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Impacts: Rising sea levels

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Rising sea levels

Summary

“…72,000 New Zealanders [are] currently exposed to present-day extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion. The risk exposure increases markedly with sea-level rise, particularly during the first metre of rise, which means long-term planning to address the risk is urgent. There is near certainty that the sea will rise 20-30 cm by 2040.” – NIWA 2019

“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

  • Sea levels rise relative to the land for many reasons (Fig. 8) including land sinking or rising (top image), sometimes during earthquakes like the Kaikoura quake (Video 1).
  • Sea level rise due to climate change is accelerating because the ocean is warming and the cryosphere—Greenland and Antarctic icecaps, mountain glaciers and permafrost—is melting at an unprecedented pace.
  • Current IPCC models of sea level rise specifically exclude rapid and widespread melting now being observed in Antarctica and Greenland. This is like making a calculation of 1+0.1+?=1.1, where 1.1m is the current maximum estimate of sea level rise by 2100 and ? is missing data. The absence of date to fill in the unknown ? is stated clearly in the IPCC and New Zealand SeaRise tool, however this is often overlooked by the media and policymakers.
  • There is also a lag time between rising temperatures and sea levels. We are already committed to a rise in sea-levels of multiple metres (Fig. 3), but the time frames are uncertain due to uncertainty around the tipping points of melting ice sheets. For example:

“The glacier in Greenland’s largest drainage basin is thinning and its flow is accelerating. Updated simulations suggest that sea-level rise will be up to five-fold higher than previously expected.“ – Khan et al, November 2022

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Home > Climate wiki > Impacts > Rising sea levels

Summary

“…72,000 New Zealanders [are] currently exposed to present-day extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion. The risk exposure increases markedly with sea-level rise, particularly during the first metre of rise, which means long-term planning to address the risk is urgent. There is near certainty that the sea will rise 20-30 cm by 2040.” – NIWA 2019

“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

  • Sea levels rise relative to the land for many reasons (Fig. 8) including land sinking or rising (top image), sometimes during earthquakes like the Kaikoura quake (Video 1).
  • Sea level rise due to climate change is accelerating because the ocean is warming and the cryosphere—Greenland and Antarctic icecaps, mountain glaciers and permafrost—is melting at an unprecedented pace.
  • Current IPCC models of sea level rise specifically exclude rapid and widespread melting now being observed in Antarctica and Greenland. This is like making a calculation of 1+0.1+?=1.1, where 1.1m is the current maximum estimate of sea level rise by 2100 and ? is missing data. The absence of date to fill in the unknown ? is stated clearly in the IPCC and New Zealand SeaRise tool, however this is often overlooked by the media and policymakers.
  • There is also a lag time between rising temperatures and sea levels. We are already committed to a rise in sea-levels of multiple metres (Fig. 3), but the time frames are uncertain due to uncertainty around the tipping points of melting ice sheets. For example:

“The glacier in Greenland’s largest drainage basin is thinning and its flow is accelerating. Updated simulations suggest that sea-level rise will be up to five-fold higher than previously expected.“ – Khan et al, November 2022

Effects of sea level rise

Sea level rise does not look like the ocean coming at us… It looks like the groundwater coming up.” – Tada, 2020

  • Flooding (temporary, generally poor drainage)
  • Erosion
  • Inundation (permanently drowned coastlines)
  • Saltwater intrusion into rivers, hapua, and aquifers
  • Inland migration or loss of estuaries and hapua depending on topographical constraints
  • Changing coastal ecosystems; loss of biodiversity and mahinga kai
  • Loss of insurance, property, and infrastructure

Sea levels are constantly changing

Sea level is generally referred to as ‘mean sea level’ or ‘MSL’ because the height of the ocean relative to the land is constantly changing for a multitude of reasons (Fig. 8). Some changes, like waves and tides, are episodic and temporary. Others such as El Niño /La Niña and the episodic wobble of the Moon leading to higher than normal tides over longer periods, can last for months, while earthquakes can change the coast in minutes (Video 1) or over millennia. Taking water for irrigation and forcing Canterbury’s braided rivers into narrow channels to prevent floods, has also prevented the rivers from delivering sediment to the coast, replacing what was taken by waves.

