{"id":69735,"date":"2025-04-17T07:28:00","date_gmt":"2025-04-16T19:28:00","guid":{"rendered":"https:\/\/www.greenpeace.org\/aotearoa\/?p=69735"},"modified":"2025-05-20T12:48:28","modified_gmt":"2025-05-20T00:48:28","slug":"new-satellite-data-shows-nzs-major-cities-are-sinking-meaning-rising-seas-will-affect-them-sooner","status":"publish","type":"post","link":"https:\/\/www.greenpeace.org\/aotearoa\/story\/new-satellite-data-shows-nzs-major-cities-are-sinking-meaning-rising-seas-will-affect-them-sooner\/","title":{"rendered":"New satellite data shows NZ\u2019s major cities are sinking \u2013 meaning rising seas will affect them\u00a0sooner"},"content":{"rendered":"\n

Rising seas are already affecting coastal communities in Aotearoa New Zealand. On a global average, the sea level is now 18 centimetres higher than it was in 1900<\/a>, and the annual rate of increase has been accelerating to currently 4.4 millimetres per year.<\/p>\n\n

This may not seem much, but it is already amplifying the impact of storm and tidal surges<\/a>. Over the coming decades and centuries, this will pose increasingly serious problems for all coastal communities.<\/p>\n\n

But this is not the end of our troubles. Some parts of New Zealand\u2019s coastline are also sinking. In many New Zealand cities, shorelines are steadily subsiding, with growing impacts on coastal infrastructure.<\/p>\n\n

Our new research<\/a> reveals where and how fast this is happening. We found the coastlines near all major cities in New Zealand are sinking a few millimetres each year, with some of the fastest rates in coastal suburbs of Christchurch, where the land is still adjusting to the impact of the 2011 earthquake.<\/p>\n\n

Relative increase in sea level<\/h2>\n\n

Sea-level rise is happening globally because the ocean is expanding as it continues to warm and glaciers and polar ice sheets are melting.<\/p>\n\n

Meanwhile, land subsidence operates on regional or local scales, but it can potentially double or triple the effects of sea-level rise in certain places. This dual effect of rising seas and sinking land is know as relative sea-level rise and it gives coastal communities a more accurate projection of what they need to prepare for.<\/p>\n\n

To understand which parts of the coast are most at risk requires detailed and precise measurements of land subsidence. The key to this is to observe Earth from space.<\/p>\n\n

We have used a technique known as interferometric synthetic aperture radar (InSAR<\/a>). This involves the repeat acquisition of satellite radar images of the Earth\u2019s surface, tied to very accurate global navigation satellite system measurements of ground stations.<\/p>\n\n

This builds on earlier work by the NZSeaRise project<\/a>, which measured vertical land movement for every two kilometres of New Zealand\u2019s coastline. Our study uses a significantly higher resolution (every ten metres in most places) and more recent datasets, highlighting previously missed parts of urban coastlines.<\/p>\n\n

Urban hotspots<\/h2>\n\n

For instance, in Christchurch the previous NZSeaRise dataset showed very little subsidence at Southshore and New Brighton. The big differences in the new data are not due to the increase in spatial resolution but because the rate of vertical land movement is very different from the time prior to the 2011 earthquake.<\/p>\n\n

Localised subsidence in these Christchurch suburbs is up to 8mm per year, among the fastest rates of urban subsidence we observed. These areas sit upon natural coastal sand dunes above the source area of the earthquake, and the Earth\u2019s crust is still responding to that sudden change in stress.<\/p>\n\n

\"A
This map shows vertical land movement (VLM) in Christchurch, highlighting areas that are sinking. The circles around the coastline show NZSeaRise estimates (2003-2011) and continous blue shading highlights new results (2018-2021). Jesse Kearse<\/a>, CC BY-SA<\/a><\/figcaption><\/figure>\n\n

We have tracked vertical movement of the land with millimetre-scale precision for five major cities in New Zealand. The InSAR technique works particularly well in urban areas because the smooth surface of pavements, roads and buildings better reflects the satellite radar beam back into space where it is picked up by the orbiting satellite.<\/p>\n\n

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