JoNova » Cherry-picking sea level rises in Perth (a city which happens to be sinking)

“Gale Force” cherries coming your way…

Two weeks ago, it was all over the news. The ocean near Perth (where I live) was rising at the terrifying rate of 9-10mm per year since 1993, which was, shockingly, “three times the global average”. (Since Perth is flatter than flat, at this rate, in a few years everything bar Kings Park and a few sky-scrapers would be washed away). The myth started because a government department that knows a lot about our roads, trains, and buses wrote a dot point in a Fact Sheet. That was the State of Australian Cities report, and a pollie (Albanese) raved. Then the West Australian newspaper headlined it, and it all got out of hand.

In contrast, Chris Gillham got the raw data (something you’d think The Department of Infrastructure might have thought of), and shows below why its nonsense on stilts. The rate is not measured from when records began, but from 1993, which (surely it’s just a coincidence) also happens to be the lowest level in local tide gauges since 1941. (See that second last “dip” near the right-end in the graph below?) If they’d started the “rate” from the year 1999, the headlines would tell us the seas were falling…

If that’s not bad enough, the sea level data comes from two spots, 20 km apart (Hillarys and Fremantle), and Gillham points out that the rates are quite different. Apparently parts of Perth are subsiding by as much as 6mm year thanks to groundwater extraction, and one of those parts doing-the-subsiding happens to be around a tide gauge. More than half the scary rise is due to the gauge sinking.

As far as global sea levels go, Fremantle is a rare long record from the Southern Hemisphere, and is based on a very stable continent — shame about that aquifer that’s mucking up the record. Odd how mistakes, like not-correcting-for-the-subsidence make their way past scientists, bureaucrats, department heads, ministers, and then journalists too. Doesn’t anyone check these any more? Are they all incompetent, or in the grip of a mind-numbing religion perhaps?

All credit goes to Chris for doing so much research here. — Jo

100 years of tidal records at Fremantle show a fairly steady small rise. Note how high the oceans were in the 1940’s and how low it was in 1993. Given past rises and falls, why are we getting excited about a rise since the dip in 1993? (Data: Bureau of Met.)

Guest post: Chris Gillham

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Perth sea level myth swamped by a rising tide of facts

A myth was started in early December that sea levels off the Perth coastline have been rising at 9-10mm per year since 1993, three times the global average. The West Australian newspaper published a page 3 story quoting a State of Australian Cities report that Perth sea levels have been rising far quicker than anywhere else, which Federal Infrastructure Minister Anthony Albanese described as “disturbing” and “extraordinary”. The foreboding news was quickly propagated by other mainstream media and the sea level myth took hold.

The facts

Closer analysis of the claim shows it is based only on a selective comparison of 1993 and 2010 sea levels off Perth that are monitored by tide gauges in Fremantle and Hillarys. The averaged mean sea level at Fremantle in 1993 was the lowest recorded since 1941, which makes it an extraordinary year for comparison since this critical fact is ignored by a Federal Government department, a Federal Cabinet Minister and the media. The mean sea level at Fremantle and Hillarys was higher in 1999 than in 2010, the most recent year of publicly available data, and a comparison of averages in the first and second halves of the 18 years since 1993 shows sea levels have risen about .61mm per year at Fremantle and 2.2mm per year at Hillarys.

Not quite so disturbing or extraordinary, is it?

Fremantle sea levels 1993 – 2010 (Note the high point was 1999)

That sinking feeling

If you’re curious about how sea levels can rise at such different rates, it’s worth noting a university report published by the Journal of the Royal Society of WA in April 2012:

“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.”

“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.”

“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 Fremantle and Hillarys tide gauges, which have not been corrected, suggest an average 1.4mm per year rise in Perth coastline sea levels since 1993. Published university research confirms an average 4.6mm per year metropolitan land subsidence that contaminates and exacerbates the tidal measurements.

Damaging myth

On 7 December, The West Australian newspaper repaired some of the mythology it had created a few days earlier by publishing a page 6 story headlined Groundwater use sinking Perth.

“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.”

However, there’s still been no media attention to the cherry picked choice of 1993 by Federal agencies to make a sensational but demonstrably false claim, believed from the ministerial level down, that Perth is being swamped by the Indian Ocean.

That’s where policy is formulated and Australia’s coastal planning policies, which influence billions of dollars worth of property development values, are nowadays based on predicted rising sea levels.

Media bias

The media headlined an inaccurate sea change myth but had no interest in the leak of IPCC second order draft AR5 on 14 December amid claims that the heating contribution of enhanced solar forcing from cosmogenic cloud formation has been ignored by the intergovernmental panel.

The claims suggest a far greater contribution to warming by the historically high solar cycles since 1950 that finished in 1996, a blast with lingering effects on sea levels.

As illustrated in the chart below, the leaked IPCC report predicts a dangerous future for Fremantle with worst case sea levels as much as half a metre higher by 2100, based on the port city’s tide gauge which is an important southern hemisphere contributor to global sea level calculations because of its longevity since 1897.

The IPCC paints a terrifying picture for Fremantle

The leaked IPCC papers also chart different land temperature anomaly projections since the first intergovernmental report in 1990, as well as anticipated heating to 2015 and actual global temperature anomalies (relative to 1961-90) which confirm they’ve been flat since 1997/98.

Warming temperatures cause thermal expansion which is the major contributor to rising sea levels, so it’s worth comparing the IPCC’s charted global temperature records and projections overlaid with Perth’s local tide gauge readings since 1990:

Tide Gauge results from Fremantle and Hillarys are overlaid onto the AR5 IPCC graph. The two gauges look quite different to each other, but both come from Perth. Possibly also, the rises and falls in sea level lag the surface record.

Observed global temperature anomalies since 1990 are marked by black dots with error bars which suggest the climate is barely agreeing with the IPCC’s best possible projections.

An interesting correlation

Despite probable contamination from confirmed subsidence of the Perth metropolitan land area, there’s little doubt that Fremantle and Hillarys sea levels correlate with global temperatures, as would be expected, but with a lag of about a year during the rapidly heating 1990s and about two years during the stable temperature plateau of the new millennium.

The temperature/sea level correlation suggests flat to slightly higher Perth sea levels were recorded in 2011 and 2012 (data not yet available), with a drop likely in 2013/14 in response to weaker 2011 global temperatures reducing thermal expansion. With solar cycle 24 currently looking chilly, it seems a fair bet that neither Fremantle or Hillarys will be underwater by 2015.

The delayed sea level reaction to temperatures doesn’t mean tide records necessarily reflect actual thermometer readings before 1990 but it’s nevertheless interesting that, due to the low 1992 global temperature indicated in the IPCC chart above (Mt Pinatubo?), Fremantle sea levels in 1993 were the lowest since 1941.

We won’t drown

So Perth sea levels haven’t risen by up to 10mm per year since 1993, they aren’t rising three times faster than the global average, land subsidence indicates they’ve been closer to flat and possibly even fallen since 1993, and the leaked IPCC report confirms they’ve been as stable as global temperatures for well over a decade.

If Australian governments and media want to research instead of ignore facts about global warming and sea levels, they might begin at Sea Levels on Perth Coastline and at Sea Level: Not so Fast released on 14 December.

Cheers!
Chris