“The conventional wisdom is that you harvest flood water in the winter and store it until it’s needed (for agriculture) in the summer. However, floods are required to carry gravels to the coastal zone and if there’s not enough gravel, the waves get hungry and start eroding the land. Sure, it’s [storing water] a solution, but it’s also creating a problem.” – Dr Scott Lanard, NIWA

Video 1: In less than 2 minutes the Papatea Fault, part of the 2016 Kaikoura earthquake sequence, raised the seabed up to 3m in places. However, rising sea levels will still affect areas of the coast as only parts of the shoreline were uplifted and the sea can still reach inland behind the raised reefs. (Geonet NZ)

“The data show that the subsidence we observed before the Kaikoura earthquake resumed within a year after the earthquake (in fact subsidence/sinking rates are much higher). So, while the land generally went up fast during the earthquake, it has since resumed subsiding. The earthqauke reset the coastline datum (instantaneously), but the pattern of long term subsidence continues.” – SeaRise FAQs

Sea levels driven by climate change

When the climate cools, eustatic (global) sea levels drop 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. At the same time, the ocean cools and contracts. Both if these result in sea levels dropping.

When the climate warms, glaciers and ice sheets on the land melt and drain into the ocean. At the same time, the ocean warms and expands, so sea levels rise.

Today, eustatic sea levels are rising because Earth’s climate is getting warmer. While this is a global change, sea levels are not the same everywhere because of local and regional factors including salinity, temperature, currents, gravity, and sinking coastlines (Fig. 8 & Videos 2 & 3). This means rising sea levels affect different coastlines in different ways.

Video 2: What’s happening in Antarctica and what it means for sea levels around Aotearoa.

How high have global sea levels risen?

Fig 1: Instructions for interactive graphs (Credit: The 2°Institute.)

  • Mouse over anywhere on the graphs to see the changes in global sea levels over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time icon.

Fig 1: Instructions for interactive graphs (Credit: The 2°Institute.)

  • Mouse over anywhere on the graphs to see the changes in sea levels over the last thousand years.
  • To see time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, then release it.
  • To see how this compares to the past 800,000 years, click on the ‘time’ icon on the top left.
  • To return the graphs to their original position, double-click the time icon.
Video 3: Sea level rise variations 1993 -2021 (Aviso CNES). This time lapse series shows the global variations in sea level height due to some of the factors outlined in Figure 8 below. Note the large changes in sea levels across the central Pacific due to changes in ENSO (El Niño/La Niña).

How much higher will they rise?

Fig. 2: Projected rise in sea levels under different emissions pathways. We are currently on the SSP5-8.5 pathway. term 'low likelihood' has been criticised is it refers to ice-sheet instabilities now being seen in Greenland and Antarctica (image: IPCC AR6 WG1 p22). See Video 4.
Fig. 2: Projected rise in sea levels under different emissions pathways. We are currently on the SSP5-8.5 pathway. term ‘low likelihood’ has been criticised is it refers to ice-sheet instabilities now being seen in Greenland and Antarctica (image: IPCC AR6 WG1 p22). See Video 4.
Sea levels don’t instantly respond to warming, just as the climate doesn’t instantly respond to adding too many greenhouse gases into the atmosphere. Earth is large, and there’s a lag time, so we know that sea-levels will continue to rise until a new equilibrium is reached. We are now reaching dangerous tipping points, where ice sheets, particularly Greenland may soon collapse, which will dramatically accelerate rising sea levels (Videos 2, 3, 4 & 5)

“The models used by the Intergovernmental Panel on Climate Change predict a sea level rise contribution from Greenland of around 10 centimeters by 2100, with a worst-case scenario of 15 centimeters. But that prediction is at odds with what field scientists are witnessing from the ice sheet itself. According to our findings, Greenland will lose at least 3.3% of its ice, over 100 trillion metric tons. This loss is already committed – ice that must melt and calve icebergs to reestablish Greenland’s balance with prevailing climate.” – Alun Hubbard, Prof. of Glaciology, University of Tromsø, August 2022

“A sobering thought is that even if we somehow managed to turn global warming off right now, the atmosphere would keep warming for some years to come because of the heat that’s stored in the ocean.” – Dr Craig Stevens, NIWA

Fig. 3: The last time CO2 in the atmosphere was 400ppm, sea levels were approximately 25 metres higher than today. In 2019, we were around 405ppm. In 2023 we had reached 420ppm. (image: Alley et al, Ice Sheet & Sea Level Changes)
Fig. 3: The last time CO2 in the atmosphere was 400ppm, sea levels were approximately 25 metres higher than today. In 2019, we were around 405ppm. In 2023 we had reached 420ppm. (image: Alley et al, Ice Sheet & Sea Level Changes)
Video 4: “The Arctic is currently warming 3 to 4 times faster than the rest of the world. This is affecting the lives of billions of people, both in the Arctic and beyond.”
Video 5: “The world needs to prepare for multiple metres of sea level rise… Geological evidence shows that at today’s level of [atmospheric] CO2, sea levels should be about 20m higher.” – Prof. Jason Box, November 2022

