Australia is one of the most stable land masses on the planet, and has more gauges than anywhere else in the southern hemisphere, so it’s very useful for sea-level measurements. It also had a couple of rare continuous long records “… the two longest sea-level records in the southern hemisphere, Sydney Fort Denison from 1886 and Fremantle from 1897″ .
A new paper by White et al, concludes that Australian sea level rises are similar to global measurements (so not a bad proxy for the world), and that during times when CO2 levels were much lower — like before World War II, sea levels were rising at the same speed (or possibly faster) than they are today.
A generalized additive model of Australia’s two longest records (Fremantle and Sydney) reveals the presence of both linear and non-linear long-term sea-level trends, with both records showing larger rates of rise between 1920 and 1950, relatively stable mean sea levels between 1960 and 1990 and an increased rate of rise from the early 1990s.
Does a “larger rate of rise” mean larger than today, or larger than average — I think, given the error margins, that we could only be sure it was a similar rate. They do point out that the Australian sea level rise was slightly faster than the global rise from 1920-1950. But they did not say by how much (did I miss it?)
For example, Australian MSL rose slightly faster than the global average from 1920 to 1950, slower than the global average from about 1960 to 1990, and similar to the global average since about 1990.
Figure 2: (a) Overview of Australian sea-level data and comparison with global sea-level estimates, all expressed as OVMSL (section 2.3). The gauges flagged “TV” in column 10 of Table 1 are plotted in grey. The records flagged because of possible ground movement and credibility issues are plotted in cyan, but not used in calculating averages. The arithmetic mean of the tide gauges considered to be of reasonable quality is plotted as a heavy black line. The GMSL estimated from satellite altimeters (see Section 2.2) is plotted as a heavy red line, and the area-weighted mean from satellite altimeters along a line around the Australian coast is plotted in orange. The cyan trace that goes very high at the end is Wyndham, and the one that goes very low in the 2000s is Point Lonsdale. (b) As in (a) but after the signal correlated with the Southern Oscillation Index is removed (see Section 3). (c) The changing number of gauge stations available for use over time.
Figure 8 Figure 8: Upper panels show the fitted generalized additive models (GAMs) and approximate
pointwise 95% confidence intervals fitted to RMSL (adjusted for SOI and seasonality) at Fremantle and Sydney. Note the use of different RMSL scales. Lower panels show the nonconstant trends as instantaneous rates of change (first differences) and approximate pointwise 95% confidence intervals; dashed lines are the estimated long-term linear trend of 1.58 mm yr-1 for Fremantle and 0.65 mm yr-1 for Sydney.
They produce a model to try to sort out the factors that jostle sea levels up and down to find the long term underlying trend, and claim they can explain 69% of the variance, and most of it is due to ENSO.
Remember also that sea level rise started before Napoleon’s time, long before our evil CO2 emissions rose, so we can quibble over whether the pre WWII rise was faster around the world, or pretty much the same as today, but it’s all deck chairs on the Climatanic. We’ve increased our CO2 emissions dramatically in the last ten years and sea level rises have slowed. Over and over we get the message that CO2 is not the defining force of our climate, other things are changing it, and the models don’t know what they are.
This paper, though, ought to calm a few councils down. How much do we need to panic over Australian sea level rises that are the same now as they were when CO2 was ideal in the 1920’s and 1930s?
Another noteworthy point is that Australia is generally stable, but some places (like Hillarys, Port Adelaide and, potentially, Darwin) are sinking. I covered the bizarre difference between Hillarys and Fremantle here. The two spots are 20 km apart yet show vastly different rates of sea level rise. Panic-merchants prefer the shorter more exciting record from Hillarys.
In comparison to many regions on Earth, much of the Australian coastline is stable in terms of vertical land motion. The GIA component of VLM introduces only small contributions to RMSL (up to about -0.4 mm yr-1, in the sense of the land rising relative to the sea surface). However, localised subsidence at specific tide gauges is important for some sites (e.g. Hillarys, Port Adelaide and, potentially, Darwin), leading to a higher rate of RMSL rise at these locations compared to adjacent tide gauges…
Sea levels are more stable in the east and south, but they jump around more each year (by a whopping 10 -15cm) in the north and west. Blame the Pacific. The SOI and PDO have a significant influence on that. The bumps start near Indonesia and sweeps anti-clockwise around the nation, which is one of those fascinating but trivial bits of information you probably have no use for.
