There are recent scary stories about sea levels rising that will sink Fremantle, so I have looked at the tidal gauge records:
Fremantle:
Tide Gauge Data
Link to larger image of monthly data plot.
Catastrophic, about 6 inches over a hundred years. Possibly sinking as the coastal plain’s aquifer is being pumped dry. So, let’s have a look at Bunbury: Oh dear, it,s worse than we thought, about 1, yes 1 inch over 100 years. Oops, no, it’s dropped, actually lower over the last 40 years.
Tide Gauge Data
Sea levels on the rise in Perth
Sea level on the rise in PerthSea levels on the Perth coastline are rising at three times the global average, the latest State of Australian Cities report shows.
In a statistic that federal Infrastructure Minister Anthony Albanese described as “disturbing” and “extraordinary”, readings since 1993 have indicated sea levels are rising by between nine and 10 millimetres per year.
The global average is around three millimetres per year.
With temperatures rising and rainfall falling, environmental changes are having little effect on the numbers of people moving to Perth, with the city population growing by 2.6 per cent since 2001 – making it the fastest growing capital in the country.
Yikes. Between 9 and 10mm a year for 20 years. That means 400mm since 1993.
Complete and utter BS
I looked at rainfall and temperature too, all gloom and doom, I have already done some fact checking of that before. Who checks this rubbish at The West?
More alarmism found in the SMH too
Don’t blame the poor old West, Tom – it’s straight reporting of a Federal Government report http://www.infrastructure.gov.au/infrastructure/mcu/soac/files/factsheets_2012/Perth_Final_Factsheet_FA.pdf
That is why The West Should write their own stories, not reprint AAP crap.
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“Yikes. Between 9 and 10mm a year for 20 years. That means 400mm since 1993.”
Your arithmetic is out by 100%, and they’re referring to Hillarys of course.
http://www.psmsl.org/data/obtaining/stations/1761.php
SLR at Fremantle since 1993 is 4.8 mm/year. 1993 is the start of satellite sea-level measurement, and also conveniently (for those like the CSIRO) a low point in almost all sea-level records around Oz,. which exaggerates the recent rise.
yeah, I had just remembered the Broome figures which were supposed to be about that, but we have a 10 metre tide range twice daily during springs … 40cm is nothing to us here, I have seen differences of that in one day to what tide charts tell us. I used to work at the top high tide level for years with water lapping at my feet occasionally. I still don’t see any recent rise.
“Don’t blame the poor old West, Tom – it’s straight reporting of a Federal Government report…”
I assume that was said in jest since The West Australian story ignored almost all the other points in the report and only headlined the one they could link to climate change. This is standard selective reporting for the monopoly West Australian on almost all major social issues, a bias only evident to people who research.
As noted above, 1993 had the lowest sea levels in the 1990s at Fremantle.
1990 – 0.699
1991 – 0.706
1992 – 0.699
1993 – 0.647
1994 – 0.685
1995 – 0.745
1996 – 0.822
1997 – 0.704
1998 – 0.743
1999 – 0.875
2000 – 0.852
2001 – 0.779
2002 – 0.715
2003 – 0.741
2004 – 0.743
2005 – 0.760
2006 – 0.753
2007 – 0.777
2008 – 0.858
2009 – 0.807
2010 – 0.797
Directly comparing 1993 and 2010 gives a 0.15m sea level rise, which over 18 years averages 0.0083333m, or 8.3mm per year.
Directly comparing 1999 and 2010, the sea level has fallen 0.078m or an average 0.0065m pa over the 12 years. That’s 6.5mm per year.
If more accurate averages are calculated with 1993 for some reason the start year, the first nine years averaged 0.761 and the second nine years (2002-2010) averaged 0.772, an 11mm difference which over 18 years averages a 0.611mm rise per year.