Updated (2022) estimates used by the Ministry for the Environment (MfE) explicitly exclude potential impacts resulting from the extraordinary rate of ice being lost in Greenland and Antarctica these past few years. This exclusion is because the effects have yet to be modelled. Absent these unknown impacts, MfE estimates are:

  • 20cm – 30cm by 2040
  • 50cm – 1.1m by 2100

Research and observations of melting ice sheets in Greenland and Antarctica the 2022 State of the Cryosphere, make it clear that if we don’t cut carbon emissions, we should plan for a minimum 2-metre rise this century.

We know sea-levels can rise even faster because 13-14,000 years ago, they rose as much as 2 metres in 50 years following a Meltwater Pulse Event (Video 7). Today, Earth is warming far faster (see ‘How hot could it get?‘).

Video 6: “The last time the world was 4°C warmer, the Ross Ice Shelf was gone, the West Antarctic Ice Sheet was gone. Sea level was about 20m higher than it is today.” – Prof.Tim Naish, Victoria University of Wellington

Every coastline responds differently

Video 7: “Around 13,000 year ago, for several centuries, sea level was rising about 4 metres per century.” – Prof. Eric Rignot University of California and Senior Research Scientist for NASA’s Jet Propulsion Laboratory

Around New Zealand, coasts have built up over millennia by rivers that carried sand and gravel (alluvium) to the coast, ash and lava from volcanoes, peat and mud in wetlands and lagoons building up over time, and earthquakes that have lifted the land or ancient seabeds and coral reefs, or caused the land to drop.

In the future, as temperatures rise, storms will become stronger and waves are likely to become larger, so ‘soft shore’ coastlines—sand, gravel and rocks, mud, ash etc—will erode faster than hard rocky coastlines. In places where these ‘soft’ coasts are cliffs, such as South Taranaki and South Canterbury, rising sea levels means the waves will reach higher and further inland, undercutting the soft cliffs and causing them to collapse. Some of this eroded material may be carried along the shore by currents and washed up on nearby beaches, but the land above the cliffs will be lost (see Canterbury case studies) as shorelines recede.

Flooding: a risk multiplier

“By the end of the century, depending on whether global greenhouse gas emissions are reduced, it could rise by between 0.5 to 1.1 m, which could add an additional 116,000 people exposed to extreme coastal storm flooding.” – NIWA

Low-lying coasts near rivers are particularly vulnerable during storms. Low-pressure systems raise sea levels, storm waves are bigger that normal waves and reach further inland, and the water from rain-filled rivers and rain-drenched land can’t drain away. Together, this can result in widespread flooding inland as well as along coastlines, and much more intense coastal erosion.

Small sea-level rise increments of 10–20cm predicted to happen around the NZ coast in the next 20–30 years may not seem like much. But the number of times coastal areas are likely to flood is increased. According to NIWA the current exposure to coastal flooding across New Zealand is several billion dollars (Fig. 7).

Inundation maps: ‘bathtub’ estimates

Inundation maps are based on topography. They’re useful where a coastline is a hard rocky shore (like the edge of a bathtub), but they do not factor in how dynamic coastal processes (wind and waves, especially stormwaves) will change the shape of estuaries or hapua, how river flows will also change where these reform and migrate inland, or how much sand and gravel that high storm waves will erode and carry into water that’s too deep for smaller gentler waves or currents to bring back onshore.

Consequently, a 2m sea level rise ‘inundation map’ is not a true picture of what will happen. For example Fig. 4 is Pegagsa soft shore coast along the Waimakariri. These types of coasts are like sandcastles. As sea levels rise, some ‘high’ areas such as dunes will be eroded and pushed inland and will not, as the image seems to indicate, become islands. These maps also don’t factor in the effect of engineering works, such as sea-walls and groins.

See this map of Pegasus Bay showing the coastline as it was 9,500 years ago. If you live in Christchurch or the Banks Peninsula, see the city Councils’ risk hazard map here.

