Sea levels on Perth coastline
Fremantle tide gauge data from 1897-2011 can be accessed from the Permanent Service for Mean Sea Level :
Directly comparing 1999 and 2010, the sea level has fallen 0.078m or an average 0.0065m pa or 6.5mm pa over the 12 years. Directly comparing 1999 and 2011, the sea level has risen 0.051m or an average 0.0039m pa or 3.9mm pa over the 13 years. If more accurate averages are calculated with 1993 as the start year to 2010, 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 0.61mm rise per year. If averages are calculated with 1993 as the start year to 2011, the first nine years averaged 0.761 and the second 10 years (2002-2011) averaged 0.788, a 27mm difference which over 19 years averages a 1.42mm rise per year. It should be noted that in 1949 the Fremantle annual mean tide level was 0.752, meaning that sea levels on the Perth coastline fell 0.68mm per year from 1949 to 2002, or rose 0.74mm per year from 1949 to 2010. In 1955, the average mean sea level at Fremantle was 0.797, the same as 2010. It's surprising what you can find when you cherry pick the comparison year, isn't it?
Notable is the year 1942 which is disqualified in the unadjusted tide level trend line because that year had only three months of tide recordings - January, November and December which always have among the lowest mean tides of the year and which disqualify 1942 as an accurate averaged year. The second draft AR5 chart above includes 1942 and this exaggerates the ensuing rise in Fremantle sea levels around 1950. The years 1899, 1902, 1907, 1910, 1911, 1912, 1913, 1914, 1926 and 1965 are also disqualified from the tide trend line because of inadequate months of readings to represent a meaningful annual record of Fremantle sea levels during those years. The tide chart overlay was based on a visual estimate of best possible fit and does not claim to be precise.
Below is a chart based on BoM tidal gauge records at Hillarys station 62237 since 1993:
If more accurate averages are calculated with 1993 as the start year, the first nine years averaged 0.662 and the second nine years (2002-2010) averaged 0.7026, a 41mm difference which over 18 years averages a 2.2mm rise per year. Fremantle at .61mm per year and Hillarys at 2.2mm per year suggests an average sea level rise off the Perth coastline of 1.4mm per year since 1993. More recent tide level data for Hillarys reveals an increase in sea levels since 2010:
If averages are calculated with 1993 as the start year, the first 10 years (1993-2002) averaged 0.660 and the second 10 years (2003-2012) averaged 0.7375, a 77.5mm difference which over 20 years averages a 3.9mm rise per year. Fremantle at .61mm per year 1993-2010 and Hillarys at 3.9mm per year 1993-2012 suggests an average sea level rise off the Perth coastline of 2.3mm per year since 1993. Fremantle at 1.4mm per year 1993-2011 and Hillarys at 3.9mm per year 1993-2012 suggests an average sea level rise off the Perth coastline of 2.7mm per year since 1993. Fremantle tide gauge data since 2011 is unavailable. Perth Metro's rainfall was 14mm below the Perth Regional Office average in 2011 and 268mm below average in 2012, possibly contributing to lower groundwater and more Gnangara land subsidence that has accelerated the Hillarys tide gauge readings (see below).
How can Fremantle and Hillarys have different rates of sea level increase when they are so close together? A clue might be found in Anthropogenic land subsidence in the Perth Basin: challenges for its retrospective geodetic detection published by the Journal of the Royal Society of Western Australia in April 2012. 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
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.
Below is extracted from Is There Evidence Yet of Acceleration in Mean Sea Level Rise around Mainland Australia? published in 2010 by the Journal of Coastal Research: However, none of the long record Australian gauges (Fremantle, Fort Denison, and Newcastle) are currently fitted with CGPS or integrated with regular geodetic levelling to an array of deep benchmarks to accurately measure vertical landform movements. Analysis of ITRF2008 data for two surface monitoring stations within ≈30 km of the Fremantle tide gauge indicates measured vertical velocities of at least −2.6 mm/y over the past decade in the Perth region (see Table 3), similar to results attained previously by Dawson (2008). Although these measurements are some distance from the Fremantle tide gauge, they confirm concerns that subsidence in the Perth region, due principally to increased groundwater extraction, might be reflected (at least in part) in the latter portion of the tide gauge record. Given the likelihood of subsidence contaminating the historical record at Newcastle and possibly the later (or after the mid-1990s) portion of the Fremantle record, it would be essential to upgrade the Fremantle, Fort Denison, and Newcastle tide gauges with collocated CGPS to enable direct measurements of land movements at each site in order to improve our regional understanding of eustatic sea level rise.
