Sea level rise. Part II – tide gauge analysis

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?”

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Sea level rise. Part I – the stoush

A stoush on Australiasian sea level rise has erupted in the press and the blogosphere since the publication on July 22 of a story in The Australian covering a paper analysing long tide gauge records in Australasia. The details are covered comprehensively by Deltoid. The paper, by Phil Watson of the NSW Department of Environment, Climate Change and Water was published in the Journal of Coastal Research in March. It concluded that:

The analysis reveals a consistent trend of weak deceleration at each of these gauge sites throughout Australasia over the period from 1940 to 2000. Short period trends of acceleration in mean sea level after 1990 are evident at each site, although these are not abnormal or higher than other short-term rates measured throughout the historical record.

The Australian misprepresented this conclusion by calling in question 21st century projections of sea level rise (SLR). There is a very simple reason as to why this is not the case. The projections are about current and future ocean budgets, whereas the tide gauge records are more about process. Sea level budgets are known well enough to provide very high confidence that SLR will accelerate throughout the 21st century. The process of sea level rise at a site as measured by tide gauges is complex. Watson’s conclusions as emphasised by himself, his employers and real experts do not call into question the basic science about future SLR budgets.

However, as to the process of SLR I think the statistics currently being used don’t tell the full story. In part I, I summarise the story to date and in Part II, I will show an alternative method for analysing long-term tide gauge records.