Fig. 4: This is a snapshot of an inundation map of the Waimakariri River mouth and Ashley Rakahuri River estuary north of Christchurch. The coastline here is 'soft', so coastal processes (waves and wind) will erode the sediment like a sandcastle. Instead of the higher points becoming islands, they will be eroded. Some eroded material will be pushed inland or carried longshore or offshore into deep water, so the coastline will not look the same as it appears in the image. Click on the image to use the interactive online tool to view other areas around New Zealand and the world, based on different temperatures and sea level heights.
Fig. 4: This is a snapshot of an inundation map of the Waimakariri River mouth and Ashley Rakahuri River estuary north of Christchurch. The coastline here is ‘soft’, so coastal processes (waves and wind) will erode the sediment like a sandcastle. Instead of the higher points becoming islands, they will be eroded. Some eroded material will be pushed inland or carried longshore or offshore into deep water, so the coastline will not look the same as it appears in the image. Click on the image to use the interactive online tool to view other areas around New Zealand and the world, based on different temperatures and sea level heights.
Fig. 5: (Image: NIWA)
Fig. 5: (Image: NIWA)
Overall, erosion will happen faster along beaches that have bigger waves. Recent modelling  indicates that the southern and western parts of the country will be affected by higher waves, whereas wave heights along the eastern coasts may be less.  However, the number and scale of storms overall is predicted to increase as the climate warms, and it is storm waves that do the most damage. While rocky volcanic cliffs such as those around Banks Peninsular and low lying rocky beaches won’t erode much in our lifetimes, low-lying areas will eventually be inundated (drowned) by rising seas.
 
NIWA’s ‘coastal sensitivity index’ (map) takes multiple factors into consideration to map the vulnerability of coastlines to erosion (Fig. 5).
 
The impacts will be compounded in areas where the land is sinking, such as areas around Christchurch and North Canterbury (SeaRise map) (Fig. 6.)
 
Unless an earthquake lifts an entire coastline evenly, parts of the coastline will still be affected by rising sea levels. The Papatea Fault (Video 1) for example, lifted a section of the seabed at an angle to the coast, so rising seas will still reach the beach. However, the newly uplifted areas may help reduce the impact of wave erosion.
 
Fig. 6: Click on this image to be taken to the SeaRise map. Here, you can zoom in to any area, to see how much land is sinking along coastal areas. In these areas, the effects of rising sea levels will be felt sooner. These effects include: erosion, made worse by storm surges; poor drainage, especially after heavy rains or flooding; and salt water entering what were freshwater aquifers, affecting wells and coastal wetlands.
Fig. 6: Click on this image to be taken to the SeaRise map. Here, you can zoom in to any area, to see how much land is sinking along coastal areas. In these areas, the effects of rising sea levels will be felt sooner. These effects include: erosion, made worse by storm surges; poor drainage, especially after heavy rains or flooding; and salt water entering what were freshwater aquifers, affecting wells and coastal wetlands.

Current coastal flooding exposure by region: this risk increases as sea levels continue to rise

Fig. 7 (image: NIWA)
Fig. 7 (image: NIWA)

Reasons why sea levels change, and why they are not the same everywhere

Fig. 8 below: Sea levels rise and fall relative to the land for many reasons. Every strip of coastal land responds differently to the interplay between complex and ever-changing dynamic forces, from wind, waves and storms to melting ice caps, and rivers no longer delivering enough gravel to the coast, all of which need to be considered when making decisions about managing and living on or close to coastal environments. A national coastal-change database incorporating multiple elements will be publicly available by the end of 2024.

1

Eustatic: contribution from terrestrial cryosphere (melting and collapsing ice caps, glaciers, and permafrost) SeaRise.nz has mapped Aotearoa coasts

Global

Months to millennia

2

Thermosteric: thermal expansion of water due to global warming, horizontally constrained by landmasses, is forced to rise

Global rise

Months to millennia

3

Steric:* El Niño / La Niña Southern Oscillation (ENSO) and Southern Annular Mode (SAM)

Regional rise & fall

Months or longer

4

Steric: Interdecadal Pacific Oscillation (IPO)

Regional rise & fall

Decades

5

Thermosteric: local and regional seasonal temperature changes

Regional rise & fall

Seasonal

6

Halosteric (salinity): changes in volume of freshwater entering the ocean due to floods/melting ice and permafrost etc.

Local rise & fall

Seasonal

7

Chaotic interactions: seiche effect, e.g. 2 hours in Pegasus Bay.