If they can cherry pick 1993 as a comparison year, so can I … in 1949 the Fremantle annual mean tide level was 0.752, in which case sea levels on the Perth coastline fell 0.83mm per year from 1949 to 2002, or rose 0.726mm per year from 1949 to 2010.
Hillarys?
1993 – 0.545
1999 – 0.770
2010 – 0.730
1993 to 2010 = 10.277mm per year
1993-2001 average – 0.663
2002-2010 average – 0.702
i.e.1993-2010 average = 2.17mm per year
Fremantle .611mm pa. Hillarys 2.17mm pa. Average 1.39mm pa.
How can Fremantle and Hillarys have different rates of sea level rise? A clue might be found in a study not reported by The West Australian … Anthropogenic land subsidence in the Perth Basin: challenges for its retrospective geodetic detection (published by the Journal of the Royal Society of Western Australia April 2012 – http://www.cage.curtin.edu.au/~will/Featherstone53-62.pdf).
Extracts:
Proper quantification, mapping and monitoring of recent-past subsidence in the Perth Basin also have implications for sea-level change measurements, because the Fremantle and Hillarys tide-gauges are located on it. Fremantle provides a long-term record (since 1897) that has been given substantial weight in global sea-level projections (Church & White 2006), notably because it is one of relatively few long-term records in the Southern Hemisphere. However, tide-gauges only measure sea level change relative to the land, so if the land is subsiding, the relative sea-level change will be contaminated (Belperio 1993; Aubrey & Emery 1986), as will be any future projections (Morner 2004). In short, coastal land subsidence causes sea-level rise measurements to be exacerbated, but it also makes lowlying coastal areas more vulnerable to seawater inundations (Brunn 1988).
…
CONCLUSIONS
First, there is no independent supporting evidence for the -50 mm/yr subsidence reported by Ng & Ge (2007) and sensationalised by the Western Australia news media. Evidence from reprocessed InSAR imagery and independent CGPS suggest that the subsidence is closer to -5 mm/yr, but the exact values are spatially and temporally variable (Table 1).
…
There is good correlation between changes in the depth of the water table in the confined Yarragadee Aquifer and the rates of subsidence of the CGPS installation at Gnangara (Figure 3). Depending on the time-span chosen over which linear regression is applied, different subsidence rates can be obtained. Fourteen years of data give a subsidence rate of -4.6 mm/yr, but this increases to -6.1 mm/yr during the 2000–2005 period of increased groundwater extraction. This demonstrates that the rate of subsidence is not linear, which needs to be taken into account by GPS analysts who do not necessarily have such local knowledge (Bouin & Woppelman 2010). Perth will need a dedicated subsidence-monitoring program if future water shortages necessitate recommencement of increased groundwater extraction from the Yarragadee Aquifer. This would also be necessary to correct relative sea-level change measurements at the Fremantle and Hillarys tide-gauges.
The past Fremantle and Hillarys tide gauge measurements have not been corrected even though Perth’s coastal planning policies are effectively based on them due to projected rising sea levels caused by climate change.
So the average annual sea level rise from the two tide gauges since 1993 is well below the global average at 1.39mm and studies pretty well confirm an average 5mm per year drop in Perth land levels over that timeframe, the study authors warning that this subsidence contaminates the tide gauge records.
Despite this, the Federal Government, the media and most West Australians now believe sea levels off the Perth coastline are rising up to 10mm per year, three times the global average.
Perth sea levels have been doing much the same as global temperatures since the 1990s – just about nothing.
Thanks, Chris, the devil is in the detail, as usual the media missed again.
Page 6 of The West Australian newspaper 12 December 2012:
http://au.news.yahoo.com/thewest/a/-/breaking/15610336/groundwater-use-sinking-perth/
“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.” etc
You’ve scooped the media by a week (not particularly difficult) but The West is only exposing a bit of the damage it caused with last week’s ridiculous page 3 story. More detail at http://www.waclimate.net/perth-sea-levels.html
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