Australia's "Poster Children" updated and analysed

I read a great deal of absolute rubbish on the 'net about sea-levels. Someone speaking at the American Geophysical Union meeting recently claimed that Miami was suffering more frequent flooding due to sea-level rise. Ignoring for a moment that much of Miami was already below tidal high-water before it was built, proper examination reveals that the rate of subsidence is greater than the rate of sea-level rise. At Cape Canaveral, further north, the measured rate is just over 5mm/year, but CGPS (Continuous GPS) data shows the land close to the tide gauge to be sinking at 2.8mm/year - the greater part of the rise is due to subsidence along the coast.

A couple of years ago, I read what I considered to be an outrageous claim - that the Tidal Unit (formerly National Tidal Centre) of the BOM was publishing "exaggerated data" in the ABSLMP (Australian Baseline Sea Level Monitoring Project) series. Worse, that the data was "homogenised". Well, it's impossible to "homogenise" tide-gauge data. Each gauge uses a different base-line for measurements, referred to as "Tide Gauge Zero". It's a "virtual" reference level, referenced to a physical benchmark (the TGBM or "Tide gauge Bench Mark") at the gauge site, one of a set, the remainder being on the adjacent dockside or land.

I'll relate the full story in a later post, but for now just let's say it concerned the record for Sydney, and that for Port Kembla, some 64Km to the south. Here's the full chart for Sydney, to October 2016.

Sydney 1914-2016.   Source: BOM

While the long-term rate for Sydney is 1.02 mm/year, it's easy to see that any short-term rate depends entirely on the start point. Start in 1997 or 1998, when there was a "dip" due to the 1997-8 El Niño, and you get a very high trend; start in 1990, a much lower rate. I've plotted the rate for a 20-year sliding window, using annual average data:

Rate (mm/year) for 20-year sliding window; end year on X-axis.

Port Kembla (an ABSLMP station), installed in 1991:

Port Kembla 1991-2016   Source: BOM

Sydney 1990-2016 for comparison.

Sydney 1990-2016  Source: BOM

Both together, after Port Kembla data adjusted by +58mm - the offset due to the different "Tide Gauge Zero" benchmarks.

Sydney & Port Kembla compared; 1991-2016

The claim of "homogenistation" is egrarious bullshit, invented by the morally, evidentially and statistically challenged. Put even more forcibly, it's a lie.

On to Fremantle, another source of claims and misinterpretation. The full plot first:

Fremantle, WA 1897-2016

This is the chart for Hillarys (Boat Yard), on the far side of Perth from Fremantle, to the north.

Hillarys, WA 1992-2016

Fremantle 1992-2016 for comparison:

Fremantle, WA 1992-2016

And both together, with Fremantle adjusted down by 64mm:

Hillarys/Fremantle over-plotted; Fremantle adjusted down by 64mm

Hillarys starts somewhat lower than Fremantle, but they plot together from 2002 onwards. It's easy to show such correlation between adjacent stations around Oz, even if they're several hundred Km apart. Even if it were possible to "homogenise" gauge data, the tides do a perfectly good job already.

Data is sourced from The main BOM Tidal Unit for Sydney and Fremantle, and from the AMSLMP project for Port Kembla and Hillarys.

NOTE: I've just discovered that SONEL have updated with data for many Oz stations. The CGPS pillars are often very close to tide gauges. At Hillarys they're co-located, and the latest plot shows a drop of 2.78 mm/year:

Source: SONEL

Updates include data for many of the Pacific islands, including Sceptic poster-child Tuvalu. Watch this space.

Damp Data from Down Under - Australian Sea Level Update

Australia's Bureau of Meteorology finally go their act together and published sea-level data (to the end of 2012) at the end of May. Prior to that, the latest data (other than for the newer ABSLMP stations) was to the end of 2010. I've been hard at work updating and expanding my database, and will update my reference page soon. In the meantime, I present charts for the two long-term "poster-children", Fremantle and Sydney. Both much analysed, much discussed, and often misrepresented. A certain Andrea Boretti spent many hours, produced many spreadsheets and charts, wrote many pages, and tortured the Sydney data at length until it confessed that there wasn't much of note going on there - in other words, there was no sign of any significant acceleration in the rate of rise.

In my humble opinion, if your intention is to analyse the rate to identify any significant change, then do just that. There's no need for "sliding windows", spectral analysis, polynomial curve-fitting or anything else. For example, calculate the rate from the start year to a succession of years, e.g. 1900-1910, 1900-1911, and so on to the last year of data. I've made it a standard analysis in almost all of my spreadsheets, and shown examples in a number of posts. While the amount of variation in the long-term rate reduces with the data length, significant year-to-year changes are still clearly represented in addition to longer-period change. Here's the chart for Sydney - I've used a 121-month (10-year) centred running mean. The rate has increased a little from the previous chart's 0.89 mm/year (to 2010).

Sydney, NSW - Sea level 1914-2012  (Data source: BOM)

The plot of the annual rate shows a break in slope at 1997, and a small increase thereafter.

Sydney, NSW - trend in mm/year from date on lower axis.

Fremantle has seen a definite change after 1994 - the running mean shows an uptick after then. This is not surprising, all West and North-West Australia stations show similar upticks, an acceleration in fact; in the Fremantle case after around 40 years of little change, even a slight reduction after the middle 70's.

Fremantle. WA - Sea-level 1897-2012   (Data source: BOM)

The long-term trend plot reflects the change, with a break in slope at 1994, and an increasing upward trend after 1998.

Fremantle. WA - trend in mm/year from date on lower axis.

There are those who would have you believe that nothing of the sort has happened; "situation normal, no change", but the evidence is clear. More on this in a post in preparation. However, the fairly sharp increase in rate over the last couple of decades around the west of Oz and the far-western Pacific in general is matched by virtually no change on the eastern shores - North and South America, and little or no change in the central Pacific. It's quite clear to me that what the satellites have recorded over the last 20 years is fairly accurate - those who would have you believe otherwise never actually compare tide-gauge data for exactly the same period as the satellite timespan. If you've seen such claims, check out what was actually compared - nothing, just a few longer-term charts produced. "See, there's no comparison!" they say, when they've done no comparison whatsoever. More (in detail, with real comparisons) on that in a future post too.

Extending the record - a sea level reconstruction for Sydney

On my reference page Sea Levels in Australia (top right sidebar) I state that Fremantle has the longest record of any Australian tide gauge, from 1897. The following is shown for Sydney:

However, it's little known that for a long period, there were two active gauges on Fort Denison, the older of the two recording from 1886, so predating the second by some 28 years. Unfortunately the two gauges used a different benchmark datum (reference level)  for measurements, preventing a simple backward extension of the record. By plotting the records together, I've been able to establish a good estimate of the difference (-127mm).

Adding that offset to the earlier data to line up with the data from the second gauge, resulted in the following reconstruction.

As the 5-year (61 month centred) moving average shows, sea level was dropping between the turn of the century and the middle 1940s, then there was an upward jump to 1950. The overall rate of rise is somewhat less than that shown for the single-gauge record, which was "pulled down" by the slight dip to the late 1940s. I've produced a reconstruction for Newcastle, NSW which extends the record back from 1966 to 1925, using the same technique, and will post it very soon.