The next big wave

We know erosion. We know coastal accretion [build-up]. As a seabound nation on a plate boundary, we know the tilt and shift of tectonic movement. Add climate change to the mix and our changing coastline is in for more change.

Under present projections, the sea level around New Zealand is expected to rise between 30cm and 1m this century as warming ocean waters expand, mountain glaciers retreat and polar ice sheets in Greenland and Antarctica shrink. Even if global emissions were to stop today, more warming over the next few decades is inevitable, bringing a trail of storms, ocean surges, flooding and erosion.

The Ministry for the Environment says extreme coastal water levels, currently expected to be reached or exceeded once every 100 years, will, by 2050-2070, occur on average at least once a year.

In her 2015 report on coastal hazards, former Parliamentary Commissioner for the Environment Jan Wright said rising seas will lead to flooding on low-lying coastal land, erosion of many beaches and “soft” cliffs, and higher and possibly saltier coastal groundwater. Inevitably, she wrote, both central and local government will begin to face pleas for increasing financial assistance. Large-scale managed retreat will be costly, but the alternative will be leaving people in homes “that become uninsurable and then uninhabitable”.

Evidence is already piling up. Waihi Beach in the Bay of Plenty, Beach Road south of Ōamaru, and small seaside towns in Taranaki and the West Coast all bear the signs of coastal erosion. Low-lying areas in Napier, Whakatane, Tauranga, Motueka, Nelson, parts of Auckland and Wellington have all been inundated by storms. Just before Christmas, the Whakatane District Council declared 34 properties in Matata in the Bay of Plenty “unliveable” due to severe flooding risk.

“We are a coastal nation so we are going to get whacked by sea-level rise,” says GNS climate scientist Tim Naish, head of a new Government-funded programme set up to assess the magnitude and rate of sea-level rise. “We’re talking places we will not be able to live in because a so-called one-in-100-year flooding event becomes a daily event. You can defend if the assets are worth defending – I suspect Auckland will defend parts of its coastline, Wellington will, New York is planning to. Or you can retreat, which comes with massive disruption and social and economic issues.”

Worst-case scenario, he says, is an average 2m sea-level rise by the end of the century. Best-case scenario, if we achieve the goals of the Paris climate agreement and keep temperature rise well below 2°C, is 50cm of sea-level rise.

Impacting on the severity of these events is the movement of the Australian and Pacific plates as they move over and under each other. Recent research from Victoria University of Wellington, based on GPS data, shows parts of New Zealand are sinking at faster rates than others and will be subjected to higher levels of future sea-level rise. The North Island’s east coast, for example, has subsided up to 3mm a year for the past 15 years, exposing the region to twice the global average maximum sea-level rise. Other parts, says Victoria University of Wellington geophysicist Tim Stern, are rising, meaning sea-level rise in these areas will be less than the global average.

“I suspect Auckland will defend parts of its coastline, Wellington will, New York is planning to.”