Understanding Climate Risk

Science, policy and decision-making

Sea level rise. Part II – tide gauge analysis

with 9 comments

Sea level rise Part I covered the stoush resulting from a paper on long-term tide gauge records for Australasia. The author was Phil Watson of the NSW Department of Environment, Climate Change and Water and the paper was published in the Journal of Coastal Research in March. Tamino has pointed out the limitations of the statistical methods used, showing that the conclusion of decelerating sea level cannot be sustained. Tamino removed the annual cycle then used 20-year and lowess smoothing to show that the opposite conclusion – recent sea level rise is accelerating – is probably true for the Australasian region. A conclusion I strongly support.

It’s generally accepted that long-term climate records are analysed using trend analysis; either as a linear or non-linear trend, usually quadratic. The use of a particular statistical method assumes a specific model of how a system behaves. That model can be made explicit but if not, there is still an assumed model being used. Sometimes the assumption won’t be declared because it’s a widely accepted paradigm.

So what is the model sitting behind trend analysis – measured as either a straight line or a curve – and what paradigm of change process does it support? By analysing single tide gauge records, I am asking “How does sea level respond to externally-driven warming at a given location?”

The use of trend analysis to measure climate change through variables such as air temperature assumes that the climate change signal is a smooth trend and deviations from this trend are climate variability (This depends on the ability to rule out other causes). An example, using artificially-generated data representing an air temperature anomaly is shown below. This type of change is also assumed for sea level rise, but longer-term processes in the ocean compared to the atmosphere recommend a minimum 50-60 years for trend analysis compared to 30 years for the atmosphere.

This is pretty much what Watson did with the 20-year smoothed records of tide gauge records, shown below for Fremantle and Auckland.

20 year smoothed tide gauge records from Fremantle and Auckland shown with quadratic lines of best fit (Watson, 2011)

To be meaningful, a statistical method needs to adequately represent the underlying physical processes. Two lines of reasoning suggest that climate change may not be smooth: one is that climate is a complex system with non-linear behaviour, exhibiting abrupt changes (a theoretical argument that dates back to Lorenz) and the other is that palaeoclimatic reconstructions suggest abrupt changes also occur in the sea level record (reconstructed climate proxies). Although the latter are generally assumed to result from non-linear ice sheet dynamics, this may not be the whole story.

Here, three of the long-term time series used by Watson, Fremantle, Sydney and Auckland, are analysed for abrupt changes. Newcastle is omitted because it is several records stitched together and shows clear signs of artificial inhomogeneity. Tide gauge data was downloaded from psmsl and only annual data used, although gaps were filled by averaging incomplete years. That left some gaps where whole years were unrecorded. The method used to assess abrupt changes is the bivariate test of Maronna and Yohai, updated to test a series of step changes in a single record. The method is described in a paper submitted to Journal of Geophysical Research (draft supplied on request).

Pictured below is Fremantle with trends and shifts dating from 1920 (pre-1920 is gappy and may also be inhomogenous).

Fremantle annual average tide gauge data analysed for step and trend changes from 1920, also shown with a linear line of best fit.

This chart shows two step changes. One in 1945 is about 75 mm and the other in 1996 is about 65 mm with little intervening trend. Both are statistically significant to the 1% level. They show little intervening trend. The Auckland record, which I couldn’t easily get data for after 2000 (Watson obtained it directly from the port authority), shows a signficant step change of about 75 mm in 1947. Figure 3 above shows that if I did have the last decade’s data, it would likely kick again in the late 1990s. The Sydney record is shown below.

Fort Denison tide gauge records with step and trend analysis, combined with simple trend and 11-year running mean.

Sydney shows a series of statistically signficant step changes (1% level) in 1909, 1923, 1950 and 1998. None of the intervening trends are signficant. Interestingly, 1923 and 1948 mark changes in the decadal variability of ENSO in the Pacific. Perhaps this is decadl variability associated with changes in the East Australian Current and the Pacific more generally. The shift in the late 1990s coincides with the “El Niño of the century” and a statistically significant increase in global mean air temperature of 0.3°C.

So, the reason why Phil Watson found a deceleration was because he started the analysis in 1920, found a large shift in all records in the late 1940s, static conditions until the late 1990s when another step change occurred. Put a curve through that and it will decelerate. It also misses the changes from the late 1990s which I interpret as step and trend and others interpret as trend change.

So how widespread is such behaviour in tide gauge records? I decided next to analyse one of the longest running continuous records in the world: San Francisco. This record is three sites combined and adjusted to create a homogenous, uninterrupted record. The results are shown below.

