coastlines and islands.

Figure 20: Global MSL variation according to TOPEX/Poseidon satellites. [8]

The TOPEX-Poseidon project also has established a set of tide gauges on islands in the Pacific Ocean fitted with GPS equipment. Their purpose is to calibrate the satellites to a still greater level of accuracy. In time, they should render tide gauge data redundant.

The IPCC predicts that its sea level rise rate estimate of +1.8 mm/yr will accelerate during the 21st Century to around 5 mm/yr. As Figure 20 shows, the current sea level rate of rise after cycle 276 is 0.9 mm/yr, half the rate claimed for the last hundred years, and less than one fifth the rate claimed for the 21st Century.

Not only has recent sea level rise not matched the 20th Century rate claimed by the IPCC, the idea that an acceleration might be underway as a result of climatic warming is contradicted by satellite temperature data. Although the IPCC claims the 1990s have been characterized by human-induced warming, the accelerated sea level rise they anticipate from that warming simply has not materialized.

As the graph in Figure 20 also shows, even the small amount of sea level rise there has been is due primarily to the intense El Niño of 1997-98 which has been demonstrated to cause a temporary rise in global sea level (although it causes a local fall in level in the western Pacific). Sea level following the El Niño is now much the same as it was prior the event. The CNES, based in Toulouse, France, and co-sponsor of the satellites, also attributes the sea level rise since 1992 to the 1997-98 El Niño [9].

Once there is a longer period of satellite data, the effect of that El Niño will diminish in the statistically-averaged sea level result. If the effect of the El Niño is excluded from the sea level result (as was suggested in the case of the Nauru tide gauge data), scarcely any sea level rise is registered by TOPEX/Poseidon.



People have been given the impression that the 18 cm sea level rise claimed for the 20th Century is an observed quantity. It now should be clear that it is not. The 18 cm figure is the product of combining data from tide gauges with the output of the ICE-3G de-glaciation model. A simple logical equation can be constructed for this:

an observed quantity ± a modeled quantity = a modeled quantity

Thus, the claimed 18 cm sea level rise is a model construct, not an observed value. Worse still, the model is primarily focused on the North Atlantic Basin, which exhibits relative sea level trends quite unlike any observed outside the North Atlantic. Thus, global estimates should not be inferred with any confidence from modeled trends that mainly affect only that basin.

In the world’s remaining oceans there clearly is a lack of evidence of sea level rise during the 20th Century. This is particularly true around the Australian coast – a coastline representative of three oceans – where good quality records of tide gauge data are available. The rise recorded along the Australian coast is an insignificant 1.6 cm for the entire century. That’s just over half an inch in a hundred years!

The absence of significant sea level rise around Australia is confirmed by a similar absence of sea level change as measured since 1888 against the Ross-Lempriere benchmark carved on a natural rocky cliff on the Isle of the Dead in Port Arthur, Tasmania. It also is possible that a significant sea level fall occurred between 1841 (when the benchmark was struck) and 1888 (when its height was accurately measured). The only other tide gauge records of similar age are few in number and come from regions severely affected by PGR within the North Atlantic basin. Thus, they cannot be considered as conclusive evidence disputing a possible global sea level fall during that period.

Outside the North Atlantic Basin, most other tide gauges with long-term records have been mounted in tectonically active areas, especially along the west coast of North America and New Zealand. Thus they are unsuitable for measuring global trends. Many others are subject to local subsidence.

As to the future, the IPCC suggests accelerating sea level rise to nearly 5 cm/yr [25]. However, the TOPEX-Poseidon satellites now show sea level rise to be only 0.9 mm/yr [8], all of which has been attributed to the 1997-98 El Niño event [9]. Sea level was largely unchanged before and after that event. Thus the 0.9 mm/yr rise merely is a statistical artifact and does not represent a true rise in the background sea level.

Finally, it should be remembered that no matter what is said about sea level, it depends entirely upon how global climate responds to greenhouse gases – whether the planet warms significantly or not [45]. Sea level rise is contingent on atmospheric warming. If there is no warming, there is no sea level rise. The record of atmospheric temperature as recorded by satellites since 1979 reveals no significant warming despite numerous model predictions to the contrary.



(This references list is common to both the main article and the appendix)

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[2] BBC Sci/Tech Online News, "Mark of a Hot Dispute", Oct 7, 1999.

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[3] Belperio, A, "Land Subsidence and Sea-level Rise in the Port Adelaide Estuary: Implications for Monitoring the Greenhouse Effect", Australian Journal of Earth Sciences, v.40, p.359-368, 1993.

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[4] Bolt, A, "Maybe we aren't all going to drown in rising seas after all", Herald Sun, Melbourne, Feb 14, 2000.