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. On 12 December 2012, The West Australian newspaper published Groundwater use sinking Perth, with the opening editorial: 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. Commonwealth survey body Geoscience Australia used GPS monitoring to conclude Perth's land heights had fallen up to 6mm a year on the back of increased extraction from the Yarragadee aquifer. Pumping from the aquifer, which is below bedrock deep under the city, has increased greatly since the 1990s as its importance to Perth's drinking supplies has grown.
Perth sea levels have been doing much the same as global temperatures since the 1990s – very little. The only thing disturbing or extraordinary in the newspaper reports about Perth sea levels is that they reflect a biased editorial attitude toward the climate change issue, also apparent in the November 2012 production by ABC Catalyst of Taking Australia's Temperature (see Taking Catalyst's Temperature), and an ignorance or disregard for facts at a Federal ministerial level.
The main difference between the 12 averaged tide gauges and Fremantle/Hillarys alone is that sea levels peaked in 2000 rather than 1999 in response to the 1998 peak in land air temperature (or solar influence that caused the high land temperature). Based on the proxy average annual mean of these 12 tide gauges, sea levels around Australia either rose 5.3mm per year from 1992 to 2010 or fell 1.8mm per year from the year 2000 to 2010.
The tidal mean movements seem to correlate with global temperatures anomalies, with a one to two year lag. The actual temperature does not necessarily mirror sea volume or vice-versa before 1990, but it is worth noting that 1993 was the lowest sea level recorded at Fremantle since 1941. The chart below, courtesy Who Turned on the Heat? author Bob Tisdale, helps explain the increase in average Australian sea levels in 2011 as charted above. Surface temperature trends are similar to sea level trends, as would be expected through thermal expansion of the top layer of global oceans. The chart provides a hint at likely sea levels into 2012 and 2013.
It is possible that the inflections in 1920, 1960, and 1990 are caused by a multi-decadal climate oscillation with a strong peak between 50 and 60 years (e.g., Holgate, 2007; Jevrejeva et al., 2008). This is seen in all reconstructions (Figure 3.14; Church and White, 2011; Jevrejeva et al., 2008; Merrifield et al., 2009; Ray and Douglas, 2011) as well as in tide gauge records around the world that extend back to 1900 (Chambers et al., 2012). Although the calculations of 18-year rates of GMSL rise based on the different reconstruction methods disagree by as much as 2 mm yr–1 before 1950, all do indicate 18-year trends that were significantly higher than the 20th Century average between 1930 to 1950, with rates comparable to those measured in the recent period with satellite altimetry. Multi-decadal fluctuations with periods around 60 years appear in many climate indices (Section 2.6.8), including the AMO and PDO. Whether these quasi 60-year oscillations represent a real global variation or are related to sampling regional oscillations at the tide gauge locations is still unknown, but this should be accounted for when computing acceleration terms, especially over a portion of the cycle. When a 60-year oscillation is modeled along with an acceleration term, the estimated acceleration in GMSL since 1900 ranges from: 0.000 [–0.002 to 0.002] mm yr–2 (90% confidence) in the Ray and Douglas (2011) record, 0.013 [0.007 to 0.019] mm yr–2 in the Jevrejeva et al. (2008) record, and 0.012 [0.009 to 0.015] mm yr–2 (90% confidence) in the Church and White (2011) record. Thus, while there is more disagreement on the value of a 20th Century acceleration in GMSL when accounting for possible multi-decadal fluctuations, two out of three records still indicate a significant positive value. The much higher trend in GMSL calculated since 1993, however, likely reflects, in part, a multidecadal oscillation.
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