Local rise & fall

2 -4 hours in Pegasus Bay

8

Atmospheric pressure: storms/cyclones

Regional rise & fall

Hours to days

9

Tides: lunar and solar

Regional rise & fall

Episodic daily

9

Tides: lunar and solar

Regional rise & fall

Episodic daily

10

Tsunami: tectonic & underwater landslides

Regional or local rise & fall

2 -4 hours in Pegasus Bay

11

Tectonic: earthquake, e.g. Kaikoura

Regional or local rise & fall

Seconds to minutes

12

Tectonic:  volcanoes creating land (e.g. Iceland & Hawaii) or destroying land (e.g. Hunga Tonga-Hunga Ha’apai, not including the impacts of tsunamis)

Regional rise & fall

Minutes to millennia 

13

Vertical land movement: land slowly rising or subsiding due to compression, slow earthquakes, water or oil extraction etc.  SeaRise.nz has mapped Aotearoa coasts

Regional or local rise & fall

Minutes to millennia

14

Vertical land movement – Isostacy: the lithosphere (and sometimes the crust) is either compressed when a large load of ice (on land) or water (in the ocean) is added. It rebounds when ice or water is removed. The process is so slow that rebounding or compression may continue for thousands of years after weight is removed or added.

Regional rise & fall

Several millennia

15

Changes in terrestrial water storage: non-cryospheric water held in rivers, lakes, dams, and aquifers.

Global rise or fall

Decades to millennia

16

Gravitational: changes in local gravity due to mass changes in terrestrial ice-sheets.

Regional rise or fall

Centuries to millennia

17

Dynamic coastal processes – waves and swash: in general, possibly reduced along the Canterbury coast as the climate warms#, however more destructive storms waves possibly increased due to increasing storminess## NIWA coastal sensitivity index map.

Regional/local rise or fall

Hours to days

18

Dynamic coastal processes – sediment budget gravels from braided rivers restricted or carried into deep water means it becomes unavailable; excess = accretion; insufficient = erosion. NIWA coastal sensitivity index map.

Local rise or fall

Hours to millennia

19

Dynamic coastal processes – wind/vegetation: high winds over soft coastlines not vegetated with native plants erode. Well vegetated areas can do the opposite by accumulating sediment (accretion).

Local rise or fall

Hours to years

20

Dynamic coastal processes – currents: longshore transport of sediment.

Local rise or fall

Ongoing

*  Thermosteric (heat) and halosteric (salinity) are together referred to as ‘steric’ changes. Where ocean waters are water and/or saltier (more dense), sea levels are higher relative to regions of cooler and/or less saline (less dense).

# 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).

New Zealand stories

Several well-research stories that cover the impacts on New Zealanders are published by Stuff, some using a storytelling multi-media platform. These stories breath life into what is now an everyday reality, not a distant problem, for many Kiwis:

Fig. 9: After debris flow from the valley behind hit homes in 2005, Matatā residents were asked to shift due to the risk. (Image: Dominico Zapata/ Stuff)
Fig. 9: After debris flow from the valley behind hit homes in 2005, Matatā residents were asked to shift due to the risk. (Image: Dominico Zapata/ Stuff)

More information

  • Explainer: Abrupt sea level rise (Meltwater Pulse events)

    At the end of the last glacial (not the end of the ice age: we still are in the Pleistocene Ice Age) the ice sheets that covered much of the planet began to melt. While some meltwater flowed into the ocean, gradually raising sea levels, huge volumes of water also pooled into vast meltwater lakes dammed by the natural shape of the land, the (still melting) glaciers, and/or ice shelves along coasts. As the Earth continued to warm, the lakes kept filling until they eventually collapsed and burst open (Video) and the meltwater rushed into the ocean.

    In other areas, coastal glaciers (ice shelves) collapsed, allowed glaciers and ice sheet covering the land to quickly disgorge into the ocean. These ‘Meltwater Pulses’ (MWP) raised sea levels rapidly. Some events, supported by geological evidence, match stories of great floods from the cultures known to have inhabited the affected regions at that time. This includes the land bridge between Alaska and Russia (Beringia) and the UK and Europe (Doggerland).

    There is still some uncertainty about how many of these meltwater pulse events occurred, as sea levels continued to rise at different rates until about 4,500 years ago. After this date, sea level and the global climate remained relatively stable.

    Some researchers suggest there may have been 15 or 16 MWP events between 17,500 and 4,500 years ago. The fastest appears to have been MWP-1A (Image below: NASA).

    • 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. The average rate was 40–60 mm/year, however there were periods of extremely rapid rise of up to 4 metres/century, for several centuries.
    • MWP-1B: ~11,300 – 11,000 years ago sea levels rose 40 – 80 metres.