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Rising sea level

What

Our region’s seas will continue to rise as greenhouse gas emissions continue to increase and the earth’s climate warms.

Sea level rise happens because rising temperatures warm ocean waters making them expand, and because more water is being added to the oceans from melting glaciers and ice sheets. The melting glaciers in the Southern Alps are part of the process driving sea level rise – although scientists believe we can still save our high-altitude glaciers, if we act now.

There are uncertainties about how fast the sea will rise and when it might reach certain levels. This is because it depends on greenhouse gases and human behaviour, and inter-linked natural processes. Scientific projections therefore cover a range of possible futures.

NIWA climate change projections for the Canterbury/Waitaha region highlight that our sea levels may rise by an average of 30cm in the next 30 years, and by 80cm in 60 to 80 years.

The effect on our coast won’t be like filling a bath, with the water coming up in a tidy line. Low-lying land is more exposed, like the parts of Christchurch that sank during its earthquakes.

Although the picture of how different parts of Canterbury/Waitaha will be affected by sea level rise is still being developed, some communities are already affected.

In the coming decades, tides could slowly stretch further inland as the sea gradually claims low-lying land. Saltwater may reach into rivers and underground aquifers, where our drinking water comes from. Coastal flooding during storms - whether damaging or nuisance - could happen more often, and coastal erosion could affect more properties. Environment Canterbury is advancing a Regional Climate Change Risk Assessment to better understand what will change, where and when.

Impacts

If greenhouse gas emissions are not slowed down or limited, sea level rise could have profound consequences for Canterbury/Waitaha’s coastline. Coastal settlements - including Christchurch, our region’s largest city – have been built on low-lying or flat land which is flood-prone. In those places, flood risk will increase over time, both from sudden storms and from gradual changes to the shoreline and tides. Some pockets of Canterbury are already exposed to coastal erosion and coastal flooding. These areas will become increasingly vulnerable as the sea level continues to rise, and communities will need to decide how to respond.

Rising sea level may lead to more saturated soils beneath the water table, increasing the chance of liquefaction in coastal areas during an earthquake.

Someday, aspects of our heritage may be under water. Canterbury/Waitaha has a rich history and many of us have parts of the coast that are dear to us or to our families. This includes places where memories have been made, as well as places of archaeological, cultural and historical importance.

Our relationship with the coast is part of our identity, and it creates economic opportunities for investment and jobs in tourism, conservation, research, education and the arts. As we adapt to our changing coastline, we will be writing new chapters of history. This will be a story of loss and reinvention.

There are Ngāi Tahu marae and settlements all along the coast. Rising sea levels could affect buildings, land and investments that provide for the tribe. Land, water, taonga species, mahinga kai and landscapes are all important to history and identity. Although consequences may be minimal now, these could become more marked in 30 years, affecting cultural, spiritual and economic wellbeing.

Sea level rise will change our foreshores, estuaries, dunes, and coastal lakes and wetlands. Although climate change is having consequences on these environments now, scientists think transformations could become more extreme by the end of the century. This would affect flora and fauna, including migratory and coastal birds. Erosion of dunes, changing tides in rock pools, and water quality may drive changes in species distribution and threaten vulnerable populations of coastal species.

Sea level rise - and rising, saltier groundwater tables – could affect septic tanks and water bores near the coast. If wastewater systems rely on gravity to work, sewers may back up or spill. We could be dealing with public health and environmental issues well before waves are lapping at waterfront properties.

From time to time, flooding, storms, slips and wash outs affect coastal roads and railway lines. For example, in 2020 a series of high tides gave the railway south of Timaru a battering, disrupting freight and requiring KiwiRail to undertake emergency repairs.

We can expect more of this as the sea rises. We may need to build or enhance protection such as seawalls or rock revetment. New, safer routes may be possible (if expensive) in some places, but not all. Ports, wharves and boat ramps could be affected but those consequences are not considered imminent and might be addressed as marine facilities are renewed.

Sea level rise could increase the risk of flooding during coastal storms, particularly in low-lying parts of Christchurch. Some coastal homes may become more expensive to insure, or even uninhabitable. A recent study says about 4500 coastal homes in Canterbury that are currently within 1km of the coast could face insurance difficulties in coming decades, with flow on effects for the ability to raise a mortgage. This will depend on how insurers and banks respond to climate change risk.

Sea level rise will increase the frequency and scale of coastal hazards. Coastal storms may flood larger areas and erosion may accelerate because waves are able to reach higher and further inland than they could under lower sea levels.

Coastal inundation / flooding may occur when high tides combine with a storm surge and larger than normal waves or swell. Just how big and deep a flood gets depends on the size and timing of the storm and the characteristics of the coastline. Coastal flooding will be exacerbated by the effects of climate change, such as sea level rise.

Coastal erosion can occur either suddenly or slowly and can be temporary or permanent. Erosion only becomes a hazard when it threatens people’s activities or settlements, or other things they value.

One of the impacts of climate change is that councils and landowners need to undertake more complicated hazard assessments that look at how land might behave under the different possible futures under climate change.

For example, a recent coastal erosion assessment for Timaru showed that, generally, eroding areas will continue to erode, possibly faster. In some parts of the Timaru coast, sea level rise has little influence on erosion rates.

In 2019, the Intergovernmental Panel on Climate Change advised that globally, sea levels have risen by 1.5 millimetres each year during 1901-90, accelerating to 3.6 millimetres per year during 2005-15.

Measurements taken at Lyttelton confirm this is happening here.

Although sea level rises and falls naturally depending on the Earth’s climate - the ocean was few metres higher 125,000 years ago and 120m lower 20,000 years ago when ice sheets were super huge– scientists agree that greenhouse gas pollution is a significant driver of the change we are seeing now.

One way to glimpse the shoreline that our children and grandchildren may experience is to observe what happens during very high tides. What we call a very high tide now – something equivalent to king tide level - may become a normal high tide by the end of the century because of sea level rise. These very high tides happen about twice as often today as they did in 1900, increasing the potential for coastal inundation.