Ancient Tasmanian tidemark helps scientists track sea levels

Australian scientists have gained new insights into rising sea levels thanks to a benchmark carved by an amateur meteorologist 160 years ago in cliffs near the infamous convict settlement of Port Arthur.

Australian scientists have gained new insights into rising sea levels thanks to a benchmark carved by an amateur meteorologist 160 years ago in cliffs near the infamous convict settlement of Port Arthur.

The benchmark, the oldest in the southern hemisphere and one of the earliest anywhere in the world, was made by Thomas Lempriere, a senior officer at the penal colony in south-eastern Tasmania.

It was gouged out of a sandstone rock face on the Isle of the Dead, off Port Arthur, where convicts were buried seven deep in mainly unmarked graves. The discovery of the carving at this lonely spot, looking out across the Southern Ocean towards Antarctica, was greeted with excitement by oceanographers.

But the benchmark had no scientific value without accompanying tidal records, and they were lost. Then, by a stroke of luck, Lempriere's original data, logged over several years, was found in the Royal Society's archives in London. The records, compiled long before the issue of global warming reared its head, enabled marine scientists from the University of Tasmania to compare sea levels from the mid-19th century with present-day levels. Their research, to be published next month, established that the waters off Port Arthur have risen by an average of 1mm a year since 1841.

They believe that the increase was minimal in the early decades, and that it jumped to 1.4mm a year in the 20th century thanks to global warming caused by carbon dioxide emissions. That figure conforms with records kept later in Sydney and in Fremantle, Western Australia.

Dr John Hunter, a British-born oceanographer who led the research, said the results were consistent with those of similar studies elsewhere. He said they were an important contribution to knowledge on past sea levels because there was a dearth of historic data, particularly from the southern hemisphere.

"People all over the world are trying to estimate sea level rise over the last century, but there have been very few observations in this region," he said. "This is another piece of the jigsaw." Dr Hunter took readings for three years after installing a tide gauge on the jetty at Port Arthur, where the ruins of the convict settlement are one of Tasmania's biggest tourist attractions.

A British tidal expert, David Pugh, from the Southampton Oceanography Centre, also took part in the study. He found Lempriere's data at the Royal Society. The documents had been feared burnt by a family member. Lempriere was the Deputy Assistant Commissary General at Port Arthur. His duties were apparently not arduous and he had plenty of time to pursue his other interests, which included painting, natural history, the weather and tides.

The roughly hewn benchmark – a horizontal line surmounted by a broad arrow – was made by Lempriere at the suggestion of Sir James Clark Ross, the British explorer who visited Hobart during an Antarctic expedition in 1841.

European scientists were not interested in climate change; they wanted to test the theory, fashionable at the time, that the continents were moving up and down. They knew that the ocean marker needed to be backed up by sea-level measurements. Lempriere set up a rudimentary tide gauge at Port Arthur and conscientiously recorded water levels, sending the results to the Admiralty in London. All the evidence suggests that his efforts were disregarded.

Dr Hunter said there was no indication that Lempriere was taken seriously or his data ever put to practical use. "Maybe he was regarded as unreliable because he was not a naval officer," he said.

"We know that he wrote to the Admiralty, asking if the data had been of use, but there is no record of any response. Soon afterwards he stopped recording the measurements. It looks like he gave up," Dr Hunter added.