From 1880 to 2012, global average temperatures warmed by 0.85ºC, as reported in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report [IPCC website].
The ocean is absorbing 90 per cent of the heat added to the climate system. This warming is causing an expansion of ocean water which, in combination with water from the melting of land-based ice, is causing sea levels to rise.
The global average sea level rose about 19 cm between 1901 and 2010, at an average rate of 1.7 mm per year. From 1993 to 2016 the global average sea level rose at an average rate of about 3.4 mm per year.
Due to the influence of regional climate trends and gravitational effects, sea level does not rise uniformly around the globe. Sea levels in New Zealand rose on average by 1.7 mm per year from 1900 to 2008.
Much of New Zealand’s urban development and infrastructure is located in coastal areas. This makes it vulnerable to coastal hazards such as coastal erosion, inundation (flooding) by the sea and sea-level rise.
Climate change is likely to bring the following changes:
- increased frequency, duration and extent of coastal flooding
- coastal defences are overtopped by waves or high tides more often
- severe storms increase in intensity and storm surge levels rise
- some sandy beaches may continue to accrete, but more slowly
- some gravel beaches are more likely to erode
- in areas with smaller tidal ranges (eg, Wellington, the Cook Strait area and the East Coast) the historic high tide mark may be exceeded more often
- the potential for saltwater to enter underground freshwater aquifers increases.
It is important that we start planning for future sea-level rise now.
The Ministry recommends developing flexible adaptation plans, rather than relying on a single sea-level rise value or scenario. This is because there is a wide range of possible coastal futures with ongoing sea-level rise, particularly heading into next century.
The Ministry’s Coastal Hazards and Climate Change guidance provides four scenarios of future sea-level rise to use in conducting hazard and risk assessments.
It also provides minimum transitional sea-level rise values for use in planning processes for two out of four broad categories of development.
Category A – Coastal subdivision, greenfield developments and major new infrastructure
The New Zealand Coastal Policy Statement 2010 emphasises locating such development away from areas prone to coastal hazard risks (including climate change) and avoiding increasing the risk. Therefore, councils considering coastal subdivision, greenfield developments and major new infrastructure should avoid the hazard risk by considering sea-level rise over more than 100 years and using the highest sea-level rise scenario (H+).
Category B – Changes in land use and redevelopment (intensification)
When considering changes in land use and redevelopment (intensification), councils should conduct a risk assessment using all four sea-level rise scenarios and the adaptive pathways planning approach.
Category C – Existing coastal development and asset planning
For planning and decision timeframes out to 2120, councils should use a minimum transitional value for sea level rise of 1 metre relative to the 1986-2005 baseline.
Category D – Non-habitable short-lived assets
For planning and decision timeframes out to 2120, councils should use a minimum transitional value for sea- level rise of 0.65 metres relative to the 1986-2005 baseline.
The New Zealand Coastal Policy Statement 2010 (NZCPS) provides further direction on planning for development in the coastal zone.
Each regional council must prepare a regional policy statement, and this regional policy statement must give effect to the NZCPS. Other regional and district plans must also give effect to the NZCPS.
Under Policy 24 of the NZCPS, “Hazard risks, over at least 100 years, are to be assessed having regard to physical drivers and processes including… sea level rise…”
For further information, including a copy of the NZCPS, see New Zealand coastal policy statement 2010 [Department of Conservation website]
Many local authorities have already started to plan for sea-level rise. Some councils have completed coastal hazard assessments and have developed maps showing areas which are expected to be affected over the next 50-100 years.
Other activities being undertaken by local government include:
- restricting development in coastal erosion areas
- planning for managed retreat
- rejecting consents for alterations or extensions to existing buildings in the coastal zone
- discouraging the construction of defences such as sea walls.