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Summary
The 2021 IPCC Sixth Assessment Report (AR6) projected that by 2100 global sea levels will rise at least 0.38m and as much as 0.88m relative to 1995-2014 levels (Fig. 1). Adding that: ‘a 2m sea-level rise by 2100 cannot be ruled out due to potential rapid ice-sheet instabilities’.
…multiple lines of evidence show that +1.5°C is too high and if sustained, is likely to generate several metres of sea-level rise over the coming centuries, causing extensive loss and damage to coastal populations and challenging the implementation of adaptation measures. To avoid this requires a global mean temperature that is cooler than present. – Stokes et al 2025
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Prof. Naish, one of the developers of the NZ SeaRise tool, explains that Antarctica may be melting faster than anyone realizes, and the implications for humanity are potentially disastrous.
Global temperatures reached 1.5°C in 2024. A sea-level rise of multiple metres is now ‘locked in’, but the timing is still uncertain.
Aotearoa’s oceans are warming 34% faster than the global average, accelerating sea level rise (due to thermal expansion). In 2025, 219,000 homes worth NZ$180bn were located in coastal inundation and inland flood zones, along with $26bn worth of infrastructure was at risk.
A SLR of 0.3 m – projected to occur by the second half of this century – would mean that high water levels, currently only experienced once in 100 years, would happen every year in Christchurch. For 0.7 m of rise, current 1-in-100 year levels would occur daily in some locations. – p22 National Climate Change Risk Assessment 2026 Climate Change Commission
The impacts differ from coast to coast. Vertical land movements (VLM) where the land is rising or falling relative to the ocean is a crucial factor for understanding how some coasts will be impacted sooner than others. This is referred to as ‘relative sea-level rise’ (RSLR), that is, SLR+VLM. The NZ SeaRise tool (see below) includes VLM to determine RSLR around Aoteaora. Earthquakes have also altered some coastlines in recent years.
A common response to increasing climate risk is to “harden the coasts” to defend property from inundation. However, engineering solutions like seawalls, stopbanks, and levees only delay damage at best and might even be counterproductive, as it encourages intensification in hazardous locations. Responses to sea level rise insurance retreat should attempt to eliminate the underlying risk by moving homes out of harm’s way. – Storey et al 2020
What’s in this page:
What causes sea levels to change?
Sea levels are constantly changing and the effects are not the same everywhere. This is why sea levels are referred to as ‘mean sea level’ or ‘MSL’; the height of the ocean relative to the land is constantly changing. Rising global temperatures are creating long term changes while also compounding the effects of shorter-duration rises in sea levels. Video 2 is a time lapse series shows the global variations in sea level height due to some of the factors outlined in the table below. Note the large changes in sea levels across the central Pacific due to changes in ENSO (El Niño/La Niña).
The following table summarises the processes that influence sea levels. All are relevant to Aotearoa at different times and in different locations, and many interact with one another, compunding or reducing their impacts.
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Type
Scale
Time frame
1
Eustatic: the volume of the water in the ocean increases when the terrestrial cryosphere (ice caps, glaciers, and permafrost) melts. This accounts for about 54% since 1960. Conversely, when the global climate cools the volume of water in the ocean decreases because rain and snow that falls on the land builds up into glaciers instead of being carried by rivers into the ocean. Glaciers eventually merge to become ice sheets several kilometres thick.
Global
Months to millennia
2
Thermosteric*: the temperature of the ocean increases due to warming. Because warming water expands, it rises relative to the land. This accounts for about 43% since 1960. When the climate cools the ocean also cools and contracts so sea levels fall relative to the land.
Global
Months to millennia
3
Water stored on the land. This includes aquifers as well as lakes, dams, rivers, and wetlands.The capacity to store this is reduced, so the balance is now in the ocean, accounting for about 3% since 1960. (More concentrated precipitation decreases terrestrial water storage).
Global
Months to millennia
4
Vertical land movement (VLM): land slowly rising or subsiding due to compression (including from buildings), slow earthquakes, water or oil extraction etc. SeaRise.nz has an option to use VLM when looking at the effects of sea level rise across Aotearoa.
NOTE:The new Christchurch City Council VLM figures supersede the SeaRise VLM figures for greater Ōtautahi Christchurch. See below.
Regional or local rise & fall
Minutes to millennia
5
Vertical land movement (VLM) – isostacy: the lithosphere and sometimes the crust is compressed when a large load of ice is added to the land, or conversely when melting icecaps add water to the ocean. The lithosphere rebounds (rises) when the weight of ice on the land or in the ocean decreases. The process is so slow that isostatic rebound or compression may continue for thousands of years after the weight has been removed or added. In some locations, land may still be ‘falling’ relative to the ocean even though icecaps melted thousands of years earlier.
