Every millimetre counts when it comes to sea level rise.
Stand by St Clair beach long enough and you will see the tide rise and fall, the wind shift and the sun set. It feels timeless. Yet, beneath this regular ebb and flow the beach is quietly changing, subtle shifts invisible in a single afternoon but powerful enough to shape Ōtepoti Dunedin’s future. The water level creeps a little higher each year.
South Dunedin is one of New Zealand’s most flood-prone communities. As many homes and businesses are within a metre of high tide, even small changes in sea level can have serious consequences. During heavy rainfall, streets can flood within hours, putting low-lying areas at risk.
Today, sensors on Earth-orbiting satellites track global patterns of absolute sea level relative to a global reference system. Those observations are corrected regionally for vertical motion of Earth’s crust but, like a low-resolution photograph, the view from space can blur local details.
For a century or more, tide gauges around Aotearoa New Zealand have tracked the regular cycle of the tide, as well as longer-term trends. Tide gauges measure sea level relative to local features, which means they cannot tell the difference between rising water and sinking land. That is where Global Navigation Satellite Systems (GNSS), including GPS and other navigation satellite constellations, come in.
Most people are familiar with the ability of GNSS to tell you where you are and - in the case of a fitness tracker - precisely when you were there. When GNSS stations are placed strategically, they can track land motion in the same way, with millimetre precision over time.
But GNSS also has a hidden capability, and this is where my PhD research comes in. By analysing satellite signals that reflect off nearby-surfaces before reaching the receiver, the properties of those surfaces can be deduced.
Over even short periods of time, changes in the reflected signals reveal subtle environmental changes such as water level, melting snow and soil moisture variations. This method is known as GNSS Reflectometry or GNSS-R.
When GNSS receivers are placed near the coast, their signals arriving directly to the antenna can be used to measure both vertical and horizontal land motion and, at the same time, signals that are reflected first record nearby sea surface height. Putting the data together, it is possible to distinguish the effects of rising seas from sinking land.
Why does this matter? It matters because it is local. What people experience is relative sea level, the height of the sea relative to the land they live on. In tectonically active New Zealand the land itself moves, sometimes rising and sometimes sinking by several millimetres a year. The slow creep of land subsidence can transform a quiet street into a soggy hazard over time just as a small rise in sea level can gradually push the shoreline inland.
This challenge is not unique to Dunedin or New Zealand. Coastal cities from Jakarta to New Orleans face similar risks, where rising seas and sinking land combine to magnify flood hazards.
These risks are not abstract for me. In my hometown of Teluk Intan, Malaysia, my family experiences frequent flooding despite engineering efforts to reduce the impact of sea level rise. These floods shaped my perspective as a researcher and drive my work in monitoring coastal changes, providing the evidence needed to support practical responses to rising seas and flooding.
As a surveyor, I have seen how planning decisions can fail when local measurements are conducted for short periods to support engineering planning. Such observations do not capture slow land motions or changes in sea level over time. They miss extreme events such as storm surges, leaving an incomplete picture for decision makers.
Filling these gaps with long-term monitoring gives communities’ information they need to make sense of environmental changes around them.
A Māori whakataukī (proverb) reflects this: Mā te rongo, ka mōhio. Mā te mōhio, ka mātau. Mā te mātau, ka mārama. From observing comes knowledge, from knowledge comes understanding, and from understanding comes wisdom.
The clearest picture comes from combining the new GNSS methods with information from satellites, tide gauges and even community-based observations. This integrated approach ensures planning decisions are grounded in the best available evidence and gives communities time to adapt.
No single tool or discipline can solve the challenge of sea level rise and coastal environmental change alone. Kaumātua and other community elders can share knowledge of conditions decades into the past, today’s communities contribute lived experience of flooded streets, scientists and engineers provide technical details and projections for what the future may hold.
Policymakers must weigh all of this information as they plan for adaptation. Initiatives such as South Dunedin Future show how these perspectives complement one another in the process of turning evidence and community priorities into decisions communities can trust.
Sea-level rise and sinking land can feel overwhelming, but knowledge is power. With the right tools in place, Ōtepoti Dunedin has time to prepare rather than react. From better flood defences to smarter planning rules, monitoring helps turn uncertainty into decisions that protect homes and livelihoods, city-wide and at the scale of individual homes.
- Written together with Prof Christina Hulbe. Joe Heng is a PhD candidate in the School of Surveying, University of Otago and Prof Hulbe teaches glaciology, climate science and geospatial analysis at the University of Otago.
Every week in this column writers address issues of sustainability.