San Franciso tide gauge records with step and trend analysis, combined with simple trend and 11-year running mean.

The San Francisco tide gauge record shows significant step changes in 1866, 1935, 1957 and 1982. The last two coincide with El Niño events, both of which were noted as significant high sea level events in a report analysing this record. I checked the shift dates using Rodionov’s STARS method for regime shifts set at p=0.1, white noise filtered (which is cruel to sea level data because of the high autocorrelation), and obtained the same dates to within a year.

These analyses show that step and trend analysis is a credible alternative to trend analysis. Are there compelling theoretical arguments to favour one method over the other? Trend analysis follows a signal-noise construct, where the signal is thought of as a monotonic curve as in the first chart in the post and the noise is variability. Step and trend analysis assumes the climate change signal is episodic in variables such as temperature and sea level rise, at least at the regional level. I will follow this up in another post. I believe the signal-noise construct does not adequately represent how climate actually changes and that the step and trend presented here is a more realistic model.


Written by Roger Jones

August 4, 2011 at 4:39 pm

9 Responses

Subscribe to comments with RSS.

  1. Thank-you for your comments on the paper that I had published in the Journal of Coastal Research earlier this year. I would be keen to discuss some of the issues that you have raised further, though not via the blogoshere (if possible). I would also be keen to have a look at the paper that you have submitted to the Journal of Geophysical Research discussing step change methods.

    I can be contacted via e-mail.

    Best Wishes

    Phil Watson

    Phil Watson

    September 21, 2011 at 12:07 pm

  2. The difficulty we have with Roger Jones approach is his up-front stated assumption that trends are due to warming.

    It would have been more open to have simply tested that something was happening, and then explored why, or what.

    Each of the gauges Jones analysed reflected the water balance of their locations. Which is to say, their data integrated the effect of tidal movements on the one hand, and variations in flows “going past” on the other. Tide levels at Fremantle, Sydney and Newcastle are affected by flows down, respectively, the Swan; Parramatta and Lane Cove; and Hunter Rivers.

    The hydrology of these river basins have been considerably and consistently altered by human interventions; including buildings and paving; urban irrigation etc. destroying over-time the naturalness of their hydrological signals.

    For example, the Sydney Basin draws water from catchments south to nearly Goulburn; west to the Great Divide and north into the catchments of the Hawkesbury River.

    The population of Sydney step-changed after both World Wars, especially to the west of the City. For the Hunter River, we have Glenbawn Dam near Scone, which was completed in the early 1960’s; also Glennys Creek Dam, completed later + the effects of coal mining, and the potential impact of the Newcastle Earthquake.

    In all these catchments, such developments and happenings could be expected to result in an upward trend in gauge heights, due to increased base-flow and rainfall induced runoff.

    To blame warming for everything is a popular thing. I think it unfortunate that Roger set out to prove his assertion, rather than investigate a neutral hypothesis.



    Bill Johnston

    September 22, 2011 at 1:34 pm

  3. It’s not about understanding climate risk; its about understanding the interface between the climate and our built and un-built environments.

    Bill Johnston

    September 22, 2011 at 1:38 pm

  4. Bill,

    From Das, P., Marchesiello, P. and Middleton, J.H. (2000) Numerical modelling of tide-induced residual circulation in Sydney Harbour, Mar. Freshwater Res., 51, 97–112.

    Freshwater input is entirely dependent upon local rainfall runoff, and no permanent rivers or streams enter the system. Although we speak of the Parramatta and the Lane Cove ‘rivers’ these are really only arms of the great estuary forming the harbour, and carry little freshwater flux except in rare flood events.
    The Harbour contains West Central South Pacific water carried southwards by the East Australian Current and modified within the estuary by occasional local freshwater runoff and heating and cooling in situ. Ingression of plankton elements of tropical origin, brought southwards by the East Australian Current, could be traced up the estuary to about the Gladesville Bridge (Revelante and Gilmartin 1978). Tidal records prepared by the Maritime Services Board (Hamon et al. 1982) show that there is little change in tidal amplitude and phase from Camp Cove to the head of navigation in Parramatta River.

    In essence, what you’re saying is that the siting of tide gauges in each of these estuaries has been so poorly done that the records are contaminated by freshwater inflows. That is, tide monitoring in Australia is totally incompetent. Does the same incompetence spread all the way to San Francisco and Seattle (latter not shown here)? I can dig up the Honolulu record too if you like – very riverine site. It’s quite straightforward to show these estuaries are dominated by ocean tides, as the extract for Sydney Harbour above shows.

