Sea level threat escalates for NZ

Sea level rise could be affecting the homes of about quarter of a million New Zealanders by the end of the century, in a possible worst-case scenario, according to United States researchers.

Despite that, the study suggests the challenges facing this country pale in comparison with those confronting some major Asian countries and some small island states.

The study, from US based Climate Central and published in Nature Communications, puts the number of Kiwis living within 2 metres of the rough high tide line at 240,000, those living within 1m at 71,000 and those essentially at the high tide line at 15,000.

Those numbers were worked out using a new system developed by Climate Central, called CoastalDEM, for calculating land elevations. The report authors said CoastalDEM reduced the error in Nasa’s widely used satellite-based SRTM model.

According to the study, the problem with SRTM is it models the elevation of upper surfaces and not bare terrain. Densely vegetated and densely populated areas, in particular, show up as having a higher elevation than they actually do.

The study’s authors said the global mean sea level rose 11-16 centimetres in the 20th century, and even with sharp, immediate cuts to carbon emissions it could rise another 50cm in the 21st century. Under higher emissions scenarios, the 21st century rise could approach 2m, and even exceed that level.

Along with calculating how many people lived at various elevations for 135 countries, the researchers also worked out how many people could be living at elevations below projected annual coastal flooding levels.

Using CoastalDEM, in the most extreme scenario, which emphasises the possibility of more rapid sea-level rise because of unstable ice-sheet dynamics, the researchers calculated 440,000 New Zealander could be living in areas affected by one-year coastal flood return levels by 2100. Under that scenario, by 2050, 80,000 Kiwis could be living in affected areas. Under a more moderate scenario, the numbers are 50,000 by 2050 and 90,000 by 2100.

Climate Central has an interactive world map, including New Zealand, which enables users to get indications of which parts of New Zealand will be affected by rising sea levels this century.

Based on that map, the Hauraki Plains look to be the largest single area that will be affected by rising sea levels in New Zealand.

Considerable coastal areas in Bay of Plenty also appear at risk, along with areas south of Dargaville, around Napier, around Lake Wairarapa, toward the coast from Blenheim, the Canterbury coast north and south of Christchurch, the Taieri Plains, land around the lower Clutha River, and coastal Southland including parts of Invercargill.

Significant land around the lower Manawatu¯ River, and on the northern bank of the lower Waikato River are also shown to be at risk, while even in moderate scenarios sizeable areas of Dunedin and Christchurch are shown at risk.

Sea defences in New Zealand are already under pressure, with notable examples in Nelson, Dunedin and Porirua.