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This newspaper, for the second time in about a month, recently gave front-page credence to a Government-funded report that anticipated (even encouraged) insurance difficulties for some 3100 Dunedin property owners within 15 years.

The report, written by North Island "climate economists", claimed to be based on the impacts of extreme storm surge in conjunction with expected sea level rise.

Having some knowledge of the physical realities involved, and of local flooding characteristics, I questioned the authors’ approach via a series of emails. It turns out that their report has not been peer-reviewed, that the authors could not identify where the claimed 3100 affected Dunedin properties were located, and that the authors were entirely unaware of the only detailed study of storm surge/sea rise impacts along the Otago coastline. I sent them a copy, but received no further response.

There are arguably three reports in play. The first is a detailed Niwa report. It is a comprehensive 160-pager that maps, in detail, areas of the Otago coast at risk from combined sea level rise and storm surge. I refer to this as the scientific report.

A second Niwa report produced generalised nationwide tabulations of numbers of houses and utilities in cities that are situated on land believed to be lower-lying than calculated future sea levels in conjunction with extreme low-probability storm surge conditions. I refer to this as the inventory report.

Finally, there is this latest report, highlighted by the ODT and other media, whose authorship is predominantly (not entirely) based within Te Herenga Waka — Victoria University of Wellington. For clarity, I refer to this as the economists’ report. This report is funded by MBIE (the taxpayer).

Niwa describes its inventory report as utilising simple "bathtub" mapping. If an area’s surface (the seat of the bath) is below the elevated sea level in a projected storm event, it is included in the inventory.

This approach essentially ignores the elevation of any protective landform between the area under consideration and the sea (it ignores the height of the "rim" of the bath). In the case of Dunedin, the protective rim is provided primarily by the sand dune system that extends from Lawyers Head to St Clair. That system obviously needs to be maintained.

Niwa has confirmed that its scientific report takes into account local topographical detail, whereas its inventory report does not.

I can’t tell you the quantum of sea-level rise, nor frequency of extreme storm surge, nor what the condition of the sand dunes will be at any time in the intermediate to longer-term future, but I can assure readers that the insurance retreat scenario 15 years hence predicted for Dunedin is at odds with credible scientific analysis and reporting. Residents should ensure that their insurers take note.

We should all be mindful of realistic climate change impacts and of realistic solutions. The areas of the Dunedin coast that the scientific report identified as potentially at risk from storm surge are at Taieri Mouth, Long Beach, Purakaunui and possibly other small settlements. I suspect the total number of such properties at imminent risk would be closer to 31 than 3100.

■ Neil Johnstone is a semi-retired Dunedin engineer.