Impacts: Rising sea levels
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
“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.
How high have global sea levels risen?
How much higher will they rise?
“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
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?‘).
Every coastline responds differently
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.
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).