Costs of not adapting to climate change will rise along with sea levels - scientist

A red sticker at the boundary of a Dunedin property in 2024 after massive downpours caused floods, landslides and road closures.

Photo: RNZ / Charlotte Cook

Earlier this week homes on the Wellington coast saw eleven metre swells knocking at their doors, while a new study revealed that Wellington's one-in-100-year coastal floods have become roughly a twice-per-year occurrence.

We're also in store for what experts are predicting to be the strongest El Niño weather event in a century.

So what does this mean going forward?

Graham Leonard is a principal scientist at Earth Sciences New Zealand. He told RNZ's Saturday Morning the evacuations - though considered by some an overreaction - were a "a timely reminder of the myriad of coastal processes that turn into hazards at the coast because we live there, and they're getting worse with climate change".

"These coastlines, they're kind of ephemeral, constantly changing places, and they don't look the same for long. We've chosen to live at the coast for amenity, logical reasons, access to transport and sometimes floodplains where it's a good place to grow crops and increasingly we just love the view. We love to be able to go swimming but it is one of the most unstable bits of the country."

He said that because of the warming climate, what would otherwise have been a "normal" storm can be more damaging as the sea level rises, not to mention they are becoming more frequent.

"Climate change… has this multiplier effect on everything else that's going on. Our mean sea level has risen by about 0.2 metres, about 20 centimetres over the last century, you know, from human greenhouse gases. And it's projected to rapidly accelerate at the moment and reach perhaps a metre over the next century.

"But we can change that. That's the current prediction. I guess the big story for the future is that baseline will just come up and up. And it's really forecastable, so there aren't really any excuses to not do something about it. We're good at forecasting sea level."

New Zealand already had a high rate of renewable energy use he said, helping minimise the main culprit - greenhouse gas emissions. But as the government moves towards upping the use of fossil fuels under the guise of energy security, Leonard says transport is an area we could be doing better.

Even people who do not live near the coast should be concerned about rising sea levels, he explained.

"The biggest surprise is that sea level rise doesn't just affect the shoreline or the waves and the swell and the surge coming on top of that. The ocean is connected - this is quite logical when you start to think about it - it's connected to groundwater beneath our coastal towns and cities and floodplains.

"And so as sea level rises, groundwater can rise too. In some places, the first sign of sea level rise might not be water coming over the seawall or over the beach, it might be water coming up through the ground."

Research in Dunedin, he said, showed that can make flooding worse during heavy rainfall because the ground cannot absorb as much as before.

"The effects of sea level can be transferred surprisingly far inland, hundreds of metres or kilometres because of that groundwater gradient change… Research in places like this, they really are highlighting this influence can go quite a long way inland. And then it starts to intersect with your floodplains and your drainage and your infrastructure problems at the coast, on coastal plains, and then back up those floodplains."

Photo: RNZ / Samuel Rillstone

Because people like to live near the coast, this drives up the cost of infrastructure, Leonard said - because it is taking the brunt of sea level change and storm damage.

"Because we're building there and we're continuing to intensify at the coast now, we're not turning this around at all, those risks and those disasters are just getting worse…. We don't want to be paying for those cities and those infrastructure again twice in rapid succession because we just can't afford it. So we need to transition to this long-term horizon where we're thinking across all of these hazards - where is it riskiest and where is it a lot less risky, and the better ways to engineer."

That will take planning 50 or even 100 years ahead when building housing and infrastructure, he said.

"If we have a long-term plan like that, and then unluckily some place gets hit by a disaster, at the moment we are recovering in quite a reactive way. We're saying, 'Okay, we need a recovery plan after a cyclone or an earthquake.' And we rapidly come up with a plan to recover, often to look quite similar to the way we were before.

"But if we did have that long-term reduction and adaptation plan, our recovery can surge that. That's a moment where we're spending billions or tens of billions of dollars in new building, and let's not spend it twice. Let's pick a plan that we hopefully already have and surge that during our recovery."

It was not cost-effective to constantly buy out owners whose properties are no longer suitable to live in, he said, like has happened in places like Hawke's Bay and Auckland after recent weather events.

"If that becomes a precedent and a norm, we're not going to be able to afford that across all of these places. So we need a much more subtle and inclusive-of-society decision about how we do that, slowly over decades and with renewal to move inland.

"It's not as simple or as affordable as we just buy people out and move somewhere else."

Wellington Harbour swell from Port road, Seaview.

Photo: RNZ/Reece Baker

As for the upcoming El Niño, Leonard said we need to be ready for "spiky temperatures, dryness and winds".

"The forecast is for a super El Niño, you know, one of these five biggest since 1973 or bigger, quite possibly bigger… the El Niño is probably going to dry the North Island, dry the eastern and northern part of the South Island. And that means less water. And actually, already we're seeing drying, especially in Eastern Canterbury, for example… so people and industries who are reliant on water supply really need to be taking note now."

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