Quakes accelerate sea-level rise
Land sinkage in some areas of the country could double the impact of sea-level rises, writes Ged Cann.
The impact of sea-level rises could double in some areas as a result of sinking land levels, according to a Victoria University study.
While the findings had the potential to wipe thousands of dollars off the value of some seaside properties, for some it will be music to their ears, with shoreline uplift in other areas negating sea-level rises.
Study co-author Professor Tim Stern said this new information could be used by homeowners to decide land value, by the government to plan mitigation measures and the insurance industry to set premiums.
‘‘In parts of the North Island the actual surface of the land is going down 3 millimetres in relation to the centre of the Earth, you effectively double the global rate [of sea-level rise].’’
Insurance Council chief executive Tim Grafton said sealevel rise was on insurance companies’ radars, and was one of the risks insurers took into account.
‘‘Insurers draw on all available data points to inform themselves about risks.’’
Grafton had not seen the reports’ full findings, but said he would be interested in the projections.
Stern, from the School of Environment and Earth Sciences, said the global sea-level change due to global warming and the melting of polar icecaps was projected to be around 3mm a year.
‘‘The rate in New Zealand is being measured at about 1.5mm [a year] by tide gauges.’’
The study used 15 years of GPS data to predict land movements.
In other areas, including the central Southern Alps, uplift is progressing at 5 to 10mm per year, outstripping sea-level rise entirely.
Stern cautioned against anyone relying on the land elevating to safeguard seaside properties, but said it should be factored in for defence and mitigation planning.
The findings were pertinent, Stern said, because buyers of seaside properties were looking out to 50 or 100 years in the future when making a decision.
‘‘When it comes to real estate it’s about people’s perception and their confidence to invest,’’ he said.
Some local authorities have already started looking at including sea-level information on residents’ property information.
Kapiti Council previously included coastal hazard information in residents’ Land Information Memorandums (LIMS), but were forced to stop after residents litigated over lost property value.
Stern said banks and insurers would find the information invaluable.
‘‘If they have a better knowledge of what their particular area of the coastline is doing they will be in a much better shape to make good policy.’’
As the study continues and the data pool got larger, Stern said the margin of error became smaller.
The scope of the study is limited to areas with GPS stations. Stern said it was important to get better coverage, particularly in the South Island.
Parliamentary Commissioner for the Environment Dr Jan Wright said while the information was useful, sea-level rise was dictated by so many factors it would not greatly change current estimations.
She said accelerations in sealevel rise as result of greenhouse emissions were more concerning than changes in land elevation, which were generally gradual and more consistent.
What does this mean for different areas?
The Bay of Plenty coast is rising at 2mm a year while Gisborne is experiencing a slight drop.
A pattern of land sinkage in the eastern-North Island represented a subduction zone between two tectonic plates.
‘‘That can’t go on forever, that’s why we call that locked because that will release and might uplift. That might be tomorrow or that might be 500 years away,’’ Stern said.
Wellington sits on the cusp of a subduction zone and neighbouring rising areas, making the capital a wildcard.
The central Southern Alps and Dusky Sounds are seeing the greatest uplift, at 5 to 10mm per year.
Rotorua is seeing significant subsidence of about 10mm a year, however close coastal areas are seeing a lift.
Auckland is stable, while Christchurch data is unclear, with frequent quakes disrupting records.