    Roger Jones

    September 22, 2011 at 1:56 pm

  5. Your reply is a tad heavy Roger. I’m sure you do not precisely know just why any of the gauges were sited where they were. All we have are data from points in the landscape. I have not said or intimated anything about competence!

    You made an a priori assumption/claim that changes are due to warming, thus predicating the interpretation of your analysis. All that we know from the data is that there exist trends and step-changes – the data do not tell use what caused these

    I note that you have not presented an hypothesis, you presented a presumption. That is what I’m questioning. I’m pointing out that there are other plausible possibilities to do with the water balance at the location where tides are measured.

    As to the reference; it’s an opinion whether the Parramatta and Lane Cove Rivers are “rivers” or “estuaries” and essentially it does not matter. Flow in these rivers does not seem to be monitored.

    Although the turn-over time for water in the harbour is probably longer than several months, I’d expect flows into and out of the harbour (and any other tidal system) with all its attendant signals, to dominate the water itself. With our measurements, we are talking about water on the scale of millimetre depth and less – 1 mm = a mere 1ML/sqKm.

    The harbour has an area of 56 km^2: 1mm depth=56ML. This is the volume of (roughly) 100 farm dams The volume of the harbour is reported to be about 500 GL – that is larger by a factor of 10,000 (roughly). We are measuring a tiny little variable bit of this water, against a huge amount that does not vary much. In comparative terms our signal represents hardly any water at all. At that scale inflows and changes in base flow could reasonably be expected to appear as a signal. If it is a signal that steps and trends, it may confound with your warming assertion.

    The actual area of the Sydney Harbour basin is small. The City-at-large gets supplied with about 1,000 ML/day from external catchments (+ desal water). Whereas the woodlands that once existed across the actual basin had a runoff coefficient of perhaps (ratio of rainfall to runoff) 0.2; today it would be close to 1 (urban irrigation which probably would appear as base-flow is a factor). Step-changes occurred in Sydney’s rainfall and its urban development, which occurred mainly after the first, second and Vietnam wars. Development was generally to the west – along the Lane Cove and Parramatta River corridors.

    Clearly there are many other forcing factors that need to be considered when looking at estuarine sea level, aside from the priori assumption that it is all about “warming”.

    Bill Johnston

    September 23, 2011 at 7:01 am

    • Correction: The runoff coefficient is the ratio of runoff to rainfall, and it is used in estimating runoff in the so-called rational method (Runoff = coefficient* rainfall (intensity[per hour; day; year])*area).

      Bill Johnston

      September 23, 2011 at 7:26 am

  6. Bill,
    sorry if it seems heavy. It was written quickly and I stuck to logic rather than checking whether it was nice.

    When doing an analysis it not necessary to do ground-hog day and go back to the beginning every time. A priori assumptions based on well-founded science are perfectly legit. So in Part 1 there is a summary of 20th century sea level rise based on published science that gives clear attribution of the sea level rise budget to anthropogenic climate change. Since 1992, sea level rise has been monitored by satellites. Earlier records are monitored by tide gauges, a summary of which can be found here. Budgets since 1950 are covered here.

    You don’t seem to like that, so you suggest that the tide gauge records may be contaminated by changes in runoff that step up at each location where I have nominated a step change according to the statistics. That means local runoff changes have affected tide gauges in Sydney Harbour, Fremantle, Auckland and San Francisco. Also Seattle which I measured and did not show and other tide gauges up the US west coast. I have just downloaded Honolulu and it shows rises (my macro that automates the step change analysis has disappeared with the recent re-install I had to do, so I can’t do rapid analysis at present). An island in the midst of the northern Pacific. Would you like to propose local runoff changes due to coastal urban development at that site as well?

    For your supposition to be plausible, runoff changes would have had to occurred at each of these locations at times when the statistics show a significant shift. Yet the Sydney tide gauge shifts up in 1997, just at the time that inflows into all rivers along the east coast decline due to significant reductions in rainfall. In WA they experienced reduced riverine flows at around the same time. The Fremantle tide gauge is situated in the boat harbour 1 km down the coast from the inner harbour, the mouth of the Swan River – how could it be affected by river outflow if the harbour faces the open ocean?

    The logic that follows on from proposing that the tide gauge records are affected by local runoff suggests that these are poorly sited. They are not totally dominated by tidal processes but are influenced by riverine process affecting the total water budget in those locations. Therefore, the process of measuring tide gauge records is contaminated by local runoff, which has not been a feature of the many analyses done to date. By implication then, the whole scientific effort around the understanding of sea level rise is incompetent.

