Ups and downs of rising seas in a shaky nation

Local variations in land movement can double the short-term impacts of sea level rise, or slow the effects to nothing. A New Zealand research programme aims to tell coastal residents how shifting coastlines will change their experience of rising seas.

When a magnitude 7.8 earthquake struck the coast of Kaikoura in 2016, locals heard what they described as a “horrendous” noise – the sound of water rushing off a platform of rock that had suddenly been lifted out of the water.

The newly-exposed land was littered with seaweed and stranded marine life. Nasa satellite images would later reveal that a thin, rocky strip of new coastline had appeared when the quake lifted the seabed by 0.5-2m, along a 20-km stretch of the South Island.

While no-one would choose to experience a quake of this magnitude, there is one threat Kaikoura residents do not need to worry about for now: rising sea levels.

Satellite measurements since 1993 show global sea levels rising by about 3mm a year on average, a rate that will increase without serious cuts to greenhouse gas emissions.

Unlike a filling bathtub, oceans rise unevenly, but it is not only the varied spread of water that affects people. In many parts of the world, the land itself is moving, particularly in earthquake-prone nations. 

New Zealand's terrain is so dynamic that it is moving up in some places and down in others as we speak, says Richard Levy, a senior scientist at GNS Science and the leader of the NZ Sea Rise programme. In some places, the coastline is rising enough that it virtually cancels global sea level rise, at least in the short term. In other places, it is falling enough to double the rate, or worse.  

This presents a problem for scientists, who are trying to tell people what to expect based on global averages in the IPCC's estimates.

“Everyone was taking these numbers that were coming out of the IPCC report and trying to apply them to all locations," says Levy. "We are saying, 'If the IPCC says that [if emissions keep rising] sea level will go up by a metre by 2100...what does that mean for Napier, what does that mean for Auckland?’."

Helping answer that question is GNS Science's Ian Hamling, who has made a preliminary map of the New Zealand coastline using Synthetic Aperture Radar Interferometry (InSAR) data to precisely track the vertical movement of land bordering the ocean.

While the results are still being refined, they show remarkable variation across small distances -- so much so that people who are living in the same district can experience quite different amounts of sea level rise. 

Take the Heretaunga plain, a fertile apple- and pear-growing region between Napier and Hastings. The area appears to be almost static when you look at GPS data for the region, says Levy. But when researchers studied the area using InSAR, they found out that the middle of the plain was subsiding by about 3mm a year. For people and properties in that small patch of real estate, sea level rise is approaching twice as quickly as the global average. 

*Watch the video above to see the impacts of the Kaikoura Earthquake on the south end of Waipapa Bay, New Zealand* 

It's a similar story for some residents of Wairarapa. Hamling's early estimates suggest New Zealand's most significant area of subsidence is along the North Island’s lower east coast, south of Napier and down through the Wairarapa region.

This scenic piece of coastline is dropping at a rate of around 5mm a year, says Hamling -- a victim of tectonic shifts, as the Pacific plate slowly slides under the Australian plate along a fault line known as the Hikurangi subduction zone, pulling the Wairarapa coast down. Before Kaikoura's earthquake, the coastline in that area was subsiding, just as Wairarapa's is, but it rebounded dramatically during the big quake. “This pattern may well be repeated in other areas when an earthquake eventually strikes," says Hamling. 

Geologists and civil defence planners say a major earthquake on the Hikurangi fault is very likely, eventually. But a quake is hardly a pain-free way to solve a region's sea level issues, says Levy. “If you get a large quake, it will reverse the sea level problem, but then you’ve got other issues. It’s not exactly something you want to be waiting for,” he says. Other areas are lifting upwards.

New Zealand’s most noticeable uplift is on the Bay of Plenty coast near Matata where, at one point, the land was rising by up to 10mm a year, says Hamling. The rate has since dropped to about 3-4mm -- still as fast as recent sea level rise. In other places, ports or subdivisions have been built on reclaimed or swampy land, that is sinking back towards sea level.

Over the next few years, researchers in the Sea Rise programme (a collaboration between GNS, NIWA and Victoria University of Wellington) will marry the detailed InSAR measurements to global projections to give people better information for their own communities.

Swamped by ice melt?

It may not be long before the gradual local land movements Hamling and Levy are discussing will come to seem irrelevant. 

By 2100, based on current emissions rates, global average sea levels may be around a metre higher than pre-industrial levels, says Nick Golledge, an associate professor at the Antarctic Research Centre at Victoria University of Wellington. The increase is a mixture of melting ice from the glaciers, Greenland and Antarctica, and thermal expansion that happens when the ocean warms and molecules of water move further apart.

Golledge is part of a collaboration between ice sheet modellers, working together to compare models and solidify projections before the next IPCC report. He says that whether we stay inside 1m sea level rise by the end of the century depends not only on greenhouse gas emissions, but on whether global heating triggers runaway instability in West Antarctica, which would continue regardless of future warming. It is possible, though in his view unlikely, that global average sea levels could reach 2m higher by 2100. ”There might be surprises in the system."

Long-term, however, the range and magnitude of possible sea levels starts to rise dramatically. In the next two or three centuries, each polar ice sheet could release several metres of sea level rise apiece, depending how quickly and severely they melt, Golledge says. “Once you’ve put the CO2 in the atmosphere it is going to be warming the atmosphere for millennia,” he says.

Levy is well aware that the small land movements he is studying may come to seem minuscule eventually: ”If we don’t work hard to reduce emissions, and we get significant melting of the polar ice sheets, we won't be thinking about rates of subsidence or uplift,” says Levy. “Significant polar ice melting would swamp these smaller movements.”

But in the next few decades, every few centimetres matters, at least for some communities.

In a shallow, gently sloping bay, just a small amount of added sea level rise could see a storm tide surge a kilometre further inland.

 Preliminary data from GNS Science shows which parts of the New Zealand coast may be subsiding the most (blue) and which parts are lifting the fastest (red) according to InSAR measurements. GNS says the figures on the map are preliminary estimates that will be refined with ongoing research.

Modelling by NIWA has found that, on average, every 10cm of sea level rise could expose 7,000 more New Zealand buildings to the risk of flooding in the event of a severe, once-in-a-hundred years storm tide. Because so many buildings, airports and roads are on the coastline, the biggest increased risk to New Zealand property will happen within 1m of today's storm tide line.  

Whatever happens, 30cm higher sea levels are essentially locked in, says Levy, the leader of the Sea Rise programme.

Just a portion of that total, combined with a dropping coastline, may determine whether a person’s home floods in a big storm, or whether a scenic tourist road remains open. “These vertical tectonic movements amplify the impacts in the short term," he says.

“10cm, or even 20cm or 30cm doesn’t sound like very much – you think, ‘oh we can handle that’ – but it might be the difference between a storm surge that doesn’t inundate your property and one that does," he says. "The same-sized storm after 15cm of sea level rise mightn't have much impact, but at 20cm you can cross a threshold, and it suddenly does. That certainly seems to be the case in places like Wellington, where 30cm of sea level rise turns a 1-in-100 year flooding event into an annual event. Suddenly, it only takes a small storm on top of your high tide and you’re getting the…inundation," says Levy.

Losing their grip

Not only are coastlines rising and falling in quake-prone countries like New Zealand, the ocean itself is rising unevenly around the globe.

As ice melts in Greenland and Antarctica, it reduces the mass of ice at the poles, loosening the poles' gravitational pull on the oceans. Once the poles lose some of their grip on the seas around them, water will flow away from them and towards the opposite end of the Earth. Since Greenland is currently melting faster than Antarctica, this gravitational effect is expected to lead to worsening sea level rise in the Southern hemisphere in the short-term. Long-term the trend is likely to flip, when Antarctica takes over from Greenland as the faster-melting ice sheet.

“We are not only changing the amount of water in ocean, we are redistributing it,” says Nick Golledge, an associate professor at the Antarctic Research Centre at Victoria University of Wellington.

Unfortunately, ocean simulations suggest that some of the countries that can least afford sea level rise -- such as low-lying Kiribati -- will experience sea level rise about 1.3-1.4 times the global average rate, as a result of these gravitational changes, says Golledge. Given that Kiribati sits on average only 2m above sea level, this is a serious potential problem. Both Hawaii and Kiribati are in the central Pacific Ocean, a hotspot for higher-than-average increases. “The Pacific islands are really at the forefront of this, because not only are they very low-lying, they are going to see almost one and a half times the global mean sea level rise, coupled with all the additional storm surge and other impacts from a warming ocean,” says Golledge.

Once New Zealand has been mapped, the Sea Rise researchers hope to tailor their data for island countries such as Kiribati, to improve their local projections.

One thing is already clear, says Levy: Relying solely on global averages could leave some people unprepared for their local reality. A threshold people may think is decades away could be reached much sooner -- or later. "You might be in for a surprise,” Levy says.

- This story was reported with help from the Aotearoa Science Journalism Fund.

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