🤔 What am I looking at? The unlucky places are in bright pink. You might think the ocean would rise evenly around our coastline. This map shows how far from the truth that is - and how widely different the experience of sea level rise will be for different parts of the coastline. Unlucky places in bright pink could see an extra half a metre by the end of the century. Fortunate spots in shades of gold will experience less sea level rise than the rest of us, thanks to quirks of their geology.

Note: this map is based on high (but plausible) greenhouse gas emissions, well above what countries are aiming for under the Paris Agreement. However, the same geographic pattern applies at lower global heating levels, even though the amounts of sea level rise are smaller.

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A huge new public data set shows many communities are sinking, speeding up the effects of sea level rise. A lucky few are rising, making for wildly varying situations around the coast.

ELOISE GIBSON
Climate Editor

FELIPPE RODRIGUES
Data Reporter

You might not notice if you’re strolling along the harbour, but much of Auckland is sinking.

So is much of Wellington, bits of Christchurch, most of the Wairarapa coast and land around Napier airport.

In contrast, much of Tauranga is standing still or even rising gradually out of the ocean.

The tiny settlement of Pīkowai (between Te Puke and Matatā in the Bay of Plenty) is rising fast, in geological terms, while Thames is on a veritable roller-coaster ride. In the space of just 2km of shoreline, Thames goes from rising, to sinking, to rising again.

None of these movements is big enough to bother the average resident as they go about their daily lives - they amount to just 0-4mm of movement a year.

However, new measurements of these movements - released publicly today, for every 2km of New Zealand’s coastline - drastically change how long many communities have to prepare for coastal floods.

Take Lowry Bay, a narrow, rocky swimming beach in Wellington’s Lower Hutt, around the coast from Petone. 

This gravelly piece of seaside is sinking up to 3.8mm a year, coincidentally about the same amount (3.5mm) that the sea around New Zealand is rising. Add the two together and you get double the global average rate of sea level rise.

Surprisingly, small amounts of sea level rise can have devastating impacts: research for former Parliamentary Commissioner for the Environment Jan Wright found just 30 centimetres higher seas would turn a one-in-100 year flood event in Wellington into an annual event for the capital.

That’s the kind of extreme flooding that, today, is considered rare - and it’s big enough to gouge the city’s seawalls.

You might expect the oceans around Aotearoa would rise evenly, like a filling bath - and indeed, that’s what most of our coastal planning assumes will happen. National guidance published by the Ministry for the Environment projected 0.46–1.05m of sea-level rise by 2100, depending on how quickly people and governments cut carbon emissions. Like most countries, New Zealand has based its planning mainly on global average projections from the Intergovernmental Panel on Climate Change (IPCC).  

But as glaciers and polar ice sheets melt and sea water expands with heating, it won’t affect every bit of coast evenly.

Not only do gravity and local factors, such as the slope and shape of harbours, affect how far the flooding goes - it turns out Aotearoa’s coast is itself sinking and rising, by wildly varying amounts.

The new dataset from the NZ Searise project (which includes scientists from GNS Science, Niwa and Victoria University) shows how these movements affect the entire coastline. The project uses Synthetic Aperture Radar Interferometry (InSAR) data; researchers gather and compare radar “pings” received back from satellites bouncing off Earth. By comparing radar images of the same location, they measured movement of the ground surface toward or away from the satellite. They then checked the accuracy of their findings by comparing them with nearby GPS readings. 

It’s not perfect. The measurements are spaced 2km apart (meaning they miss some local changes) and some places have more accurate estimates than others. The results are median numbers drawn from a comparatively short time record (2003-2011), and they assume future land movements will be similar. 

But it’s the first time New Zealand’s whole, wobbly, shifting coast has been tracked with this kind of coverage and accuracy. 

And it shows how out-of-sync with reality our one-size-fits-all approach has been.

🤔 What am I looking at? At first glance, these two ring-shaped graphics might not put you in mind of the North and South Islands. Look closer, however, and they reveal how much the land is rising and subsiding in your region - and the huge variation around our coasts. To find your region, look at the ring representing your island (left is North, right is South). Follow the ring down to move south down and around your island. The west coast is on the left, the east coast on the right, just as they would be on a map. Some regions (such as Auckland and Waikato) straddle both coasts, so you’ll see their west coasts on one side and east coasts on the other. If the jutting lines are shaded pink and pointing towards the middle of the ring, your region is sinking. You’ll see that that’s the case for most of the North Island, aside from a few lucky points where the lines jut outwards into the grey-shaded area, indicating rising land. Each jutting line represents a point on the coast 2km from the last one, which is why Northland and Southland, with their long coastlines, seem to occupy so much of their circles. The length of each line represents how much the land at that point is sinking or rising each year. You can see that one region has the most dramatic movements (clue: it’s also where the politicians live).

“Based on current international emissions reduction policies, global sea levels are expected to rise about 0.6m by 2100,” says Tim Naish, who co-leads the programme. “However for large parts of Aotearoa this will double to about 1.2m due to ongoing land subsidence,” he says. “We have less time to act than we thought.”