Regional rise & fall
Several millennia
6
Steric*: El Niño / La Niña Southern Oscillation (ENSO) and Southern Annular Mode (SAM). El Niños are often associated with Kelvin Waves that push warm water across the Pacific and pile it up against the western coasts of the Americas, temporarily raising sea levels up to 30cm.
Regional rise & fall
Months or longer
7
Steric: Interdecadal Pacific Oscillation (IPO). Like ENSO, this also alters trade winds and ocean temperatures.
Regional rise & fall
20-30 years
8
Thermosteric: local and regional seasonal temperature changes.
Regional rise & fall
Seasonal
9
Halosteric: changes in volume of freshwater entering the ocean due to floods/melting ice and permafrost etc. Freshwater is less dense than saltwater so locally, sea levels may be lower even though the overall addition of water to the ocean raises average sea levels.
Local rise & fall
Seasonal
10
Chaotic interactions: seiche effect, where water flows back and forth in a confined space, just like tipping bathtub back and forth. This happens in Pegasus Bay where Ōtautahi Christchurch is located.
Local rise & fall
2-4 hours in Pegasus Bay
11
Atmospheric pressure: storms/cyclones. Low pressure allows the ocean to rise higher because there is less ‘pressure’ pushing down on it. Combined with wind-driven waves, these storm surges can raise sea levels significantly.
Regional rise & fall
Hours to days
12
Tides: lunar and solar. The episodic wobble of the Moon leads to higher than normal tides that can last for months.
Regional rise & fall
Episodic daily
13
Tsunami: tectonic (earthquakes and volcanoes) & underwater landslides can displace water over large distances.
Regional or local rise & fall
Hours to months, depending on the impacts
14
Tectonic: earthquakes, e.g. Christchurch and Kaikoura (see below Explainer: Effects of earthquakes). This may also change the long term rate of VLM over long periods.
Regional or local rise & fall
Seconds to minutes
15
Tectonic: volcanoes create new land (e.g. Surtsey island), extend coastal lands (e.g. Iceland & Hawaii), or destroy land (e.g. Hunga Tonga-Hunga Ha’apai; not including the impacts of tsunamis)
Regional rise & fall
Minutes to millennia
16
Changes in terrestrial water storage: non-cryospheric water held in rivers, lakes, dams, and aquifers.
Global rise or fall
Decades to millennia
17
Gravitational#: changes in local gravity due to mass changes in terrestrial ice-sheets. See the Explainer below. The impact is strong enough to affect Earth’s rotation.
Regional rise or fall
Centuries to millennia
18
Dynamic coastal processes – waves and swash: as the climate warms, wave heights may be reduced along the Canterbury coast.## However more destructive storm waves may increase due to increasing storminess and cyclones tracking further south. ### See the NIWA coastal sensitivity index map.
Regional/local rise or fall
Hours to days
19
Dynamic coastal processes – currents & sediment budget: how much sediment enters or leaves a coastal area over time is called a sediment budget. More sediment = accretion (builds vertically) and possibly progradation (builds horizontally, that is, seaward). Less sediment = erosion. So much sediment was delivered by rivers to the coast that it created the Canterbury Plains. Following major earthquakes along the Alpine Fault, large pulses of sediment carried by rivers to the West Coast created dune fields while currents carried some sediment north, creating Farewell Spit. See below and Canterbury case studies: Kaitorete Spit.
Local rise or fall
Hours to millennia
20
Dynamic coastal processes – wind/vegetation: high winds over soft (sandy/gravel) coastlines not vegetated with native plants are prone to erosion. Conversely, dunes with native plants accumulate sediment (accretion) or at least slow the rate of erosion. See Fig. 7.
Local rise or fall
Hours to years
21
Currents – global-scale circulation patterns: transferring heat, these impact regional and local weather and wave patterns that contribute to changes in sea levels. See currents.
Local rise or fall and global heat exchange
Ongoing
*Thermosteric (heat) and halosteric (salinity) are together referred to as ‘steric’ changes. Where ocean waters are warmer and/or saltier (more dense), sea levels are higher relative to regions of cooler and/or less saline water
(less dense).#There is a deep gravitational ‘hole’ over Antarctica and a part of the Indian Ocean. See the Explainer below: Gravity and sea levels.
##2022: Albuquerque et al; On the projected changes in New Zealand’s wave climate and its main drivers, New Zealand Journal of Marine and Freshwater Research
###2022: Shaw et al; Stormier Southern Hemisphere induced by topography and ocean circulation, PNAS 119 |5 (also see Shaw, Guest post: Why the southern hemisphere is stormier than the northern; Carbon Brief (open access plain English article on the above research).
How high could sea levels rise?