    Roger Jones

    September 23, 2011 at 8:33 am

  7. Roger,
    At issue here is not whether sea levels in general are rising; it is about sea level as measured at specific locations.

    For sea level rise in general, Professor Will Steffen in his “Critical decade” directed the public’s attention only to an unspecified period sometime post-1990. This corresponded to a particularly steep but non-significant gauge rise for Sydney and other places.
    It seemed unreal to me, given all the data available, that he (more likely his workers) had not done his/their homework. It is an important issue because it is impacting on the assets and life-plans of many thousands, if not millions of people.

    As you know. NSW Government and local councils are acting on this claim, which may turn out to be spurious. I note also that the satellite sea-level data is also showing a recent downtrend, which should not be happening if the theory is correct.

    I am unconvinced of the attribution of climate-warming. As time passes, model projections look less likely; their general “uncertainty” has not declined; it actually seems to be widening; critical data has been irregularly updated, rendering some recent-past published work obsolete; and indeed, scientists including Hansen; and Trenberth & Fasullo have both published, questioning the mystery of “missing energy”. (T&E’s graph suggests energy first started to “missing” in 2006.)

    You would be aware that the NODC satellite-based heat content data from about 2003 were revised downwards in January 2010, resulting in 8 or 9 years of stasis, or even decline in ocean heat content.
    There are also increasing numbers of published work that question the sensitivity of GCM’s to the doubling of CO2. I question the work of Karoly and Braganza (Meteorol Atmos Phys 89, 57–67 (2005) in particular who relied entirely on GCM outputs, despite some important doubts being raised in the peer-reviewed literature before their work was published.

    (Already, the CSIRO graph you directed me to at http://www.cmar.csiro.au/sealevel/sl_hist_few_hundred.html is out of date relative to the latest graphs from NODC.)

    These are real concerns, real questions, and real issues affecting millions of real people.

    So … I’ve been aware of step-change issues for a long time, but I’m no longer able to freely access data or information; and I don’t have statistical support. (I make that clear.)
    Getting back to the measured data. You made some important points:
    · It’s generally accepted that long-term climate records are analysed using trend analysis; either as a linear or non-linear trend, usually quadratic. The use of a particular statistical method assumes a specific model of how a system behaves. (Presuming that we know! My comment).
    · The use of trend analysis to measure climate change through variables such as air temperature assumes that the climate change signal is a smooth trend and deviations from this trend are climate variability (This depends on the ability to rule out other causes)
    · To be meaningful, a statistical method needs to adequately represent the underlying physical processes.
    · Sydney shows a series of statistically significant step changes (1% level) in 1909, 1923, 1950 and 1998.
    · The San Francisco tide gauge record shows significant step changes in 1866, 1935, 1957 and 1982. The last two coincide with El Niño events.

    Whilst GCM’s, global sea level data; global CO2 and others measures indicate “steady” and mostly rising trends; your analyses, and similar analysis that I did, indicate episodic steps with no significant intervening trends. For example, your graph for San Francisco indicates no trend from about 1859 to 1930 (90 years) and 1951-78 (30 years); Fort Denison from about 1945 to 1988 (40 years or so); Fremantle from 1940 to about 1994 (50+years).

    In your first post you mentioned a critiqued by Tamino. His analysis, which I’ve followed, predicted an acceleration of about 05.mm/yr/yr. But he did not allow for steps in 1925; 1949 and 2000. (As a compound interest proposition, 05.mm/yr/yr is pretty steep!)
    So how, if we are implicating climate-warming, can we have steps, related to known, exogenous gross climatic events; but we don’t have trends within steps; and please remember I’m discussing data for specific gauges.

    My basic contention is that I don’t think you can rule out endogenous factors that affect the water balance at points where measurements are made.
    In the case of Fremantle, which you specifically mentioned, if the breakwater were not there, the tide gauge would be virtually in the Swan River. Clearly, although the breakwater may deflect its flow, the river would still affect the gauge.

    I’m not implying any incompetence anywhere by anyone, but it possibly is time to do a bit of “ground-hog day” work as you put it.


    Bill Johnston

    September 23, 2011 at 12:35 pm

  8. The tidal gauges were changed in the periods mentioned and so the readings changed. It’s a simple explanation and I guess in the rush to prove a theory, no one asked the people who took the readings why the readings changed. I did and that was the answer. Sea level rises over the past 10,000 years have not changed annually that much. The sea has risen by about 120 metres in that time. The aborigines caused that rise by having too many fires. Or maybe god did?

    Gordon hinds

    January 3, 2014 at 12:21 pm

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: