Sinking land and rising oceans could push sea levels in  our two biggest cities and many other parts of the country to double that generally projected – and much sooner than we first thought.

Scientists behind a sweeping new stock-take – giving projections for  every 2km  of Aotearoa’s coastline, right out to the year 2300 – say councils should be preparing for higher seas right now.

The fresh estimates cover some 7,434 sites – all searchable in an online tool launching Monday – and  build in, for the first time,  the influence of  land movement on sea level.

That’s just transformed our understanding of the ever-growing risk that tens of thousands of Kiwis face.

Going by international climate commitments today, we could expect global oceans to climb by about 60cm before this century’s end.

“However, for large parts of Aotearoa his will double to about 1.2m due to ongoing land subsidence,” said Victoria University climate scientist Professor Tim Naish, who co-leads the NZ  SeaRise  programme that carried out the work.

“We have less time to act than we thought.”

The new projections show the largest increases  would happen around the southeast North Island, along the Wairarapa coast.

“Here, land subsidence rates are high and sea level could rise by well over one and a half  metres  by 2100 if we follow the least optimistic climate change scenario,” said fellow  programme  co-leader  Associate Professor  Richard Levy.

“In contrast, land is rising near  Pikowai  in the Bay of Plenty and uplift rates may keep pace with climate change driven sea level rise, causing a small fall in sea level if we follow the most optimistic climate scenario.”

Baked-in impacts of climate change meant the world faced an unavoidable 25cm to 30cm of sea level rise by 2060.

“But what may be a real surprise to people is that for many of our most populated regions, such as Auckland and Wellington, this unavoidable rise is happening faster than we thought,” Levy said.

For many parts of our coast, 30cm of sea-level rise was a threshold for extreme flooding – above which a 100-year coastal storm became an annual event.

Naish said that, as parts of Wellington and Auckland were subsiding at rates of more than 3mm each year, the threshold could be crossed as early as 2040.

“This means councils putting in place serious plans for adaptation right now.”

In projections for a set of coastal spots, the scientists drew on moderate-confidence estimates for Antarctic ice sheet melt out to the middle of  next  century.

But the amount of sea level rise that each location could expect to see hinged on what trajectory future climate change followed.

“We are currently on a pathway where global warming could be  stabilised  at about 2.7C above pre-industrial levels, however, if each country  honours  its emission reduction pledges at the recent COP26 meeting in Glasgow, then we could limit global warming to below 2C,” Naish  said.

If that happened, then, under mid-range, medium-confidence estimates, about 72cm of sea level rise could occur at Auckland’s Viaduct Harbour by 2100 and more than 1.1m by 2150.

But if warming carried on to more than 2.7C – the path we’re on now – then the same site could see about 86cm of rise by the century’s end, and 1.4m by 2150.

For Wellington, at Petone, a low-emissions future could mean 70cm by 2100 and nearly 1.1m by 2150, compared with 84cm and nearly 1.4m respectively in a status-quo scenario.

Elsewhere in the country, median end-of-century values for the two emissions scenarios were 29cm and 42cm at Mt Maunganui; 42cm and 55cm at New Plymouth; 56cm and 70cm at Whanganui; 73cm and 87cm at Napier; 63cm and 76cm at Nelson; 51cm and 65cm at Christchurch; 47cm and 61cm at Dunedin.

The scientists also provided calculations for a high-emissions scenario that results in much higher levels of warming – putting Auckland’s and Wellington’s median rise at 1.8m by 2150 – but Naish considered that prospect unlikely.

“The severity with which we will experience sea level rise, and other impacts of climate change, can be lessened if we do all we can to limit warming by reducing emissions now,” Naish said.

Rising tide, rising threat

The new data comes days after the Government put out for consultation its draft National Adaptation Plan, which aimed to determine how the country should meet climate-driven impacts, and fund the costs that came with them.