Back to Lowry Bay, which neatly demonstrates the problem. The sinking of the land there means that instead of 15cm sea level rise over the next 30 years, the area is in line to get 30 cm, says Richard Levy, the Searise project’s other co-leader. (All projections in the Searise project are against 2005 levels).

“If you're planning for that 30cm threshold that turns a 100-year [flood] event into an annual event, you might think you'd have until 2070 until that threshold was breached, if we managed to track along on the Paris [Agreement] and get our emissions under control,” says Levy.

“But with vertical land movement, you bring it forward to 2040. So you lose 30 years’ of time,” he says.

Residents plan their lives around these kinds of projections. Whether they’re investing in coastal buildings or renovations, looking to buy seaside property, or wondering whether their new abode will qualify for flood insurance, they could be looking at flooding risk maps made by councils, using average sea level rise. Councils rely on projections too, for planning and maintaining town centres, roads, airports and water pipes and writing development rules. 

Even small differences matter: An assessment in 2019 found that for every 10 centimetres of sea level rise, thousands of people and billions of dollars worth of houses - plus more roads, pipes and infrastructure - become exposed to potential flooding during extreme events.

Going up, going down

Land sinks and rises for all kinds of reasons.

There might be an old landfill whose compacted rubble is slowly subsiding beneath the surface (as there is in part of Thames). A town might be built on the slowly-compacting sediment of an old river bed. Or land could be rising due to volcanic activity underneath it, as is believed to be happening to Pīkowai.

One of our biggest cities (Wellington) is on the edge of a tectonic plate that is slowly being pulled under another plate, dragging the land’s elevation down, or “pulling down on the eastern side of the North Island,” as Levy puts it.

That same effect makes Wairarapa, up the coast from Wellington, the fastest-sinking area of the country. It is also the area where some of the most reliable measurements have been taken, says Levy. This gives the Wairarapa coast the dubious honour of having the nation’s quickest rate of effective sea level rise.

If Wellington and Wairarapa were to suffer a huge earthquake along the Hikurangi subduction zone, a faultline which lies off the east coast of New Zealand, the consequences would obviously be devastating. 

But one upside would be that the resulting uplift would probably stop the coastline sinking and allow the ground to spring back up, stalling the impacts of sea level rise. That’s what happened in the Kaikoura earthquake in 2016, when formerly underwater rock ledges suddenly rose out of the ocean. (The effect was temporary. The area is now sinking again).

Pīkowai in the Bay of Plenty is rising so fast that it could outpace sea level rise if the world cuts emissions quickly enough, says Levy. Under the most optimistic scenario, Pikowai may see a small fall in sea levels. (Under less-cheery scenarios, when emissions rise more, the rate of sea level rise will outstrip the land movement.)

The Searise team brought in the latest sea level science, including recent modelling of the Antarctic ice sheets, adding that to a map of the New Zealand coastline made using InSAR.

Layered on top of that is the biggest wildcard of all - planet-heating emissions.

Stuff looked at two scenarios - a plausible-but-pessimistic one (where countries don’t make much effort and various factors conspire against emissions cuts) and a more likely, middle-of-the-road future (where the world acts faster but still misses the goals of the global-warming-fighting Paris Agreement), to show how much emissions matter to sea levels (with or without the coastline moving). You can read more about this in the text box below.

🤔 Two possible futures. Climate projections are uncertain because no one knows how much climate pollution people will make from now. The IPCC gets around this by modelling a huge range of social and economic futures - population, technology, education, international rivalry, inequality and more - and combining these with heating models to show a range of scenarios. We’ve used two scenarios - middling climate pollution, where countries take some climate action but miss their Paris Agreement target of keeping the heat under 2C, and a pessimistic (but happily less likely) scenario, where collective action fails and global heating reaches 3.8-4.2C. In IPCC terms, these paths are known as SSP2-4.5 and SSP3-7.0. There are many other possible futures, including much better ones, and nobody knows which one will eventuate. We’ve used these to show how much difference it will make if governments rein in emissions.

The effects of ongoing climate pollution get particularly noticeable after 2050.

Until 2050, our coast will largely experience the effects of what we’ve already done - pollution we’ve already created. Sea level rise takes a while to unfold, so the heating already caused by our gases is still working its way through the ocean system.

After 2050, though, the paths diverge, depending on emissions. 

At Lowry Bay, for example, by 2100, the difference from middling to higher emissions adds up to about 20cm - enough to make an impact on those people and assets that would be safe under one future, but not the other. 

“20 centimetres matters. If we can get on top of climate change, by 2100, we've saved ourselves 20 centimetres,” says Levy. 

The country is on a pick-a-path adventure, where nobody quite knows what they are planning for - only that they may need to plan for it sooner than they previously thought. 

“How do we adapt to a future where sea level is rising, but our land is also constantly evolving. It's tricky,” says Levy. “We want to build permanent structures on the coast and we expect the land to stop moving and shifting when we've done that. And it's just a bit crazy trying to control and constrain nature.”