Special Report: Sea-level rise could hit parts of New Zealand twice as quickly as expected due to the movement of tectonic plates.
A massive new research project has estimated sea-level rise every couple of kilometres along the country’s coast by incorporating the projections of global climate models with satellite data on how quickly the land is rising or falling. The results will be crucial as local and central government start to prepare for the impacts of climate change.
“Being a New Zealander, we know that the land goes up and the land goes down and we have earthquakes,” Tim Naish, a paleoclimatologist at Victory University’s Antarctic Research Centre and one of the leads of the new SeaRise project, told Newsroom.
“So we certainly expected that, by the time we put these vertical land movements into the sea level predictions, they would have an effect. But I think we were quite stunned by the magnitude of that effect and how much it really mattered in the very near term, in the coming decade.
Parts of the North Island are sinking by nearly a centimetre each year. Wellington, where Naish and co-lead Richard Levy spoke to Newsroom, is sinking by between two and four millimetres annually.
“Globally, the sea level is going up at about 3.5 millimetres per year. If the land is going down at that rate, as it is right here, it essentially doubles the amount of sea-level rise we will get,” Naish said.
“Another way of looking at that is that we’re going to get sea-level rise a lot sooner than we thought.”
Levy, who also leads environment and climate work at GNS Science, said relatively small changes in sea level can have an outsized impact on the likelihood and severity of extreme events. Previous research has found that every 10 centimetres of sea-level rise increases storm surge potential by a metre – turning a one-in-100 year flood in Wellington into a one-in-10 year flood.
“In the near term, 30 centimetres of sea-level rise – small amounts of sea-level rise – have a big effect on our region. They turn that event that happens once every 100 years into an annual event,” he said.
“With 50 centimetres, it will happen twice a month. With 70 centimetres, it happens every single tide. Relatively small amounts of increase in sea level have a big, big impact on this location. What we’re learning is all of those increases are not going to happen 50, 60, 70 years down the track, they’re going to happen within the next 20 to 30 to 40. It’s not a future problem, it’s a now problem.”
Much of the country could cross that first 30-centimetre threshold in a matter of decades, the SeaRise research found. The fastest sinking places, along the southeast coast of the North Island, will cross the 50-centimetre line by the middle of the century in even the most optimistic climate scenarios.
In just eight years, parts of the Wairarapa will see 31 centimetres of sea-level rise. Two decades later, the coastline could disappear under between 57 and 61 centimetres of sea-level rise, depending on whether global emissions decline rapidly or continue to rise.
Parts of the country that are rising, like Stewart Island and the Bay of Plenty, would experience much slower sea-level rise. In scenarios consistent with limiting warming to 1.5 degrees, Pikowai near Matatā might even see the sea level fall. But if global emissions aren’t constrained, the sea would still rise by between 10 and 36 centimetres by the end of the century.
The new figures will form a reference for planners and councillors seeking to protect existing infrastructure from climate impacts or identifying safe locations for new development.
“Even the minimum we’re going to get is the 50 or 60 centimetres. That’s knowledge, that’s power, we can plan for that right now and we must plan for that right now,” Naish said. “We can’t avoid 30 centimetres of sea-level rise by mid-century. That’s baked in, that’s unavoidable, so we can plan for that.”
Local authorities have been required to plan for sea level rise since 2010, through the country’s Coastal Policy Statement.
Canterbury has the most public infrastructure at risk from coastal hazards – flooding, erosion and rising groundwater – of any province in the country.
At 1m of sea-level rise, about 25,000 properties, mainly residential, worth about $6.7 billion, in low-lying parts of Ōtautahi Christchurch, are at risk. That’s more than in either Auckland or Wellington.
Christchurch’s city council is pursuing a district plan change to limit future development in areas of high risk. It has just signed off a coastal adaptation plan, and is consulting with existing communities considered most at risk, and the appropriate response – everything from building seawalls to property buyouts.
“There are areas in Christchurch which already have issues with surface flooding and groundwater, and we know that those areas will be also impacted by sea-level rise over time,” said Sara Templeton, a councillor in the city’s coastal Heathcote ward, speaking ahead of the SeaRise data’s release.
“The way that we have done our programme of work means that we’re able to take into account new information. We knew that there would be, over time, and so the programme is designed in a way to be able to cope with that.”
Ōtepoti Dunedin is grappling with its own vulnerable areas, including South Dunedin. Aaron Hawkins, a first-term mayor who’s a member of the Green Party, said SeaRise data largely confirms the level of sea rise his city council has been planning for.
“Things like the vertical land movement data are really helpful in terms of breaking down how it might look at different points along the coastline. The better the information that we can get, the better informed our planning decisions can be.”
Climate change adaptation will require local and central government to work closely together, on policies and funding, at a scale that’s never been seen before, he said.
“It’s going to be important to get that right if it’s going to achieve what we need it to achieve. They’re not easy conversations. That’s why I think it’s going to be vital to have a more coordinated approach.”
Last week, Climate Change Minister James Shaw released a draft national adaptation plan.
Levy, of GNS Science, said infrastructure in New Zealand is built to withstand one-in-100 year events, but that those events are becoming much more common.
“A lot of our coastal regions are designed for a sea level that was 50 years ago – much lower than even now. We’re living on an antiquated human surface,” he said. “We’re not dealing with a static system anymore. We’re dealing with a system that’s changing and the rate at which it’s changing is increasing.”
The solutions aren’t just building walls. In the most vulnerable locations, entire communities might have to be moved. And while conventional flood defences have a role to play, they can also worsen extreme events if they’re breached.
“We need to seriously consider alternatives to concrete, I think we need to seriously consider alternatives to the hard infrastructure we tend to build and, where it’s possible and where it makes sense, let natural systems help us,” Levy said.
The researchers say the release of the dataset is just the beginning.
“It doesn’t end here,” Levy said. “We’ve taken that major leap forward, I think this is really valuable, important information, but we’ve still got work to do.”
SeaRise combines satellite measurements of vertical land movement with global projections for sea-level rise from the latest Intergovernmental Panel on Climate Change report. But it doesn’t show how the coastline might change as land sinks and the ocean rises.
That’s one of the next big tasks for Levy and Naish. They also hope to understand how the sedimentary system on the shoreline will respond and how coastal hazards to human and natural infrastructure might change.
“We’ve got to figure out exactly what’s going to happen along our coastline,” Levy said.
