To understand Perth sea-levels is a complex issue – much of the Perth plain has subsided in recent decades

Recent media publicity around a “State of Australian Cities” report has beaten up claims that Perth sea-levels are rising at three times the global average, or near 10mm per year. A matter that Federal Infrastructure Minister Anthony Albanese described as “disturbing and extraordinary”. Chris Gillham shows that the claims in the State of Australian Cities report re Perth sea level rise are over-hyped.
This chart fom a 2010 WA Govt planning report shows long term sea level changes at Fremantle.

The State of Australian Cities report concentrates on shorter term more sensational trends in the last 20 years but omits to quote recent research measuring rapid subsidence of large areas of the Perth coastal plain.
A 2012 paper – Anthropogenic land subsidence in the Perth Basin: challenges for its retrospective geodetic detection – says subsidence at Gnangara for the 14 years 1997-2011 has averaged 4.6mm per year – see Table 1 below.

In recent decades subsidence has been worsened by increased drawdown ground water for Perth water supplies – a process which has eased post 2005.

This online chart shows CGPS readings from the Hillarys tide gauge installation and indicates steady sinking of ~3mm per year 2006-2012. There are no CGPS readings I can find from Fremantle.

Hillarys has CGPS data from 1997 and Featherstone et al say those data – “…are not in a form that can be readily analysed in a short time frame, particularly because of the multiple equipment changes that make the time series very discontinuous.” Other published versions of the Hillarys CGPS data quote a 1998-2009 subsidence averaging just over 5mm per year. Watson PJ – Is There Evidence Yet of Acceleration in Mean Sea Level
Rise around Mainland Australia?
Journal of Coastal Research, Vol. 27, No. 2, 2011
Featherstone et al 2012 discuss surveying to detect level changes and say – “There is some scope for detecting subsidence in the Perth Basin if levelling is repeated now or sometime in the future. However, the costs are likely to be prohibitive.”
I fail to see how a few repeat levelling traverses from the Darling Ranges to the coast costing what ? – a few $million ? – should be beyond the wealth of a First World nation like Australia. Apparently some traverses were carried out in the 1980’s.
There is no shortage of Govt grant monies to waste on shonky Greenhouse inspired “research”.
Tide gauges and harbours are often located in places where the crust is sinking due to say sedimentation from a river. Any harbour structure attached to piles will slowly sink into the muddy substrate over the decades – factors like traffic vibration, a century of ships banging into piles – all can contribute to piles sinking. Dock areas where tide gauges tend to be installed are often the sites of reclaimed land, Auckland is an example. This tends to quietly settle for decades which can confound tide gauge data. At Fremantle 150 years of urbanization – including storm-water drainage direct to the sea – can lower ground water levels and cause subsidence – quite apart from deliberate ground water harvesting.

16 thoughts on “To understand Perth sea-levels is a complex issue – much of the Perth plain has subsided in recent decades”

    In that sea levels relate to climate change, understanding them requires understanding the physics behind such climate change.

    It is still obvious that the 13-month running average in Roy Spencer’s November plot has been declining since 1998. This is totally as would be expected due to a roughly sinusoidal superimposed 60 year natural cycle, for which there is now compelling evidence. See for example, the linked references to such in my current paper about planetary surface temperatures, which is on the PROM* system at PSI for a month or so.

    When you remove the effect of the 60 year cycle (with, for example, a trend for a 60 year running average) you get down to analysing the underlying long term trend which has periodicity of about 1,000 years, maybe a bit longer. This was the cause of fairly regular warming periods observed for at least the last 7,000 years, the most recent being the Roman and the Medieval W.P. which have both now been confirmed to have been worldwide and at similar temperatures to the present.

    There is however still a slight incline in this long term natural cycle. About 100 years ago the mean rate of increase was around 0.06 C/decade, whereas in recent times it has declined, but only to about 0.05 C/decade. If it is also roughly sinusoidal we should see a maximum in about 200 years, probably less than 0.8 degree above the current trend. But of course, after that there would be 500 to 600 years of long term cooling, even though the superimposed 60 year cycle will continue to cause some alarm each time it rises for 30 years, as happened from around 1970.

    Again, there is now compelling evidence that these natural cycles are the only “forcing” for our climate. There are links to evidence in my paper, and even to some evidence that the cycles are in some way controlled by planetary orbits, which makes sense because such orbits are the only “timing mechanisms” of such long duration in our solar system.

    The reasons why carbon dioxide has no effect are explained in a radically new way in my paper. Nowhere else have I seen the hypothesis which brings together evidence from different sources into what I consider a cogent argument for a completely different explanation of planetary surface temperatures, not to be found elsewhere to the best of my knowledge. Yes, parts of the explanation are elsewhere, but it has not hitherto been coordinated to give an explanation based on correct physics.

