by Sean Rush
Here are a few comments and slides about the Sea Rise study that Jim Bolger is basing his Coastal Advisory Panel’s recommendations on.
Traditional survey techniques based on tide gauges and datum points show Kapiti Coastal properties are 6 metres above mean sea level. This house on Otaki beach, with the red X, is about as close as anyone’s in the district to the shoreline. The survey peg from a month or two back shows it to be 6 metres above mean sea level. With sea level rising only 20 cm [8 inches] over the 20th century and very mixed responses as to whether that has changed at all, this is well above any sea level for 21st century planning purposes.
On 1 May 2022, a new online tool was released to the public. Known as the ‘SeaRise’ project, it used satellite radar to measure land subsidence around NZ. They added the subsidence to estimates from the UN’s IPCC of ‘absolute’ sea level rise, the increase in the volume of water, to conclude that sea level was rising much faster (Petone underwater in 20 – 30 years).
It was big news all week, interviews with the PM, Minister James Shaw etc, etc.
But the dataset was short – 2004 to 2011, chosen specifically due to its relative seismic quiescence (an ‘inter-seismic’ period). This means it is biased towards subsidence. We know that since 2011 the Christchurch, Kaikoura and Seddon earthquakes raised most of the Lower North Island and Marlborough by several metres in places. Wellington in 1855 and Napier in 1931 did the same.
Is it a coincidence that the Ministry for the Environment released its ‘managed retreat’ consultation in the same week?
The answer is “NO”. Documents from months before show Victoria University and Ministry for the Environment colluding with other agencies. A VUW Comms plan from October 2021 specifically directed that the launch on the website go ahead even if the work had not passed the peer review. This is not in line with the scientific method.
On the SeaRise Twitter feed, among howls of anguish and media frenzy, only one word of caution was given: by Dr Dan King, a member of the SeaRise team whose published work in 2020 had warned against using satellite data for forward planning.
“A slight word of caution: It’s worth noting that the SeaRise maps only give a short snapshot of the VLM rate based on InSAR data. At sites like Pauatahanui, our upcoming salt-marsh data suggest that this shouldn’t be trusted to represent the long-term signal over many decades.”
The model has already been proved to result in extreme sea level projections. At the location of the Wellington Tide gauge the model projects 11 cm by 2020.
But the tide gauge records that we got 2 cm, in line with average rates.
The project participants have routinely ignored my requests for clarification, even when I was a Wellington City Councillor (until 2022) with thousands of affected residents and with several experts in support. I extracted the below exchange via the Official Information Act. It is from a GNS expert to the Project Leads, that notes the model has ‘major holes.’ Sadly it also notes she ‘continues to ignore’ me, before acknowledging I am right.
Coastal residents are concerned that in combination with the ‘managed retreat’ report prepared by the Environmental Defence Society (and curiously funded by AIG, BECA, ASB and WCC), hopelessly extreme inundation lines will devalue coastal properties and affect insurance.
The study is now part of NZ’s costal management planning documents courtesy of MfE, who have similarly stopped engaging with me.
But there is a plethora of peer-reviewed and well-established science that warn against adopting such a simplistic model.
Beaven and Litchfield in 2012 specifically warn about ‘slow slip’ events which are tiny earthquakes that lift Kapiti over periods of months to years, eliminating subsidence.
Dr Dan King’s study specifically warned against using satellites for long term planning:
“We therefore interpret that the slow subsidence in the cGPS record likely indicates a short-term signal which, over the course of the past century, has been counteracted by the combined influence of events such as slow-slip associated with the Kapiti Coast source region, post-and co-seismic uplift, and possibly variations in subsidence rate. This, combined with the lower than expected rates of sea-level rise at the Wellington tide gauge from nearby cGPS stations, suggests that it is unwise to base local sea level projections on the observed recent net subsidence alone without factoring in the long-term effect of slow-slip and co-seismic uplift.”
NIWA said the same in 2018: “The net effect is that the subsidence due to the subduction of the Pacific plate under the Australian plate and coseismic displacement to date was mostly cancelled out by the current day Kaikoura earthquake postseismic deformation and the upwards ratcheting effect of the SSEs.”
Dr Wallace makes clear slow slip events (SSEs) are a fundamental part of any modelling of land movement, particularly in the Lower North Island area. This year she said (on the GNS website): “Some of the GNSS sites closest to the Manawatū and Kapiti SSEs have undergone as much as 1.5 cm of horizontal displacement and 1 cm of upward displacement since the beginning of 2023.”
–Dr Laura Wallace, GNS, 2023
And actually the ‘best available science’ still seems to be to use the tide gauge network for 20 year+ policy making. The below will shortly be published from Victoria University’s Professors Stern, Lamb and Otago Uni’s Paul Denys. It was presented favourably at last week’s NZ Geoscience conference.
With $20 million in funding at stake and the prospect of having funded a lemon, MfE have refused to accept the science is flawed and continue to press on with reliance on the SeaRise study as a foundation for coastal planning as well as the more extreme IPCC scenarios that are now regard by most as ‘implausible.
Sean Rush is a Barrister and runs Sean Rush Energy & Infrastructure Law Limited.