Local sea level (also called relative sea level) refers to the sea’s position in relation to the land at a certain place. Local sea level determines how, when and where waves interact with the coastline. This means that changes in local sea level can cause coastal change.  Sea level is arguably the most important driver of coastal change at a range of timescales. Over long geological timescales (millions and thousands of years), sea level change has forced global-scale retreat and advance of the position of the coast. Every day, changes in local sea level occur due to tides, and the presence/absence of waves/storms. On longer timescales, tectonic factors, such as subsidence/uplift, or earthquakes, can force slow and progressive or rapid changes in local sea level. 

Globally, sea level is rising due to warming ocean temperatures and melting ice sheets associated with human-caused climate change. In New Zealand, sea level has risen on average 20 cm since the 1900s, which is similar to the global average rate of sea level rise over that period. Sea level is continuing to rise, and it is expected that sea level in New Zealand will be 20-40 cm higher than present by 2060, and between 40-100 cm higher than present by 2100. This background trend of ongoing rising sea levels will heavily impact New Zealand’s coast and coastal communities. Despite sea level being an important driver of coastal change, there are other important factors that interact with sea level change to result in coastal change, such as wave regime or storms, sediment supply, human modifications of the coast, and coastal vegetation. 

Example: In February 2023, Cyclone Gabrielle coincided with a neap tide, a period of moderate tide heights and caused around 5 – 10 m of erosion (landward movement of the beach) at certain beaches across the east coasts of Northland, Auckland, the Coromandel Peninsula and the Bay of Plenty. If Cyclone Gabrielle had occurred during a spring tide, a period of higher-than-average high tides, the water level would have been ~50 cm higher. This increase in water level would have extended the reach of wave impacts, probably increasing the amount and extent of coastal erosion. 

NZ SeaRise recently published sea level rise projections at 5 km intervals around New Zealand. Check out the link below if you want to explore sea level rise projections around New Zealand.