environment

125,000 buildings at risk from first metre of sea level rise

Insurers were told today that the first metre of sea level rise is where much of the cost and pain will hit, reports Eloise Gibson.

At the Insurance Council conference at Sky City, insurers were given updated -  albeit draft - estimates that say $38 billion of New Zealand’s residential and commercial buildings may be at risk of flooding if sea levels rise one metre.

While the final report won't be in until next year, scientists have been improving their cost estimates by factoring in detailed satellite elevation data from more regions and adding comprehensive rebuilding costs to get a picture of possible exposure to flooding.

The $38 billion represents 125,600 homes and other buildings that could be in the flood zone if seas rise from today’s levels to up to 1m higher.

The figures were presented by NIWA sea level expert Rob Bell based on work he and his collaborators are doing to improve on their previous estimates for the Parliamentary Commissioner for the Environment.

An earlier report for the Commissioner used land elevation as a proxy for flood risk and estimated that about 9000 homes were vulnerable from seas just 0.5m higher than today. The previous estimated value of buildings at risk from seas up to 1.5m higher was $19 billion, but that was based on incomplete information.

Since then, scientists from NIWA, GNS and elsewhere have been refining the numbers, funded by the Deep South National Science Challenge.

Bell told the insurance conference that the newer estimates had been through one round of peer-review but were still in draft form. He told Newsroom a more detailed report breaking the risk down into regions and 10cm increments of sea level rise would be out in early 2019.

He and his collaborators have incorporated natural hazard risk modelling from GNS, more extensive LiDAR satellite imaging showing how high land is above sea level (barring a few parts of the coastline where the data isn’t yet available, such as the West Coast and Marlborough) and aerial maps showing the number of houses and commercial buildings in the at-risk area, to produce a more comprehensive picture of the potential cost of climate change on the coast.

They estimated the size of the risk zone using what insurers and councils call the 1 percent AEP flood risk, which indicates that a building currently has a one-in-a-hundred-year likelihood of flooding based on what’s happened during the past few decades. However, the frequency of floods will increase on the coast as sea levels rise. The study will model how the potentially at-risk area increases after 10cm, 20cm, 30cm and so on of sea level rise up to 3m. 

... hard decisions will face coastal communities sooner rather than later in the process.

Re-construction costs for homes and commercial buildings were estimated using details like roof style and cladding and the number of storeys each building has. The models couldn’t be certain that a particular building would flood in a particular storm or tide because they don't have all the local details and didn’t take account of the protective effects of structures like roads, tide gates and stop banks, which might reduce flooding. For that level of detail, regional councils will need to produce their own reports, as regions such as Hawkes Bay have done already.

Bell told the conference that adding the effects of sea level rise to today’s flood risk in 10cm increments revealed that some areas – like Tamaki Drive in Auckland – were at imminent risk already. That news is unlikely to surprise East Aucklanders, who are already experiencing road closures regularly.

Bell’s main message was that the potential cost of sea level rise is heavily weighted towards that first metre, meaning hard decisions will face coastal communities sooner rather than later in the process. He told Newsroom that is partly because New Zealand’s topography tends to have flat areas right on the coast, rising quickly to hilly country inland. The quick change in steepness means there isn’t as much housing at risk between, say, 1-2m as there is below 1m.

Between today’s sea levels and 1m of sea level rise, the estimated total of $38 billion of commercial and residential buildings in the risk zone represented 125,600 buildings.

Another $26 billion worth of buildings (a further 70,490) would be in the risk zone if seas rose between 1-2 m.

And a further $20 billion worth would be at risk if seas reached 2-3m higher, representing another 65,530 buildings, Bell’s slides showed.

In other words, almost half of the value of buildings affected after 3m of sea level rise was affected by the first 1m, with costs tapering after that.

Earlier in the day Robert Muir-Woods, the chief research officer of science and technology at catastrophe-risk-modelling company RMS and a former IPCC lead author on insurance and climate risk, told the conference that past policies had artificially concentrated buildings in the area just above today’s risky zones.

For example, to get insurance, people often needed to build above the line of a 100-year average probability of flooding, or 1 percent AEP. “So everybody builds a few centimetres above that line and when a big flood hits everybody gets hit at once because of this concentration of risk.”

... half of the $2.7 billion in insurance payouts over the past 42 years had been paid in the 11 years since 2007.

Muir-Woods said container ports, hotels, marinas, airports and other assets also tended to cluster just above sea level. Globally, he estimated $2 trillion dollars’ worth of assets were concentrated about 1m above sea level.  “If you assume the climate system is stable…then the consequences of a small increase in rainfall intensity can dramatically increase the loss, as we saw with Hurricane Harvey,” he said.

Bell wrote a comprehensive Ministry for the Environment guide to councils on planning communities around rising seas. He told the conference there had been 20cm of sea level rise already since the 1930s and 1940s and “that’s when a lot of our urban areas were developed. Unlike jurisdictions like the UK we have very few engineered or walled coastlines.”

After re-processing insurance council data to 2018, Bell estimated that half of the $2.7 billion in insurance payouts over the past 42 years had been paid in the 11 years since 2007. “So things are changing,” he told insurers. “We are seeing increase in sunny day flooding, which coincides with king tides, so there’s no margin in a lot of our harbours and estuaries for a storm to occur at same time.”

Early 2018 was the “summer of coastal storms” with floods in January, February and March, said Bell. “There is still ongoing development around hazard prone land and we need to be careful we are not locking in future risk. Past events are not a reliable guide to future.”

He reiterated the advice in the Ministry for the Environment guidance to councils against trying to plan by “picking a number” for sea level rise.

Although 20cm to 30cm is virtually certain over the next two decades, after that much depends on whether the Paris agreement succeeds.

Places such as south Napier were already at the point of having to choose what to do, while others had more time. If the globe kept under 1.5C of warming, median sea level may reach about 1m by 2300, while continued high emissions would see people having to deal with 5-10m by 2300.

The result would be uncertainty for a long time yet, because of the long lag time between emissions and sea levels rising, said Bell.  “We have to live with that uncertainty. So we monitor and we move when we have to.”

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