The RMSL signal is transmitted from the Western Pacific Ocean through the Indonesian Archipelago to Australia’s north-west coast (where the variability is highest), from where it propagates anti-clockwise around Australia (Wijffels and Meyers 2004) with magnitude decreasing with distance. Sea-level variability from the western equatorial Pacific Ocean is weaker along the Australian east coast where westward propagating Rossby wave signals from the subtropical Pacific Ocean directly impact on coastal RMSL (Holbrook et al. 2011). The Indian and Southern Oceans have a much weaker influence on Australian RMSL
than the Pacific Ocean.
Abstract
There has been significant progress in describing and understanding global-mean sea-level rise, but the regional departures from this global-mean rise are more poorly described and understood. Here, we present a comprehensive analysis of Australian sea-level data from the 1880s to the present, including an assessment of satellite-altimeter data since 1993. Sea levels around the Australian coast are well sampled from 1966 to the present. The first EmpiricalOrthogonal Function (EOF) of data from 16 sites around the coast explains 69% of thevariance, and is closely related to the El Niño Southern Oscillation (ENSO), with thestrongest influence on the northern and western coasts. Removing the variability in this EOF correlated with the Southern Oscillation Index reduces the differences in the trends between
locations. After the influence of ENSO is removed and allowing for the impact of Glacial Isostatic Adjustment (GIA) and atmospheric pressure effects, Australian mean sea-level trends are close to global-mean trends from 1966 to 2010, including an increase in the rate of rise in the early 1990s. Since 1993, there is good agreement between trends calculated from
tide-gauge records and altimetry data, with some notable exceptions, some of which are related to localised vertical-land motions. For the periods 1966 to 2009 and 1993 to 2009, the average trends of relative sea level around the coastline are 1.4 ± 0.3 mm yr-1 and 4.5 ± 1.3 mm yr-1, which become 1.6 ± 0.2 mm yr-1 and 2.7 ± 0.6 mm yr-1 after removal of the signal
correlated with ENSO. After further correcting for GIA and changes in atmospheric pressure, the corresponding trends are 2.1 ± 0.2 mm yr-1 and 3.1 ± 0.6 mm yr-1, comparable with the global-average rise over the same periods of 2.0 ± 0.3 mm yr-1 (from tide gauges) and 3.4 ± 0.4 mm yr-1 (from satellite altimeters). Given that past changes in Australian sea level are
similar to global-mean changes over the last 45 years, it is likely that future changes over the 21st century will be consistent with global changes. A generalized additive model of Australia’s two longest records (Fremantle and Sydney) reveals the presence of both linear and non-linear long-term sea-level trends, with both records showing larger rates of rise
between 1920 and 1950, relatively stable mean sea levels between 1960 and 1990 and an increased rate of rise from the early 1990s.
Conclusion
A large part of the inter-annual and decadal variability in sea level around the whole coast of Australia is coherent and highly correlated with the Southern Oscillation Index and can be represented by a single EOF. Removing this coherent variability from both tide-gauge and altimetry records around Australia significantly reduces uncertainties of sea-level trends with
more uniformity in regional trends than previously reported. An assessment of the two longest tide-gauges records (Sydney, 1886-2010; Fremantle, 1897-2010) shows that the rate of rise has been non-linear in nature with both records showing large rates of rise around the 1940s, relatively stable RMSLs between 1960 and 1990 and an increased rate of rise from the early 1990s. From 1966 to 2010, when there is good coverage of most of the Australian coastline, the average Australian relative rate of rise is slower than the global mean prior to about 1985, but the mean Australian OVMSL rate from tide gauges is close to the global mean. Since 1993, MSL trends are considerably higher than the global mean around Northern Australian
and similar to the global mean around southern Australia. Higher sea-level trends in northern Australia are largely associated with natural climate variability. Even after attempts to remove the effects of this natural variability, trends around most of Australia, show an increased rate of rise from the early 1990s, consistent with global mean trends.
h/t to Willie. Thanks.
REFERENCE
White, Neil J., Haigh, Ivan D., Church, John A., Koen, Terry, Watson, Christopher S., Pritchard, Tim R., Watson, Phil J., Burgette, Reed J., McInnes, Kathleen L., You, Zai-Jin, Zhang, Xuebin, Tregoning, Paul: (2014) Australian Sea Levels – Trends, Regional Variability and Influencing Factors, Earth Science Reviews, doi: 10.1016/j.earscirev.2014.05.011