According to our most recent national risk assessment, some 675,500 Kiwis already live in areas prone to flooding – with a further 2,065 living in the firing line of where some of the most dramatic effects of sea level rise could hit.

Buildings, too, were at extreme risk – nearly 50,000 of them were currently exposed to coastal flooding, and at the highest range of warming scenarios, that could rise to nearly 120,000 this century.

Rough estimates showed the cost of losing properties and assets in coastal and floodplain areas to be threatened by climate change impacts was about $145 billion – posing wide-ranging implications for the banking, insurance and property sectors.

Modellershave warned how properties in flood-prone areas could quickly become stranded assets.

Given more than 60 per cent of the four largest banks’ loan portfolios were made up of housing loans, even a relatively small proportion of stranded assets could cause a big blow.

Another recent report warned how thousands of seaside homes could face soaring insurance premiums – or even have some cover pulled altogether – within only the next 15 years.

International experience and indications from New Zealand’s insurance industry suggest companies start pulling out of insuring properties when disasters like floods become one-in-50-year events.

By the time that exposure has risen to one-in-20-year occurrences, the cost of insurance premiums and excesses will have climbed sharply – if insurance could be renewed at all.

In Auckland, for instance, around 540 exposed homes with current median coastal flood premiums for once-in-a-century events of $2000 could reach that one-in-20-year threshold with just 15cm of sea-level rise.

If insurance was still available at that point, the report found, premiums would have soared to $10,000.

The expert who led that work, Belinda Storey of Climate Sigma, described the latest projections as a “good step forward” – but stressed  that, for it to be of best use for planners,  it needed to be  integrable with other accessible data.

Levy said the projections were already being incorporated into the next Ministry for the Environment guidance for  councils on coastal hazards and climate change.

“We hope everyone takes a good look at the mapping tool, but we expect councils and planners will be the primary users,” he said.

“The finance and insurance sector have already been asking for the data – in part driven by the Task Force on Climate Related Financial Disclosures reporting requirements that need to be met by 2024.”

The sector has taken some early direct steps in confronting the risk.

in November, Tower revealed it would be  investing  in detailed flood risk modelling, and making ratings public.

The step meant that its customers would receive either a low, medium or high rating for their home, reflecting the potential risk of a flood and the estimated cost of replacing or repairing damage caused by flooding.

“While insurance underwriters already have some of the best data available against which insurance risk assessments are completed and risks priced, this data will be welcomed,” Insurance Council of New Zealand (ICNZ) chief executive Tim Grafton said of today’s projections.

The ICNZ also wanted to see councils and landowners using the data to inform their own planning.

“Part of that is to stop making matters worse through the continued consenting of building in high-risk areas,” Grafton said.

“Another part is using this data to model the effectiveness of any proposed adaptation investment designed to protect against natural hazards.

“Insurers do like to see firm evidence that any flood  defences, for instance, have been designed, built, and are being maintained, against good data.”

Local Government New Zealand’s policy and advocacy director, Grace Hall,  agreed that taking a planned approach to climate change adaptation was vital.

“The tool will help councils assess and identify risks, and take a strategic approach to better protect our communities from sea level rise in the short and long term.”

Many mayors have turned to the Government to provide  councils  the laws, guidance and help they need – much of which is expected to arrive through a legislative overhaul.

While the  adaptation plan aimed to tackle the complex legal and technical issues, funding and financing for adaptation would come through a new Climate Adaptation Act, aimed to be in place by the end of 2023.

Still, Climate Change Minister James Shaw has made it clear that central government  wouldn’t  be bearing all of the costs.

Shaw told the Herald that, given the risk of sea level rise had been known for some time, planners and developers should have been taking a precautionary approach, even in the absence of detailed data.

“But the thing about this new tool is, it actually changes what we thought about the nature of sea level rise,” he said.

“Where we thought we might have had four decades, we now have two. So, the change in the data makes a massive difference when it comes to planning.”

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