In 1841, a mark was cut into the rocks of the Isle of the Dead at Port Arthur, in an attempt to record the height of the sea in the area, and to provide a benchmark for future studies of the movements of the Earth's crust, relative to sea-level. It was made at the instigation of Captain Sir James Clark Ross, with the support of Thomas Lempriere (Deputy Assistant Commissary General at the Port Arthur penal settlement). Records indicate Lempriere had studied the tidal levels in the area for several years before the mark was made.

The mark at Port Arthur is among the earliest benchmarks in the world against which to scientifically measure changes in sea-level. Until recently, a lack of actual data from the time prevented a proper understanding of the site. However, some of the original data has been found, and together with some recent detailed monitoring at Port Arthur, a clearer understanding of sea-level changes in the area are now possible. It also aids in the understanding of global sea-level changes, as there are very few good long-term benchmarks in the southern hemisphere.

Setting the site in stone

Thomas Lempriere was a keen observer of his surroundings, and kept records of a variety of features, including meteorological conditions such as the wind, rainfall and temperature. He also operated a tide gauge, though the exact details of where and how the gauge worked are not known.

When Captain Ross visited Tasmania in 1841 during his voyages to Antarctica and the Southern Ocean, he visited Lempriere at Port Arthur. Ross was keen to have a sea-level benchmark installed in an area free from large wind and river flow influences, and Port Arthur appeared to be an ideal location. Lempriere's monitoring of the tides at the site added to its value, contributing several years' of important data. Together, they provide a baseline of great value to the contemporary science of sea-level and climate change.

On 1 July 1841, Ross and Lempriere had a standard survey mark cut into a sandstone cliff on the Isle of the Dead. The mark is a horizontal line with a broad arrow touching and pointing down at the horizontal line. A plaque was installed above the mark, but unfortunately it has not survived.

Ross' journal of the event is confusing. It is not clear whether the mark was made of the mean sea-level, or high water. Ross did make two more marks on the Falkland Island on the same voyage, and these were both above mean sea-level.

A paper published in 1889 by Captain Shortt recorded the wording of the plaque, including the time the mark was struck and the height of the sea given by Lempriere's tide gauge. By taking a measurement of the height of the sea, and estimating what the tides were when the mark was made, Shortt determined that the mark was made near high water.

An article in The Australasian in 1892 also recorded the wording of the plaque. While almost the same as the version published in Shortt's paper, it differed in the time the mark was supposed to have been made, although both reports were consistent regarding the reading of Lempriere's tide gauge when the mark was struck. Taken on its own, the reported time of the striking would suggest that the mark was originally near mean sea-level.

Significant work has gone into determining which of the accounts is correct, including a current major study by a collaboration of international scientists, as knowing whether the mark was originally placed near mean sea-level or high water is crucial to being able to compare sea-levels of 1841 with today. This study has concluded that it is almost certain that the benchmark was originally placed near high water. The conclusion is based on other estimates of sea-level made later in the 19th century, and on the fact that, if the mark had originally been placed near mean sea-level, then the Penitentiary building would have suffered flooding every few years (there is no record of this having happened).

The old and the new

It was thought that Lempriere's original data had been entirely lost, making it very difficult to understand how the sea-level in 1841 related to the sea-level of today.

However in late 1995, Dr David Pugh of the Southampton Oceanography Centre, found Lempriere's data for 1841 and 1842 in the archives of the Royal Society in London. In mid-1998 Dr John Hunter, then of CSIRO's Division of Oceanography, found data for December 1839 and February 1840 to January 1841 in the Australian Archives at Rosny.

In 1999, the universities of Canberra and Tasmania, and CSIRO, set up a sea-level monitoring station at Port Arthur. Referencing this data with the benchmark on the Isle of the Dead, and utilising Lempriere's original data, has enabled the first comprehensive study that compares the Port Arthur sea-level of 1841 with that of today.

It is unfortunate that a continuous record of data has not been collected from Port Arthur, but having such an early benchmark to compare to is still of great importance.

Global importance

The Port Arthur site is particularly important because of the few long-term sea-level records in Australia and the southern hemisphere.

Records spanning many decades are considered necessary before long-term trends can actually be identified. Some highly accurate monitoring stations have been established in Australia during the last 20 years, but it will be some time before these show clear long-term trends. There are also a number of Australian ports that have reasonably long series of data, but still do not go back as far as 1841.

Two of the longest continuous Australian tide gauge records are from Fremantle in Western Australia (92 years) and Fort Denison in New South Wales (83 years) indicate that the observed rate of sea-level rise relative to the land has been 1.38 mm/yr and 0.86 mm/yr respectively (see Internal linkDPIWE 2004).

As the earliest sea-level benchmark in Australia, and possibly the southern hemisphere, the Port Arthur benchmark gives a great opportunity for assessing long-term sea-level change in this region and contribute to the understanding of global sea-level rise.

Dramatic changes in sea-level are possible during the next hundred years as the release of greenhouse gases enhance the Earth's natural greenhouse effect. While there may be some extra water added to the oceans by melting of ice caps and glaciers, much of the initial sea-level rise will be caused by the oceans expanding as they warm up (thermal expansion).

Sea-level projections for the 21st century from the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) of 2001 and the Fourth Assessment Report (AR4) of 2007 identify that observed sea-level is currently tracking near the upper limit of the IPCC model projections from the start date of the projections in 1990 (see Internal linkRahmstorf et al. 2007; Internal linkIPCC 2001; Internal linkIPCC 2007). The IPCC TAR Report model projections estimated a sea-level rise of between 9–88 cm by 2100. The IPCC AR4 model projections (with a 90% confidence range) revised this estimate and calculated a sea-level rise of between 18–59 cm by 2095 plus an allowance of another 10–20 cm for a potential dynamic response related to the melting of land ice in Antarctica and Greenland (see also Internal linkChurch et al. 2008; Internal linkIPCC 2007; Internal linkChurch et al. 2006; Internal linkEarth Observatory 2006). However, more recent published work suggests that a rise of up to 2 m is possible (Internal linkACE CRC 2008) and it could be more than 5 m by 2100 (Internal linkHansen 2007). CSIRO promotes the range 18–79 cm for Australia (Internal linkCSIRO 2009). Even with a rise of mean sea level of approximately 50 cm, it is predicated that events which now happen every few years could occur every few days in 2100 and that larger increases in the frequency of extremes could occur in Bass Strait, along the coastline of Western Australia and capital cities such as Sydney, Brisbane and Hobart (Internal linkHunter 2007).

Already the Port Arthur benchmark is showing a rise in sea-levels of at least 13 cm since 1841, with an average annual rate of 0.8 mm/yr ± 0.2 mm/yr relative to the land in the southeast of Tasmania during the period 1841 to 2002 (Internal linkPugh et al. 2002; Internal linkHunter et al. 2003). Hence the site will be an important benchmark to continue measuring the changes in the average level of the sea.

This discussion was compiled from a number of studies and articles including: Department of Primary Industries, Water and Environment 2004; Hunter, Coleman and Pugh 2003; Pugh, Coleman and Hunter 2002; Bowden, Houghton and Ding 2001; and Hunter and Pugh 1997. John Hunter (Antarctic Climate and Ecosystems Cooperative Research Centre) in particular provided valuable advice.