A new review has slated the sea-level findings used by Christchurch City Council to assess risks to coastal properties.
While council has scrapped fast-tracked plan changes based on the findings, coastal residents want hazard warnings removed from their properties' LIM reports. The findings were in a report on 50-to-100-year climate change risk, written by consultants Tonkin & Taylor. The report identified 18,000 properties as being threatened by rising sea levels, and 6000 by coastal erosion. LIM reports were amended to match.
Mathematician and policy analyst Simon Arnold has now reviewed Tonkin & Taylor's report. He considers it was statistically flawed, based on outdated law, and exaggerated the effects of sea-level rise.
"Scientists and engineers are good at talking about what is happening, but they struggle with this level of forecasting - it's too complex," Arnold said. "You really need to get a specialist statistician involved."
Arnold said the report was not fit for purpose and the council should never have relied on it. He urged them to back away from it.
"The Council is in an untenable position. This exaggeration of risk is costing homeowners now, a lot of people are affected by it," he said.
Arnold is a mathematician with experience as a policy analyst and forecaster for government , and has worked as an advisor to the McDiarmid Institute, and previous Prime Ministers. He lives on the Kapati Coast but said his property is not affected by coastal hazard projections.
He sent his review to both the Christchurch City Council and Tonkin & Taylor last month.
A spokeswoman for Tonkin & Taylor said they had already spoken to Arnold about his review, and did not want to wish to comment publicly. No-one was available from the Christchurch City Council to discuss the review.
Arnold's review questions the statistical methodology of the report, which he calls misleading. He asserts much of it is based on 1994 coastal policy statements in the Resource Management Act, rather than the updated 2010 version.
The review also says while the Tonkin & Taylor report is based on possible hazards, the law requires recognition of likely hazards only when assessing risk.
Arnold concludes the expected sea level rise in 50 or 100 years could only be half or a quarter of what was forecast. He also says the report's figures may be inaccurate as they were based on global, rather than local, climate change forecasts.
On the threat of coastal erosion, Arnold forecast Christchurch coastlines would ebb and flow, and would likely be "in about the same place" in 100 years.
He also pointed the city council towards a report written this year by retired principal Environment Court judge Joan Allin, which criticised how coastal risks were increasingly over-estimated.
Allin's report said she had "developed concerns about what other NZ coastal experts are doing. It seems that a number of them consider that it is appropriate . . . to provide only results that are very unlikely, or overstated."
In a letter of reply to Arnold last month, Tonkin & Taylor said they were still in the process of evaluating his comments. They did accept there had been "a rather straightforward error" in their report where a number was added rather than subtracted, but believed it did not "materially affect" their sea level estimates, the letter said.
"We will review the other matters raised in your letter as part of our ongoing work for Christchurch City Council," the Tonkin & Taylor letter concluded.