Sea Level Change
QUESTION
I have been a ships super cargo and agent for sixty years, what is the story on sea level changes.
As i need sea levels for our ships, we have many hundreds of them, to dock and discharge our D Class ships were once aloud into Port Pirie.
South Australia to load zinc, loaded draft for departure was nine meters, we are not allowed to go in with these ships for some fifteen years now.
As Flinders Ports claim there is not enough water. And it’s not the seabed as it is limestone.
As i am involved with tides and sea levels of many ports around the world, I find in this time some sixty years the sea levels haven’t changed.
A great example the old slack tide sea level mark in Port Arthur Tasmania put there one hundred and eighty years ago shows very little sea level change????
ANSWER 1
Written response:
The short story is that global mean sea level is rising. Since 1880, global sea level has risen more than 20 cm (https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level). About one third of this rise is due to the ocean expanding as it warms, and about two thirds is due to water from melting glaciers and ice sheets flowing in to the ocean (https://sealevel.nasa.gov/understanding-sea-level/global-sea-level/overview/). There are also some small changes due to dams and other water storage on land.
However, this headline figure hides lots of important details and complexity. Concerningly, sea level rise is accelerating; between 2006 and 2015, global sea level rose 2.5 times faster than the rate of rise in the 20th Century.
It’s important to remember that we don’t really experience the global mean sea level. You already mentioned one of the most important reasons: tides. With a tidal range of \~3 m, Port Pirie is hardly the most tidal place in the world, but the daily cycle of high and low tide makes it harder to notice a 20 cm change over more than a century (https://www.bom.gov.au/australia/tides/#!/sa-port-pirie).
Another challenge is that the relative sea level change in a specific location also depends on what the land is doing. For example, some places in Scandinavia are rising up out of the ocean because the land was pushed down during the last ice age when it was covered in ice. Now that the ice is gone, the land is rebounding and the local sea level appears to be falling. In Stockholm, the sea level has fallen by about 50 cm over the last century (https://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?id=050-141).
The Port Arthur tide marker you mention shows that the local sea level has increased by more than 15 cm since 1841 when the mark was cut (https://www.abc.net.au/news/2021-12-04/sea-level-climate-change-damage-port-arthur-tasmania/100665988).
The shipping changes you describe at Port Pirie are fascinating, but I can only guess at what has happened. You say that the bottom is limestone, so silt accumulation seems unlikely. Flinders Ports may reduced their risk tolerance and decided that the larger ships should no longer be permitted despite no significant change in the physical environment. Or perhaps the modern ships are more affected by squat, thus increasing their draft while underway.
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