Coastal sea-level rise

  • Image, Coastal sea-level rise.

    Sea-level rise is a consequence of climate change. Increased global temperatures lead to rising sea levels because warmer waters take up more space and increased meltwater and ice from glaciers and polar ice sheets enter the ocean. Sea level varies naturally from place to place due to local ocean circulation and temperatures and the movement of the land relative to the sea. For example, earthquakes can lift or drop the land.

    Rising sea levels will affect our coastal communities, environments, and biodiversity. Coastal flooding and erosion will increase, coastal groundwater may rise and lowland freshwater systems become saltier.

    We classified Coastal sea-level rise as a national indicator.

     

    Key findings

     Image, Increasing trend, declining state.  Increasing trend (declining state)

    Our coastal sea level (relative to land) is rising.

    • Most sites experienced higher sea levels in the last 20 years and lower sea levels in the first 20 years for which we have data.
    • Available tide gauge data showed rising linear trends (with standard deviations given in parentheses) in all long-term monitored sites throughout the period for which data are available.
      • The Wellington tide gauge showed the most marked trend with a rate of increase of 2.23 (±0.16) mm/year for 1891 to 1893 and 1901 to 2015.

      • Other sites with less marked increases were:
        • Auckland: 1.60 (±0.08) mm/year from 1899 to 2015
        • Dunedin: 1.42 (±0.08) mm/year from 1899 to 2015
        • Lyttelton: 2.12 (±0.09) mm/year from 1901 to 2015
        • Moturiki (Mount Maunganui): 1.9 (±0.25) mm/year from 1973 to 2015
        • New Plymouth: 1.37 (±0.16) mm/year for 1920 and 1955 to 2015.

     

    Note: Sea levels are relative to a local vertical datum for a particular gauge station. Therefore the annual sea level for each station is calculated relative to the average mean sea level for that station over the 20-year period (1986–2005) to enable a comparison between sites.
    Data are not available for all sites for all years. The longest records are from ports in Auckland, Wellington, Lyttelton and Dunedin, with limited data availability pre-1901.

     

     

    Definition and methodology

    We report on the annual mean sea level relative to the average mean sea level over 1986–2005 (baseline) for six locations:

    • Auckland – data from 1899 to 2015
    • Wellington – data from 1891 to 1893 and 1901 to 2015
    • Lyttelton – data from 1901 to 2015
    • Dunedin – data from 1899 to 2015
    • New Plymouth – data for 1920 (this is a mean value for the four years from 1918-1921) and from 1955 to 2015
    • Moturiki (Mount Maunganui) – data from 1973 to 2015.

    Note that data were not available for all years for Auckland, Wellington, Lyttelton and Dunedin. We chose the 1986–2005 baseline as it is used by the Intergovernmental Panel on Climate Change (IPCC) as the zero baseline for future sea-level rise projections. It is also long enough (20-years) to cover the range of tidal combinations (18.6 years) and some of the climate variability (e.g. El Niño-Southern Oscillation or ENSO).

    Linear trends were provided by NIWA and Emeritus Professor John Hannah (previously University of Otago). Ideally, linear trends in sea level would be reported if there are at least 50 years of data to account for climate variability from climate oscillations such as the 20–30 year Interdecadal Pacific Oscillation (IPO) and the shorter ENSO cycle. Such climate variability can be seen in the increase in annual mean sea level in 1999–2000, when the IPO across the entire Pacific Ocean changed to a negative phase. While the Moturiki data cover 43 years, it was considered appropriate to apply a linear trend to further extend the number of reported sites. Further detail on the data processing (including adjustments for historic datum changes) and methods used for the trend analysis can be found in Hannah (1990), Hannah (2004), and Hannah and Bell (2012).

    We report on relative sea-level rise, which includes local and regional changes in vertical land movement and ocean conditions, as well as absolute rise in ocean sea level. Wellington in particular includes recent slow-slip tectonic subsidence (measured separately by continuous GPS sensors) that has increased relative sea-level rise since around 1998.

