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1870 photograph helps researchers chart sea level change

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An 1870 photograph of the Royal Naval Dockyard, with a “black zone” of cyanobacteria highlighted. (Image from the Currents newsletter)

An historic photograph of Dockyard is helping researchers to gauge changing sea levels in Bermuda.

While the picture, taken in 1870, was intended to capture buildings in the West End including the Commissioner’s House, researchers noticed that it also captured a dark line representing high tide.

The January edition of the Currents newsletter, published by the Bermuda Institute of Ocean Sciences, highlighted the use of the image to measure historic sea levels on the island.

Philippe Rouja, who led the team of researchers, said: “Sea-level rise is not a mystery and certainly this paper does not tell us something scientists and many people don’t already know.

“That said, I think the publication is significant as an interdisciplinary piece of work that shares the less-communicated but equally important aspects of the scientific process.

“Creative happenstance, camaraderie of good teams and being with like minds can lead to discovery, innovation and invention and make doing science, even when dealing with serious subjects, quite enjoyable.”

Dr Rouja, cultural and medical anthropologist with the Department of Environment and Natural Resources and adjunct faculty member at BIOS, teamed up with a number of other researchers to find historical records of sea-level changes.

During their search, the Bermuda Archives found a high-quality military photograph that captured the south-facing dock wall, which is still in existence at the site owing to its construction from dense Bermuda limestone and conservation efforts to protect the historic site.

While the researchers did not know what the tide level was at the time of the photograph, they were able to identify the “black zone” stained by cyanobacteria, which peaks just above the mean high-water mark.

In 2007, Dr Rouja was able to confirm that while the wall is still marked by the black-zone, it was about one block higher than it was in the 1870 photograph.

They estimated that the black zone had risen between 33 and 41cm since the original photograph was taken.

Photograph indicating the suspected upper movement of the “black zone” covered with cyanobacteria. (Image from the Currents newsletter)

While the upper end of the estimation was close to the average annual sea-level rise of 3.1mm calculated by the Intergovernmental Panel on Climate Change, at the low end it was similar to the 2.17mm annual change measured by the St George’s tide gauge installed in 1932.

In 2017, the team decided to utilise new technology to create a “digital twin” of the seawall to allow for relative measurements to be made to further gauge sea change levels at Dockyard.

Dr Rouja said: “It was only because we were digitising shipwrecks with Falko Kuester and his team at the University of California San Diego that the idea came up of modelling the wall and creating a modern digital twin that could be compared almost perfectly with the historical image.”

The new, more precise approach backed up the earlier estimates, finding the black zone was 31.2cm higher in 2007 than in 1870 — an average rise of 2.28mm per year.

However, from 2007 to 2017, the black zone had risen by another 8.8cm — roughly 305 per cent more than the average annual sea rise level as recorded in St George’s.

The report said the difference may be due to “local impact factors” such as sea floor topography or ocean circulation.

It also noted that the 2017 line mirrors the highest spring tide recorded in Bermuda, suggesting that while in the long term the black zone shows the average change in sea level, in the short term it shows how high the tide rises above the average sea level.

Dr Rouja said: “We worry about averages over the long term but increases in amplitude, or dramatically higher tides, can impact us much sooner than the average change might suggest.”

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Published February 03, 2023 at 7:53 am (Updated February 03, 2023 at 7:53 am)

1870 photograph helps researchers chart sea level change

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