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[5] Bryant, E. "The Greenhouse Conspiracy", The Optimist, Nov/Dec 1988.

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[6] Bugg, R., (1990), "Waves and Coastlines" in "Behind the Scenery - Tasmania's Landforms and Geology", Dept. of Education & the Arts, Hobart, Tasmania, pp 96-97.

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[7] Canadian Hydrographic Service, Tidal Manual, 1999.

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[8] CNES online MSL data for TOPEX-Poseidon cycle 276 - (click on ‘mean sea level monitoring’ link)

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[9] CNES online "El Niño is behind rise in sea level".

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[10] Daly, J. "The Isle of the Dead - Zero Point of the Sea?", Sept 1999.

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[13] Daly, J. The Greenhouse Trap, Bantam Books, Sydney, 1989.

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The Origins of the ‘Isle of the Dead’ Benchmark

by John L. Daly

In the attached article, the main focus on the 1841 Ross-Lempriere sea level benchmark was to demonstrate how sea levels had scarcely moved since 1888 when it was first investigated and its exact height measured by Capt. Shortt [39]. But where the mark was struck originally and the circumstances surrounding it is still the subject of some dispute and makes a fascinating story in its own right.

The benchmark is engraved on a rocky natural cliff on a small isle (the Isle of the Dead) within the harbor of Port Arthur in southeastern Tasmania, an undeveloped harbor which opens directly to the Southern Ocean. The idea for the benchmark came from Capt. Sir James Clark Ross, the renowned British Antarctic explorer and marine scientist, acting in collaboration with Thomas Lempriere, an official of the convict colony at Port Arthur.

Here again is what Ross said about the benchmark in his 1847 book.

The fixing of solid and well secured marks for the purpose of showing the mean level of the ocean at a given epoch, was suggested by Baron von Humboldt, in a letter to Lord Minto, subsequent to the sailing of the expedition (Ross' own expedition of the `Terror' and `Erebus'), and of which I did not receive any account until our return (to Tasmania) from the Antarctic seas, which is the reason of my not having established a similar mark on the rocks of Kerguelen Island, or some part of the shores of Victoria Land (in Antarctica).

Having missed that opportunity, he went with Governor Franklin to Port Arthur in 1841 to see Thomas Lempriere who had observed and recorded tidal, astronomical, and meteorological observations over several years. Ross goes on -

My principal object in visiting Port Arthur was to afford a comparison of our standard barometer with that which had been employed for several years by Mr. Lempriere, the Deputy Assistant Commissary General, in accordance with my instructions, and also to establish a permanent mark at the zero point, or general mean level of the sea as determined by the tidal observations which Mr. Lempriere had conducted with perseverance and exactness for some time: by which means any secular variation in the relative level of the land and sea, which is known to occur on some coasts, might at any future period be detected, and its amount determined.

The point chosen for this purpose was the perpendicular cliff of the small islet off Point Puer [the Isle of the Dead], which, being near to the tide register, rendered the operation more simple and exact. The Governor [Sir John Franklin - a naval man], whom I had accompanied on an official visit to the settlement, gave directions to afford Mr. Lempriere every assistance of labourers he required, to have the mark cut deeply in the rock in the exact spot which his tidal observations indicated as the mean level of the ocean.

Explaining why he chose Port Arthur for a mean sea level mark instead of in the Derwent estuary closer to Hobart Town, where his ships `Erebus' and `Terror' were moored, he wrote -

The tides in the Derwent were too irregular, being influenced greatly by the prevalence of winds outside and the freshes from the interior, so that we could not ascertain with the required degree of exactness the point of mean level.

The emphases have been added to highlight key points. Ross refers to the intended mark as being "mean sea level" or "zero point", no less than 5 times in these short extracts. His intention in respect of the benchmark is therefore clear and unmistakable. The mark was intended as a mean sea level mark, not a mere tide mark which would only be of use in Port Arthur, nowhere else. The CSIRO did not mention any of Ross' words in a lecture they gave to the Hobart Royal Society in April 2000, and repeatedly used the expression `tide mark' instead of `sea level mark' throughout the lecture.

The exact date on which the Ross-Lempriere benchmark was struck is known - July 1st 1841, because a small stone tablet was placed above it on the clifftop. The tablet went missing around 1913 [28], but there are two witnesses who reported what was inscribed on it. The first, a Mr Mason [39] acting for Capt. Shortt, complained the tablet was badly eroded and difficult to read. He quoted its words in 1888 as follows -

On the rock fronting this stone a line denoting the height of the tide now struck on the 1st July, 1841, mean time, 4h. 44m. p.m.; moon's age, 12 days; height of water in tide gauge 6 ft. 1 in.