The relationship between the amount of CO2 in the atmosphere, global average temperatures, and sea levels is well understood, well documented, and goes back many millions of years. During cold glacial epochs (colloquially called ‘ice ages’) the amount of CO2 in the atmosphere was around 180ppm. During warm interglacial epochs CO2 was around 280ppm. That extra 100ppm of CO2 corresponded to a 6-8°C increase in temperature and 25-metre higher sea levels (Fig. 2).
In April 2026 CO2 was 430ppm. That’s 150ppm more than it was 125,000 years ago when sea levels were 5-10m higher (Last Interglacial). It’s even higher than it was 3 million years ago (Mid-Pliocene Warm Period) when sea levels were 20-25m higher (Fig. 2).
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Terms: ‘CE’ = Common Era, ‘ka’ = thousand years ago, ‘Ma’ = Million years ago.
It takes time for temperatures and sea levels to catch up with the effects of so much CO2 but the rate has doubled (Fig. 3). The world is now committed to irreversible sea level rise for centuries to come. Recent observations of Greenland and Antarctic ice sheets (summarised in the 2025 State of the Cryosphere Report) adds to the growing body of evidence that melting ice sheets are reaching dangerous tipping points. That is, abrupt sea level could occur sooner than anticipated.
Could sea levels rise 2m within our lifetimes?
Following the Last Glacial Maximum ~20,000 years ago the world began to transition into a warmer climate. The ice sheets that covered much of the planet began to melt. Some meltwater flowed into the ocean, gradually raising sea levels. There were also several periods when huge volumes of meltwater pooled into vast lakes dammed by the natural shape of the land, the walls of the retreating glaciers, and/or ice shelves along coasts. The lakes kept filling until they eventually collapsed, and the meltwater rushed into the ocean. Up to 16 of these sudden meltwater pulses (MWP) abruptly raised sea levels over the past 19,000 years. The average rate was 40–60mm/year, however there were periods where it rose much faster (Video 3 & Fig. 4).
Around 13,000 year ago, for several centuries, sea level was rising ~4 metres per century. – Rignot Video 3
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MWP-1Ao: ~19,600 – 18,800 years ago sea levels rose at least 10 metres. MWP-1A: ~14,700 – 13,500 years ago sea levels rose 16 – 25 metres. Much of this occurred over a 400–500 year period. MWP-1B: ~11,300 – 11,000 years ago sea levels rose 40 – 80 metres.
In 2022, the State of the Cryosphere report concluded that under the high emissions scenario:
WAIS [West Antarctic Ice Sheet] collapse would be inevitable and potentially rapid, with sea-level rise of 2 meters possible by 2100, and up to 5 meters by 2150; 10 meters sea-level rise from all sources is likely by 2300. Sea-level rise will continue for many centuries even with temperature stabilization and slow decline, with the eventual complete loss of the Greenland ice sheet. Such a rapid rise in atmospheric CO2 concentrations and temperature has no counterpart in Earth’s geologic record, but Antarctica is known to have had essentially ice-free conditions at +6°C above today’s level. Restoration of the polar ice sheets would only begin with temperatures well below pre-industrial (i.e., substantial global cooling). – p7
In 2026 CO2 levels were 431ppm and temperatures have exceeded 1.5°C for the past three years. So yes, sea levels could rise 2m within the lifetimes of people today.
Using the online SeaRise NZ mapping tool
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This is a composite of four screen grabs of the NZ SeaRise tool taken 25 April 2026. When using the tool online, zoom to a location and click one of the dots on the map. Blue dots indicate the land is falling (-VLM) relative to the ocean. In this example, the four screen grabs show four possible scenarios at site #4299. In the menu ‘Projection to 2300’ (low confidence) +VLM was selected. This was chosen because the Coastal Hazards and Climate Change Guidance 2024 recommends using the ‘worst case’ SSP5-8.5H+ (p83) scenarios:
This guidance takes a risk-based approach to the use of projections and/or increments of RSLR (relative sea levels). In this respect, the upper-range SSP5-8.5 H+ should continue to be used in screening and detailed hazard and risk assessments to identify coastal areas potentially affected (Policy 24, NZCPS, DOC, 2010) and high-end stress testing of adaptation options and pathways... – p53Select a year; in this example the years 2050, 2100, 2200, and 2300. The terms in the horizontal breakout boxes: ‘p17’ is the lower end of the estimate, ‘p83’ is the upper ‘worst case’ scenario, and ‘p50’ is the mid-range scenario for each year selected. So, at site #4299 by 2050 sea levels might rise as little as 0.12m (p17) or as much as 0.57m (p83) with 0.32m (p50) the mid-range scenario. This rise in sea levels is relative to the baseline period ~2005. See another example: Southshore Spit or click this image to go to the website and select any coastal area around Aotearoa.
What are the effects of sea level rise?