    Sea-level rise is mainly caused by thermal expansion and glacial and polar ice-sheet meltwater or ice discharge from land (IPCC, 2014). Melting sea ice has little direct effect on sea-level rise. Global mean sea levels have risen at a rate of ~1.7 (±0.2) millimetres per year) since the beginning of the 20th century until 2010 (IPCC, 2014). However, regional rates can be considerably larger or smaller than the long-term global average due to the influence of ocean circulation and wind patterns (IPCC, 2014). This rate is comparable to that in New Zealand, where the average relative sea-level rise was around 1.7 millimetres per year (Hannah, 2004; Hannah & Bell, 2012). Ocean warming and glacier and polar ice-sheet melting, key drivers of sea-level rise, will continue for several centuries even if atmospheric greenhouse gas concentrations are stabilised (IPCC, 2013), but reductions in greenhouse gas emissions will make a difference to the future rate of rise in sea level.

    Seaside communities and infrastructure will be directly affected by rising sea levels, mainly through threats from flooding, erosion, and salinisation of groundwater (PCE, 2015). What were once very rare events will become more common. For example, the current one-in-a-hundred-years storm-tide in Wellington’s and Christchurch’s ports are expected to be exceeded once every year on average when the sea rises another 30 centimetres, which has been projected to occur by 2065 (PCE, 2015). Drainage or stormwater in low-lying areas will be increasingly more difficult to maintain as gravity systems as flooding, wave overtopping and groundwater levels increase on the back of the rising sea level.

    Rising sea levels will also affect biodiversity and habitats in estuaries, coastal wetlands, and intertidal and sub-tidal zones. Habitat will likely be lost due to coastal squeeze – when natural habitats are eroded by the sea and cannot retreat due to arable, built, or unsuitable land on their landward edge (Wong et al, 2014).

     

    Data quality

    We classified Coastal sea-level rise as a national indicator.

    Relevance

    Image, Direct relevance.    This national indicator is a direct measure of the ‘Sea level, temperature, and circulation’ topic.

    Accuracy

     accuracy-high  The accuracy of the data source is of high quality. 

    See Data quality information for more detail.

     

    References

    Hannah, J (2004). An updated analysis of long-term sea level change in New Zealand. Geophysical Research Letters, 31(3): L03307. http://doi.org/10.1029/2003GL019166

    Hannah, J. (1990). Analysis of mean sea level data from New Zealand for the period 1899-1988. Journal of Geophysical Research 95(B6): 12399-12405.

    Hannah, J & Bell, RG (2012). Regional sea level trends in New Zealand. Journal of Geophysical Research: Oceans, 117(1): C01004. http://doi.org/10.1029/2011JC007591

    Intergovernmental Panel on Climate Change (IPCC) (2013). Summary for policymakers. In: Climate change 2013: The physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the IPCC. CEUR Workshop Proceedings, 1542, 33–36. http://doi.org/10.1017/CBO9781107415324.004

    Intergovernmental Panel on Climate Change (IPCC) (2014). Climate change 2014: Synthesis report (PDF, 11MB). Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Retrieved from www.ipcc.ch.

    Parliamentary Commissioner for the Environment (PCE) (2015). Preparing New Zealand for rising seas: Certainty and uncertainty. Retrieved from www.pce.parliament.nz.

    Wong, PP, Losada, IJ, Gattuso, JP, Hinkel, J, Khattabi, A, McInnes, KL, … Sallenger, A (2014). Coastal systems and low-lying areas. In Climate change 2014: Impacts, adaptation, and vulnerability. Part A: Global and sectoral aspects. Working Group II Contribution to the IPCC Fifth Assessment Report (pp 361–410). Cambridge and New York: Cambridge University Press. http://doi:10.1017/CBO9781107415379.010

     

    Archived pages

    See Coastal sea-level rise (archived October 2017) and Coastal sea-level rise (archived October 2016).

     

    Updated 19 October 2017

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