Three years later in 1891, a second witness, a yachtsman cruising aboard the yacht Wanderer wrote an account of his voyage in a Melbourne magazine under the pen name ‘Eight Bells’ [16]. He visited the Isle of the Dead and was interested in the numerous gravestones, one of which he sketched and quoted (the gravestone he quoted from is still there today, bearing the exact words he reported) [28]. Then he noticed that -

...a few yards away, on the rocks fringing the shore and facing the east, was a curious little stone, erected it is said, by Captain Ross, of the ‘Erebus’ and ‘Terror’ Antarctic expedition, during the interval between the visits to the south polar regions. It bears the following inscription:-

On the rock fronting this stone a line, denoting the height of the tide, was struck on the 1st July, 1841. Mean time, 2.44 p.m. Moon's age, 12 days. Height of water in the tide gauge, 6 ft 1 in.

Both quotes from the tablet are presented above exactly as published, word for word, comma for comma. The differences between the two versions are clearly evident. The grammar is different (Mason's "now struck" versus the yachtsman's "was struck"), the time formatting is different (Mason's "4h. 44m. p.m." versus the yachtsman's "2.44 p.m.") and as is now obvious, the big 2-hour disagreement over the time. Lempriere's tide gauge data for 1841 and 1842, has only recently been discovered in the Royal Society archives in London, and here is a section from his tide log in his own handwriting for July 1841, including July 1st. [27].

Figure 21: Lempriere's tide entries for July 1841. Note the similarity of 5's and 6's.

From the data shown here for July 1st 1841, low tide was at 11.28 a.m. with a height of 3 ft 10 in. High tide was at 5.58 pm with a height of 6 ft 4 in. It was sometime between these two tides that the benchmark was struck.

We can infer that mid-tide for that day occurred at exactly 2.43 p.m. with a tide height of 5 ft 1 in. These are the exact mid-points between the two times and the two tide heights logged. The reported tide gauge height of 6 ft. 1 in. is only 3 inches short of high tide and could not possibly represent MSL. However, 5ft 1in certainly could be Ross' "zero point of the sea", as this height was not only the mid-tide height for that day, but is also the MSL average for the preceding month of June just prior to the striking of the mark on July 1st. If the yachtsman was right about the time, he merely misread the "6’"' in the "6 ft 1 in" entry on an eroded tablet when it should have been a "5" to give 5 ft 1 in.

Mistaking a 6 for a 5 and vice-versa is a common mistake we all make even today, especially when reading poorly reproduced photocopies and faxes. A 50-year old tablet exposed to the elements would be just as difficult to read as any bad fax copy. In the unlikely event of Mr Mason being right about the time being 4.44 p.m. that would have coincided with a tide of 6 ft 2 in, not 6 ft 1 in. - close, but not exact.

Figure 22: High and Low tides for Port Arthur in June/July 1841, with a two-tide smoothing (in red).

This chart of the tides in Port Arthur for the month preceding July 1st shows that 5ft 1in was clearly the closest height to MSL for that month, 6ft 1in being outside the range of possible estimates.

Four factors point to the yachtsman's time of 2.44 p.m. being correct, with a consequent height of 5 ft. 1 in, instead of the 6 ft. 1 in. he actually reported.

1) "2.44 p.m." reported by the yachtsman is formatted exactly as Lempriere does it in the above log. Mr Mason says it is "4h. 44m. p.m.", not the formatting style used by Lempriere at all.

2) Mason reported difficulty reading the tablet due to erosion. Erosion could easily make a 5 look like a 6 to both observers.

3) Mason's time of 4.44 pm was local mean time, equivalent to 4.55 p.m. Australian Eastern Standard Time. Being only nine days past the winter solstice deep in the southern hemisphere, the sun had already set, leaving the Isle in twilight, hardly suitable conditions for survey work. The yachtsman's time of 2.44 p.m. was mid-afternoon in bright sunlight (confirmed from Lempriere's meteorological log), conditions much better suited to such a precision task.

4) The tide height of 5 ft. 1 in. is not only the height of mean tide for that day, but is also the mean sea level one could calculate for the entire month as shown in the graph. It is also the exact height consistent with the time of 2.44 p.m. reported by the yachtsman.

There was clearly no point in striking a benchmark at 6 ft. 1 in. when it does not mark mean sea level (as insisted upon by Ross several times), does not mark high water (which occurred over an hour later), does not mark low water, and indeed does not mark anything at all except an arbitrary height on a tide gauge which no longer exists. A benchmark which depends on the continued existence of a local tide gauge for its meaning, is not really benchmarking anything.