    For example, I contend that there is no other valid explanation for the surface temperature on the planet Venus. That surface receives less than 10% of the amount of Solar radiation which we receive on Earth’s surface. It’s not correct to assume that the CO2 atmosphere caused a massive GHE, because the surface could not have been heated in the first place to over 700K with so little energy being received through the thick and dense atmosphere. Nor was it heated by radiation from what is still an atmosphere that is at much lower temperatures, less than 230K at an altitude of 50Km, for example.

    Until people come to grips with what I believe to be the correct physical mechanism which produced (and maintains) the temperature of the Venus surface, they will never correctly understand what is the same process working on Earth, Jupiter, Saturn, Uranus and Neptune – all the planets in our solar system with qualifying atmospheres.

    As I have said, the paper is up for worldwide open review on the PROM* system at Principia Scientific International. It has already been reviewed by several of our 150 members, but if you wish to submit any comments, criticism, rebuttal or support, you may do so through our CEO John O’Sullivan or our Chairman, Dr Timothy Ball, a retired professor of climatology. You may also contact me via the email address on my website which opens when you click my name above.

    However, I will only respond to those who have clearly read and understood the whole paper, whether or not they agree with the conclusions reached.

    (*Peer Review in Open Media.)

    Doug Cotton

  2. OT

    Record rainfall in the southwest of WA and it is still raining around Collie as of 9am WST. Maybe the Water Corporation will stop running those irritating ‘our drying climate’ ads.

    I understand the 170mm at Yourdamung Lake is the highest 1 day rainfall ever recorded in the south of WA.
    Ed note: I wonder what the years of “our drying climate” propaganda has cost taxpayers.
    I see on the radar now – 1pm EST – there looks to be a low in the Gingin – Lancelin area which has rain patches sweeping from the east across points south.
    Long may it carry on. I will be watching to see if there is enough stream flow to register an uptick in dam levels.

  3. I’m not sure drawdown of water from the shallow Perth aquifer is causing subsidence. Where I used to live, we had one of the few creeks on the Perth sand plain out the back. It ran all year and 3 or 4 months without rain had little effect on the water level so clearly it was fed by groundwater. I know a bit about riverine geomorphology and I’d say the water levels of that creek had changed much in decades or longer.

    More likely IMO is the tectonic subsidence that formed the Perth basin still continues.

  4. Thanks for posting on Perth sea levels, Warwick. You’ve provided a link but if I might summarise progress of the Perth myth so far …

    On 5 December The West Australian newspaper published a page 3 story claiming Perth coastal sea levels had been rising up to 10mm per year since 1993, three times the global average. This story was propagated by other media and the WA public now mostly believe that a sea level disaster is looming.

    On 7 December I uploaded which exposes some facts.

    Yesterday, 12 December, The West Australian published a page 6 story ( which only partly repairs the damage caused by its 5 December story.

    “Sea levels in Perth appear to be rising faster than elsewhere because the city’s heavy reliance on groundwater is causing it to sink, scientists believe.
    Just days after a Federal Government report claimed Perth’s sea levels had risen at three times the global average, prominent research and scientific institutions pointed the finger at the city’s thirstiness.”

    Perth sea level records agree with the recent Morner paper ( and if confirmed land subsidence is factored in, suggest levels have either been flat or possibly even fallen since 1993.

    The CGPS chart for Hillarys adds to the sea/land formula and suggests that if Hillarys soon finds itself underwater, it’ll be because of land subsidence rather than rising seas. The Fremantle tide gauge is about 30k south of Hillarys and south-east rather than east of the Gnangara mound, so the impact of that subsiding water table might not be corrupting Freo’s gauge readings as much. My calculation of averages suggests a 2.2mm per year rise at Hillarys since 1993 and .61mm per year at Fremantle, which correlates better with uneven land subsidence than sea levels that defy the laws of nature by balancing at different levels.

    I’m holding my breath wondering if any media will now point out that it’s ridiculous to compare just the year 1993, Fremantle’s lowest annual sea level since 1942, with 2010. To then suggest Perth sea levels are rising up to 10mm per year is either intended deception or blind ignorance.

    This episode raises various questions:

    • Why does a Federal department publish claims about sea level that are demonstrably false?
    • Why does a Federal Cabinet minister in a government committed to climate change believe those claims?
    • Why does the media cherry pick a report unrelated to climate change and sensationalise a foreboding claim without basic journalistic research?
    • In light of apparent government acceptance of these claims, are they the basis of coastal planning policies that devalue billions of dollars in coastal property values on the predication that sea levels are rising?

