A little detective work on tidal gauges, and of course, on sea-level rise

The question of sea-level rise continues to fascinate me, mostly because what we are taking about here are very small changes in the relative height of seawater, measured by satellites from hundreds of kilometres away and by inspection of tidal gauges. I’m not the only one to be fascinated. Philip Muscatello, a reader and occasional commentator, shares my interest, and thought he would do something about it. He sent his material to me, and it forms the basis of this essay. A few years ago he noticed tide gauges dotted along the foreshores and bays of Sydney Harbour. They seemed to have year markings on them: 1920, 2006 and 2050, and he wrote  the accompanying signs show warnings of imminent sea level rise caused by man-made global warming. They encourage recycling and cycling. I suppose these activities are meant to keep the water level within manageable levels.

A king tide was forecast for the night of July 13, 2014, and Philip decided to go the nearest such gauge and see what the apparent height of that tide was.  The water level was 9cm above the 2006 marker and 15cm below the 2050 marker. The gauges seemed to demonstrate a clear rise between 2006 and … 2014 of about 1.125cm per year — an enormous acceleration of sea level rise. How could this be, he wondered, and set off to find out how these gauges were calibrated, and what they were supposed to be showing.

It turned out that the gauges had been put there at the request of local government councils in Sydney, and in the case of Ashfield Council, on the motion of three Green councillors, who in 2007 had argued like this: It is a undisputed scientific fact that burning fossil fuels such as coal, gas and oil and clearing forests have dramatically increased the amount of carbon dioxide in the Earth’s atmosphere and hence, temperatures are rising. The majority of scientists agree that global warming is real, is already occurring and it is the result of human activities and not a natural occurrence.We’re already seeing changes. Glaciers are melting, animal and plants are presently being forced from their environments, and the number of droughts and storms is escalating. Global sea levels could rise by more than 6 metres (20 feet) with the loss of shelf ice in Antartica and Greenland, overwhelming coastal areas globally.

In our own small way, Ashfield Council will be significantly touched by rising sea levels especially at its coastal fringes along the Parramatta River. I’ll come back to this ‘information’ later.

The Green councillors therefore moved that:

1. Ashfield Council recognises the actual and potential consequences of rising sea levels caused by climate change.

2. Ashfield Council installs tide line marker/s displaying summer and winter solstice tide lines in strategic points of the Iron Cove Bay, Hawthorne Canal and Dubroyde Canal;

3. Ashfield Council install signage beside these tide markers specifically aimed at raising public awareness about climate change and the potential for rising global sea levels.

4. that this signage will explain how the line will continually rise if we do not seriously reduce our carbon emissions. This tide marker will also show where the line was in 1950, and where it could be in 2100 with indicators at ground level showing where the shoreline could potentially end up.

5. That signage will also have dot points on what councils are doing to address the impact of climate change, and how we can all reduce CO2 emissions

But who decided what those levels were, or should be? Philip discovered that Leichhardt Council had been similarly approached, and had decided further as follows: The heights proposed are based on the highest king tide level for 2006 which was 2.01m on 31/1/06. 1920 levels are based on subtracting an average 1.2mm rise in sea level for each year (as determined by CSIRO) from the 2006 level i.e. 1.91m. The 2050 level of 2.41m is 400mm higher than the current level, and is a predicted worst case scenario if no mitigation action is taken.

To which one can only say Wow! Why didn’t the Council project forwards at the rate of 1.2mm as well? Well, see above, for what those Green councillors asserted were ‘the undisputed scientific facts’. Philip, now alert to the need for exact measurements, discovered that the king tide in 1920 was 2.050 metres, compared to the 2.078 metres of 2006, which is a tiny difference. Because the Leichhardt Council minutes specified that the tidal marker had to be placed in a way that satisfied the Maritime authority Philip rang it up. No one there had been contacted by any council about placing or calibrating any such gauges.

Philip placed all his data on the table of a friendly statistician, who said that the longer the time period was the more confidence one could have in the rate of average tidal increase. Fort Denison is the standard tidal gauge for Sydney Harbour, and after looking at the rises and falls over time there, he provided  the following: … the straight line fit is the most useful way of looking at it. The equation in metres is y = 0.0000779017x + 0.8836422580, which means that mean sea level increased by 0.0779 mm per month or 0.935 mm per year. So over 100 years, mean sea level increased by 93.5 mm or 9.35cm.

I’ve used this figure before, but it is nice to have someone else independently derive it. There is a pleasant end to this story. All but one of those gauges have since disappeared, including the one he photographed (which I was unable to download). But how were they placed, with what scientific or practical acumen? And who removed them? Philip says, correctly, that  this is not the greatest example of environmental exaggeration ever revealed, and that several thousand dollars of of council expenditure is small beans. However, it goes to show that the argument may be made at many levels and that catastrophists need to be called out at every opportunity.

But it’s difficult to call them out, isn’t it. I can just imagine the Council meeting where the three Green councillors came up with their dire warnings, and called for tide gauges that would show what would happen if we didn’t all do the right thing. Rather than have a stand-up shouting match, other Councillors were likely to have shrugged and let it pass. It’s not going to cost a lot of money, they would have thought. Let’s get to the more important items. Anyway, if we don’t we’ll have the Greens yakking at us about the future. And so on. The Council staff, having been told to do it, read the minutes, made up the signs and placed them where it was easy to do so. The citizens noticed the signs, shrugged, and moved on.

Go back to the introductory stuff that the Green councillors put forward as the context for their motion. Yes there has been a strong increase in the amount of CO2 in the atmosphere, according to measurements and estimates. But nobody knows what a majority of scientists think, because they haven’t been asked. Temperatures have not risen in any significant way for nearly two decades now, despite the increase in CO2. Nor are storms and droughts increasing. And ‘global sea levels could rise more than 6 metres’. Really? Over what period of time?

The trouble is that the Greens are a passionate minority, and the rest of us are a tepid majority. That brings me to want to say something about the advantages of democracy, but that will have to wait for another time.