climate change wa

Sea levels on Perth coastline


On 5 December 2012, The West Australian newspaper published the following page 3 story concerning rising sea levels off the Perth coastline.


sea level rise off perth


Below are charts based on BoM tidal gauge records at Fremantle station 62230:


fremantle sea levels


fremantle sea levels since 1944


fremantle sea levels since 1993


fremantle sea levels since 1944


1993 (0.647) had the lowest mean sea levels at Fremantle since 1941 (0.630) but is used by government and accepted by the media as a benchmark comparison. The very low sea level of 1993 was almost certainly due to the delayed global cooling impact of the Mt Pinatubo volcano eruption in 1991.

Fremantle tide gauge data from 1897-2011 can be accessed from the Permanent Service for Mean Sea Level :


fremantle sea level to 2011


fremantle sea level since 1999


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
2011 – 0.926


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 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?


australian sea levels


Above is extracted from How our regional sea level has changed published by the Indian Ocean Climate Initiative in 2005, showing a time series of the Fremantle sea level anomaly (variations of annual mean levels).


fremantle ipcc predictions


Above is extracted from the second order draft of Chapter 13, Sea Level Changes from the IPCC WGI Fifth Assessment Report, showing the panel's estimation of future sea levels at Fremantle.


global ipcc predictions


Above is extracted from the second order draft of Chapter 13, Sea Level Changes from the IPCC WGI Fifth Assessment Report, showing the records of global sea level change over different time periods.


fremantle ipcc tide records


Above is extracted from the second order draft of Chapter 3, Observations: Oceans from the IPCC WGI Fifth Assessment Report. The original IPCC chart has a graphics overlay added by this website of raw Fremantle tide records which suggest adjustments slightly increasing the rate of sea level rise in the IPCC chart.

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.


Hillarys

Below is a chart based on BoM tidal gauge records at Hillarys station 62237 since 1993:


hillarys sea levels since 1993


Directly comparing 1993 with 2010 suggests that Hillarys sea levels have risen by 10.2mm per year.

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:


hillarys sea levels to 2012


Below are the annual mean sea levels at Hillarys since the station began recording full years in 1992, again showing that 1993 was the lowest level on record.


1992 - 0.5974
1993 - 0.5451
1994 - 0.5980
1995 - 0.6412
1996 - 0.7056
1997 - 0.5897
1998 - 0.6507
1999 - 0.7692
2000 - 0.7565
2001 - 0.7027
2002 - 0.6419
2003 - 0.6618
2004 - 0.6671
2005 - 0.6864
2006 - 0.6942
2007 - 0.7204
2008 - 0.7899
2009 - 0.7297
2010 - 0.7293
2011 - 0.8615
2012 - 0.8342


Directly comparing 1993 with 2012 suggests that Hillarys sea levels have risen by 14.5mm per year over the 20 years.

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).


Land subsidence

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
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 historic Fremantle tide gauge measurements have not been corrected for land subsidence. Hillarys tide gauge readings are corrected by 0.1mm per year to account for vertical land movement.

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.


At 1.4mm, the average annual sea level rise from the two tide gauges since 1993 is well below the global average and university studies confirm an average 4.6mm per year drop in Perth land levels over that timeframe, the study authors warning that this subsidence contaminates the tide gauge records so they are exacerbated.

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.


This story acknowledges the Perth metropolitan subsidence that probably contaminates and exacerbates tide readings but fails to correct the assertion that Perth sea levels have risen 8-10mm per year since 1993 - a selective choice of the lowest annual sea level recorded at Fremantle in 50 years compared against 2010, instead of more accurate average mean sea levels.

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.


temperatures and sea levels


Above is extracted from the AR5 draft of the IPCC WGI Fifth Assessment Report, showing recorded and projected global land temperatures since 1990, with the Fremantle and Hillarys tide gauge records added to illustrate the close relationship between global temperature, thermal expansion and sea levels, regardless of land subsidence which is likely to have corrupted the tidal records.


australian temperatures and sea levels


Above shows the trend lines of averaged annual mean tide levels from 12 different gauges around Australia since 1992. The averaged mean of all 12 in 1992 was 1.56m, in 2000 it was 1.68m and in 2010 it was 1.66m.

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.


australian sea levels 2011


Above shows the trend lines of averaged annual mean tide levels from seven different gauges around Australia since 1992 which have updated records for 2011, this additional year of data obtained from Permanent Service for Mean Sea Level. If accurate, the 2011 trend is a record high for the seven stations and corresponds with rising air temperatures in 2009/10.

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.


global sea surface temperatures


Also thanks to Bob Tisdale, below is an animation of scaled Nino sea surface temperature anomalies and corresponding delayed East Indian-West Pacific sea surface temperature anomalies, providing a further explanation for the 1999/2000 peak in Australian and Perth sea levels.



Albeit a Pacific Ocean phenomenon, the two charts below show the Southern Oscillation Index from 1990 to 2012 and monthly values for the Pacific Decadal Oscillation Index from 1900 to 2010 (charts courtesy The Australian Baseline Sea Level Monitoring Project: Annual Sea Level Data Summary Report July 2010-June 2011).


southern oscillation index


southern oscillation index since 1900


Below are conclusions extracted from the second order draft of Chapter 3, Observations: Oceans from the IPCC WGI Fifth Assessment Report:


It is virtually certain that globally averaged sea level has risen at a mean rate between 1.4 and 2.0 mm yr–1 over the 20th Century and between 2.7 and 3.7 mm yr–1 since 1993 (both ranges 99% confidence). This assessment is based on high agreement among multiple studies using different methods, and from independent observing systems (tide gauges and altimetry) since 1993. It is likely that a rate comparable to that since 1993 occurred between 1930 and 1950, possibly due to a multi-decadal climate oscillation, as individual tide gauges around the world and all reconstructions of GMSL show increased rates of sea level rise during this period. Although local vertical land motion can cause even larger rates of sea level rise (or fall) relative to the coastline, it is very likely that this does not affect the estimates of the global average rate, based on multiple estimations of the average with and without VLM corrections.

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.


On 14 December 2012, the Global Warming Policy Foundation published Sea Level: Not So Fast which noted "The interesting thing about this work is that it shows that sea level has remained constant this century confirming what others have pointed out. There has been little, or very small acceleration in sea levels, despite the increasing anthropogenic forcing."


In April 2013, Environmental Science published Present-to-future sea level changes: The Australian case which found:

  • The official Australian claim of a present sea level rise in the order of 5.4mm/year is significantly exaggerated.
  • The mean sea level rise from Australian tide gauges as well as global tide gauge networks is to be found within the sector of rates ranging from 0.1 to 1.5 mm/year.
  • The claim of a recent acceleration in the rate of sea level rise cannot be validated by tide gauge records, either in Australia or globally. Rather, it seems strongly contradicted.


In 1933 The West Australian newspaper reported the concerns of the Government Astronomer and Government Geologist that Perth's coastal plain was sinking, with an estimated tilt around four inches in one mile (10.2cm per 1.6km) from 1897 to 1933 and suspicion of subsidence at Waterman's Bay (now the site of the Hillarys tide gauge).


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