  5. Thanks Chris – you put things a bit better than I do.
    Today I heard from the people who run the network of CGPS geodesy stations –
    and the message was that from the 2006 antenna change Hillarys has been subsiding at ~3mm p yr.
    So my eyeball estimate was understated by 100%.
    But from 1997 to 2005 the subsidence was ~6mm p yr.
    So that just makes even more amazing any Govt people quoting the Hillarys tide gauge 1993-2011 change of 9.5mm p yr WITHOUT balancing that by stating the evidence that the installation has been subsiding over that period. Utterly scandalous in my view. But that’s Govt for you. Particularly Green/Labor.

  6. I estimate the Hillarys tide gauge registered an average 2.2mm rise per year from 1993 to 2010 so 3-6mm annual land subsidence certainly raises questions about the validity of Perth’s sea level records.

    Perth’s leading suburban newspaper declined to publish a letter I sent several days ago detailing the obvious errors in the claimed 10mm per year rise in sea levels, despite their quoting it last week in a coastal planning story. The West Australian newspaper has also fallen asleep since its p6 repair job on Wednesday.

    It’s just a small matter of clarifying an issue that supposedly threatens to drown everybody’s grandchildren and influence billions of dollars in coastal development. I suppose there are more important things to report and it’s not a problem if the public now believe local sea levels are rising at three times the global average.

    Just for laughs and to compare scales, I’ve added to my page at a couple of charts that fell off the back of the IPCC truck – one of global sea levels and the other of the Fremantle tide gauge, which is a major southern hemisphere contributor to the global sea level average because of its longevity. Spooky IPCC sea level forecast for Freo to the year 2100 but they’d better hope the land keeps sinking and the sun warms up.

    I’ve also added a chart comparing the close correlation between global temps and the two tide gauges since 1990, based on the IPCC draft chart which confirms flat temps since the 1990s. They’re not bad thermometers, albeit tending to lag a year. Judging by the 2011/12 temps and our current chilled-out sun, I don’t think you’ll need scuba gear to enjoy a coffee in Fremantle for a few years yet.

  7. Like most times, the detailed understanding of what is happening to levels of this and that around Perth is not easy because so many factors enter the equations and there is not enough evidence. For example, there seems to be a lot of sand on the plains, we can all see it, but the rate of both lateral and vertical movement of groundwater cannot be assumed to follow a simple path. I’ve not drilled the plains themselves, but one can get matters like calcification that can cause perched water tables that can be tight or leaky …. There is so much that we do not know. I’ll give an example from Darwin. At a large geology congress in the 1980s there was a paper with estimations of the formation time of the pisolites that littler the surface some of the soil profile of much of the top end. A very long age was deduced, like millions of years, I forget the figure. The permeability of soil is affected by the formation of these pisolites, so it was all interesting until a colleague noted that pisolites had formed on surplus WWII military hardware pushed off the cliffs near Fanny Bay. Both explanations, the slow way to form and the fast way, might be correct in the circumstances, but we just do not know. It would be a reasonable assumption that aquifer drawdown would lower the land surface, but the math?? Who knows? It’s also reasonable that the land west of the escarpment is still falling wrt a global datum. There’s no rule that says it should not drop. Look at the 2 metre change at Meckering in 45 seconds of earthquake in 1968, in what was thought by many to be amongst the most stable parts of the world.
    So this is merely a rambling cautionary note that one should not go too far out on the explanatory limb when dealing with aquifers. Some positives, however, are that Prof Featherstone from Curtin (who is mentioned above) has the highest of repute; and that the more accepted rates of change of soil surface and water table relative to the sea are tiny, with plenty of time for new measurements that might even show virtually no change. A problem at present is the fixing of a datum point that is considered free of error. At the moment, globally, JPL guys say the datum is a virtual space defined by the locations of a number of satellites. Normal scientific progress is refining these, maybe faster than the land is deflating at Uni of WA, Crawley, 6009.

  8. One point about surface drainage in Perth is most storm drains go to soaks. Where I live a couple of Ks south of the CBD, there is a soak on every block. Most are dry 99.9% of the time because storm water just sinks straight into the sand.

    I think its likely that urbanization has probably heightened the water table, because rainfall that would have been captured by trees, etc is now channeled straight into the ground.

  9. I agree there are places in Perth that have recharge basins but –
    I am sure I have seen quoted a figure of 150GL PA storm-water captured and drained to sea.

  10. Warwick, then you have to include the mains water used on gardens, 30.5% of the total from Water Corp numbers, just over 100 million cubic meters per annum, or a 100 gigaliters pa. Plus leaks probably add at least 5% more.

    Granted 35% to 50% of the mains water comes from groundwater, but those sources are well north and south of Freemantle.

    Figure A in the link below is a map of drains in Perth. The primary function of the main drains appears to be managing the water level of the lakes (which are groundwater fed), ie preventing flooding.

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