Since high or low tide is specific only to the location in which it occurs, only a mean sea level mark, having wider global implications, would have interested Ross, given what he said about Baron Von Humboldt's proposal. Ross and the colonial governor, Sir John Franklin, were both keenly interested in this benchmark precisely because of Humboldt's proposal, and it is inconceivable that the mark would be struck anywhere else other than "zero point" or "mean sea level" as stated very explicitly by Ross several times. Anything less would hardly have rated a mention in his book about his Antarctic voyages.

The conclusion to be drawn from the body of evidence is that the yachtsman got the time right, and that the only possible associated tide height for that time, Ross' "zero point", must therefore be 5ft 1in.

Even allowing for the CSIRO's provisional mid-range GPS estimate of 9.7 cm for land uplift over the 159-year period, there is still have an apparent fall in sea level of just over 25 cm to be accounted for (35 - 9.7 = 25.3 cm). Since most tide gauges record variations in sea level of that magnitude over months and years, as is clearly demonstrated in the tide gauge graphs in the main article, there may not really be any need to account for it at all. It is clear that a general fall in sea level did not happen in the 20th century as other tide gauges around the world (which became more numerous in the 20th century than in the 19th), would have recorded it, so we must look to the 19th century for an answer to the Isle of the Dead enigma.

Mr Mason who read the tablet with some difficulty in 1888 was acting for a marine researcher, Capt. Shortt, who was investigating the origins of the benchmark and searching archives in Hobart and Port Arthur for information about it, and without the benefit of Lempriere's tide data. Shortt reported his findings in a short (sic.) paper published by the Royal Society in Hobart [39].

In order to measure sea level under similar conditions which existed on July 1st 1841, Shortt compared MSL and the benchmark height when the age of the moon was 12 days, as cited on the tablet. His conclusion, in 1888, was that the benchmark was 34 cm above mean sea level, only a centimeter less than the CSIRO estimate of 35 cm 112 years later. Shortt also noted that -

...it is interesting to place on record, that Captain Miles has learnt from the half-casts in the Furneaux Group they have noticed within the last few years that there seems to be less depth of water over certain well-known rocks near the islands than formerly.

The Furneaux Islands are half way between Tasmania and the mainland at the eastern end of Bass Strait. Descendants of Tasmanian aborigines were resettled there during the 19th century by the colonial authorities. A general sea level fall at Port Arthur during the 19th century would certainly have been noticed there also as the aborigines frequently engaged in inshore fishing.

Further evidence for a sea level fall comes from period paintings of Port Arthur, all of which show a higher shoreline in the colony precincts than exists today. Indeed, large wooden vessels were unloading cargoes right inside the cove where Port Arthur sits, but today there is insufficient depth of water to do that.

In 1985, Hamon used "hindcasting" of the Hobart data to predict, retrospectively, the state of the tide back in 1841 [22]. Hamon's hindcasting matches Lempriere's data closely, so it can be safely assumed that Lempriere's tide gauge and his data log are also accurate.

Drs John Hunter (CSIRO), David Pugh (Univ. of Southampton, UK) and Richard Coleman (Univ. of Tasmania) jointly authored a confidential internal report for the CSIRO [24], but have not published anything in the scientific literature. Their recent conclusions, reported in this article, comes from a lecture they jointly delivered in April 2000 to a small audience at the Royal Society in Hobart [12], a lecture I also attended.

Surprisingly, the Ross written account about the benchmark was not raised once by either of the three speakers. They merely mentioned him as having an idea for a `tide' mark (as distinct from a `sea level' mark) without informing the audience exactly what he had written. When relating the details about the tablet, they informed the audience about Mr Mason's version, but not a word about the yachtsman's version, even though they are familiar with both.

When earlier questioned by the Melbourne `Herald-Sun' on the issue [4], Hunter claimed that Ross must have "mis-remembered", and in fact carved the mark above mean tide to show where he'd measured the tide in a tide gauge. This scenario would render the whole exercise pointless and contrary to Humboldt's proposal and contrary to the Governor's instructions. When questioned by the BBC, [2] Pugh said "From all the evidence we know, it was the high water level at that time". He obviously did not regard Ross' own words on the matter as constituting any part of "all the evidence".

Ross was an early pioneer of sea level research, being the first scientist to be credited with calculating a correction factor for sea levels due to variations in atmospheric pressure [32]. Thus he was no mere amateur at the science of sea levels as Hunter and Pugh seem to imply.

In keeping with the lack of publication in scientific journals about the benchmark, no written paper was offered at the lecture, the information given here being based on notes taken by myself. Without publication in the scientific literature, the IPCC can choose to ignore even of the existence of the benchmark, let alone integrate it into their assessments of past and present sea level change.