One-in-100-year coastal floods now happening twice a year in Wellington - study

Sea level rise caused by climate change is set to expose areas like Wellington to more frequent coastal flooding, climate scientists say.

Human-caused sea-level rise has made coastal sea-level extremes four times more frequent since the start of the 20th century.

A new study published in research journal Nature Climate Change claimed Wellington's one-in-100 year coastal floods were happening about twice a year.

It comes on the heels of a local state of emergency for parts of the capital this week in response to large swell warnings for Wellington's south coast and southern Waiarapa.

In other parts of the world, 100-year floods had a median increase to a one-in-eight year event over this period.

Researchers analysed tide gauges recorded between 1900 and 2005 and used climate simulations to identify the influence of human activity on flooding due to sea level rise at 130 sites around the world.

For nearly half the sites, floods of the size that had a 1 percent chance of happening there during the year 1900 became at least 10 times more likely to happen in a year by 2005, mostly due to human activity.

However, in some locations like Wellington, the increase in flood likelihood was many times higher.

The authors argued this was direct evidence that human-driven climate change had already increased coastal flood risk, and it was crucial flood protection planning used up-to-date flood frequency estimates.

Victoria University climate science professor James Renwick said the report showed global sea level rise because of human-induced climate change was causing changes in relative and extreme sea levels locally.

"Sea levels are not rising uniformly around the world, and local sea level changes depend on local vertical land movement and other factors. Yet, the climate change signal is now detectable world-wide and is responsible for the majority of daily extreme water level exceedances this century," he said.

"At many locations studied, the one-in-100-year extreme sea level event is now occurring at least once per decade. Sea levels are continuing to rise so coastal inundation events will be happening even more frequently over time from here."

Renwick said it also underlined the need for "urgent adaptation".

"This week's extreme wave event on the Wellington south coast reinforces the idea that we need to act now and develop response plans urgently. I trust our policymakers and politicians are listening."

Waikato University environmental planning research associate Dr Rob Bell said there were two factors around Wellington's coastal storm-tide level risk.

"Wellington's tide range is quite small and the range of New Zealand's extreme storm-tide levels is also relatively small on a global scale, as we don't have hurricanes or tropical cyclones. Consequently, with these small contributors to coastal flooding, even a small amount of sea-level rise (and/or land subsidence) can make rapid changes in the frequency of these events."

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