Understanding Climate Risk

Science, policy and decision-making

Sea level rise. Part I – the stoush

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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.

The Australian’s coverage was comprehensively covered by Tim Lambert in his The Australian’s War on Science series (Update: they can’t stop it – more from Deltoid), the statistics in the paper were critiqued by Tamino who showed up the method’s shortcomings, and the media issue was covered by ABC’s Media Watch program on Monday August 1. Also on the Media Watch site are expert statements from Kathy McInnes (CSIRO), John Hunter (Antarctic CRC) and four others that explain the science.

The Australian journalist Stuart Rintoul reported these results without getting comment from any experts such as those listed above. He quoted Macquarie University researcher Howard Brady as saying the recent research meant sea levels rises accepted by the CSIRO were “already dead in the water as having no sound basis in probability”.

“In all cases, it is clear that sea-level rise, although occurring, has been decelerating for at least the last half of the 20th century, and so the present trend would only produce sea level rise of around 15cm for the 21st century.”

Dr Brady said the divergence between the sea-level trends from models and sea-level trends from the tide gauge records was now so great “it is clear there is a serious problem with the models”.

“In a nutshell, this factual information means the high sea-level rises used as precautionary guidelines by the CSIRO in recent years are in essence ridiculous,” he said.

Despite having no history of peer reviewed publications in climatology, Dr Brady was presented in the article as a climate change expert. A letter from Watson’s employers protesting that his research was misrepresented in that conclusion was not printed. This was covered in ABC’s Media Watch Monday night.

Sea level rise can be understood in two ways: process and budget. Process involves how energy moves through the ocean and affects its surface level. Budget affects how much water is in the ocean. The ocean water budget tells us that sea level is rising over the 20th–21st century with very high confidence. Thermal expansion, and glacier and ice cap melt are both positive; the first basic science, the second known from mass balance estimates. Greenland is also positive (high confidence) and most recently, Antarctica has been added to list of positive inputs, though with moderate confidence. The heat budget of the upper ocean is estimated to contribute about 1.5 mm year-1 to global sea level rise between 1961–2003, showing that thermal mass balance is the largest single contributor to SLR.

The latest satellite data suggests that global SLR has been 2.8-3.2 mm per year since 1993. Although this latest paper was published this year, too late for Watson’s paper, Watson should have included a paragraph or two on recent satellite trends, which would have shown clearly the gap between regional tide gauge records and satellite measurements of the height ocean surface.

So if there is a debate about acceleration or hiatus of sea level over the past couple of decades, it is a debate about process, not budgets. Brady’s comment showed pretty much total ignorance of the issue, and that ignorance has now been eagerly broadcast across the denialosphere by others who are similarly unenlightened.

Part II shows that simple trend analysis may not be the best way to analyse tide gauge records.


Written by Roger Jones

August 4, 2011 at 9:11 am

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