Large sections of Auckland's coastline could erode more than 200 metres by 2130 unless strong action is taken on climate change, heat maps included in a new Auckland Council report show.
The report, entitled Predicting Auckland's exposure to coastal instability and erosion, was released to the public on Tuesday morning.
Stats NZ data shows Auckland has averaged 1.67mm of sea level rise each year since 1899, but the Ministry for the Environment predicts this rate will accelerate if little action is taken to reduce greenhouse gas emissions.
The council's report takes into account new peer-reviewed research looking at areas susceptible to coastal instability and/or erosion (ASCIE) - the area inland from Auckland's current coastline at risk because of erosion - and how these sea level rises might affect them.
The report forecasts sea level rises and subsequent impacts on ASCIEs across four scenarios:
- Low to eventual net-zero emission scenario (RCP 2.6 M)
- Intermediate-low emissions scenario (RCP 4.5 M)
- High-emissions scenario (RCP 8.5 M)
- Higher extreme scenario, with essentially no controls on emission by 2100 (RCP 8.5 H+)
Maps included in the research show that if emissions remain high and we track along the RCP 8.5 M scenario, large swathes of Auckland more than 100 metres back from the current coastline will be at risk of eroding or becoming unstable in the next 110 years.
Some cliffs at the entrance to the Manukau Harbour, on the west coast, around Great Barrier Island and in isolated patches of north Auckland, meanwhile, are predicted to retreat or become unstable up to 200 metres back.
Many others are predicted to erode by at least 100 metres, especially in north Auckland, the northern part of Manukau Harbour, around Waiheke Island and in Kaipara Harbour.
"ASCIE distances across Auckland cliffs vary, particularly with geological type, exposure and cliff height," the council report reads.
"For example, distances range from less than 20m for low Tauranga Group cliffs in sheltered harbour environments, to more than 200m for high cliffs along the open and exposed coast of Great Barrier Island."
Whatipu Beach and the Waionui Inlet, a low-lying spit at the southern side of the Kaipara Harbour, are subject to large medium-term fluctuations, and are predicted to retreat more than 800 metres by the year 2130 in the RCP 8.5 M scenario.
The majority of the west coast and north Auckland's Pakiri Beach, meanwhile, would become unstable or erode more than 100 metres back from the current coastline.
The report says ASCIE for beaches would mainly be caused by storms and the like over the next 30 years or so, but sea level rises will become more of a factor over longer timeframes.
"Under the 2130 RCP 8.5 M scenario, predicted erosion values for beaches across Auckland vary from less than 20m in sheltered harbour coast beaches to more than 190m along the highly exposed shorelines of the outer Hauraki Gulf and west coast," the report says.
"Sea level rise is a major contributor to the high ASCIE values, in some cases adding more than 90m to expected values."
The report says the heat maps will enable Aucklanders to understand long-term coastal change and climate change impacts, and will inform future sustainable hazard management approaches for the region.
Auckland councillor Richard Hills, chair of the council's Environment and Climate Change Committee, says the new report gives it stronger guidance.
"The report identifies areas of Auckland's coastline predicted to be susceptible to coastal erosion and instability with the findings, giving us a high-level understanding of potential erosion rates across the region."