Erosion
- Areas where parts of the coastal area is sinking, such as areas around Christchurch and North Canterbury (see also above: Using the SeaRise mapping tool).
- Along beaches that have bigger waves
- How much sediment is being taken away versus how much is delivered from other coastal areas (‘sediment budget’)
- The depth of the water off the beach.
Saltwater intrusion
As the seas rise, saltwater reaches further upstream into rivers, hāpua, and aquifers. For example the Kaiapoi River:
In low elevation areas where coastal aquifers are not sealed, saltwater intrusion may lead to rising groundwater and turn freshwater into saline water. This can result in more frequent surface flooding, particularly during high tide or storm surges. The salty water can also corrode the foundations of buildings, underground pipes and cables, and kill grass, trees, and agricultural crops.
The effect of rising groundwater will also compound the risk of flooding from rain and rivers.
Coastal Flooding
Using the Coastal Flood mapping tool (Fig. 9)
Before using this tool, check the effects of VLM using the SeaRise tool (see above). VLM along the coastline used in the above example was assessed at being -1.7mm/yr. This figure was inserted into the calculator shown on the right of Figure 9 under ‘Choose vertical land motion’. The ‘worst case’ sea level rise scenario (SSP5-8/5H highest percentile) by the year 2075 was also selected. This shows that under these circumstances, in the event of a AEP1% storm tide flood, because sea levels could be 90cm higher than they were in 2005, the yellow-coloured area could be inundated. This is based on the current ‘bathtub’ topography, and does not factor in erosion.
NOTE: Using the SeaRise tool before using this tool shows the effect of VLM across the length of this coast varies from -1.3mm/yr (Marshlands) to -0.3mm/yr (Saltwater Creek), with the area around Kaiapoi/Pines Beach being the worst (-1.7mm/yr.). Selecting the ‘worst’ value is essential because water will flow inland through the lowest, not highest points. This illustrates the range of variables that should be considered when using these tools. Screen shot of the NIWA flood layers tool. This enables users to visualise the effect of a 1% annual exceedance probability (AEP1%) extreme sea level flooding under current climatic sea conditions COMPOUNDED by the effects as sea levels rise above present-day mean sea level.
Inundation
Figure 10 is a generic inundation map. Inundation maps are based entirely on existing topography. They’re useful where a coastline is solid rock, but they do not factor in how dynamic coastal processes (wind and waves, especially storm waves) change the shape of sandy and gravel beaches, estuaries, bays, harbours, or hāpua, or how the coast would change of a sea-wall or other engineered structure changed those dynamics. However, this coastline is a sandy dune system. As sea levels rise and reach further inland, waves will undercut the front of any dunes still vegetated with pine trees (Fig. 7).
Changing & migrating coastal ecosystems: what’s in a name?
In 2023, the Environmental Defence Society (EDS) published several papers discussing policy frameworks for ‘Managed Relocation’ from the coast, suggesting that the word relocate was more appropriate and empowering than managed retreat. Retreat implies defeat and with it, a desire to protect, even retaliate through inappropriate, unsustainable maladaptive behaviours.
Similarly, marine transgression is the geological term for sea level rise forcing the shoreline to move to higher ground. This causes flooding, saltwater intrusion into freshwater systems via tidal and groundwater systems, and ultimately, permanent oceanic inundation by ‘transgressing’ coastal areas.
In the English language, the word transgression means ‘an act that goes against a law, rule, or code of conduct; an offence or sin’. Thus, the term marine transgression positions the ocean as the transgressor, rather than the ocean responding to the anthropogenic transgressions causing climate change. For this reason, the term marine migration is more useful frame of reference to acknowledge the corresponding forced migration of entire marine and coastal ecosystems.
Even if greenhouse gas emissions are reduced to zero, sea levels will continue to rise for hundreds of years until they reach equilibrium with global temperatures, which are still increasing. Both coastal and marine ecosystems will be in a continuous state of migration for generations to come. Some species will be lost. Some marine species will try to migrate south to cooler waters, or land species will migrate inland away from the salt. Welcoming these climate refugees rather than treating them them as invaders, will ensure a better transition for all. For example, saltwater marshes or mangrove forests may replace seagrasses, and freshwater lakes may become estuaries and hāpua. Removing structures on coasts that may contain pollutants and toxins will be an essential component of any adaptation strategy.
Insurance Retreat
As sea level rise changes the frequency of inundation events, 99% of properties currently within 1% AEP coastal inundation zones can expect at least partial insurance retreat within a decade (with less than 10cm of sea level rise). Our modelling predicts that full insurance retreat is likely within 20–25 years.
Since mortgages are typically conditional on insurance (especially at the time of issuance), the withdrawal of residential insurance (i.e., insurance retreat) results in an immediate drop in property values as this retreat decreases the pool of potential buyers. – Story et al 2024
