As climate change continues to drive global sea level rise, many people living in coastal areas are already seeing the effects. Coastal erosion is accelerating and shifting coastlines inland, and storm surges are getting worse. But lurking beneath the surface is another major consequence that is thus far poorly understood: rising groundwater.

Evidence suggests that in some low-lying coastal regions with shallow groundwater, rising sea levels will drive a simultaneous rise in groundwater levels, with potentially serious risks for homes, businesses, and other infrastructure.

In a new paper focused on the coastal city of Dunedin, New Zealand, Cox and team demonstrate a method for predicting how sea level rise might change groundwater levels and thereby increase inland flooding hazards. South Dunedin already experiences periodic flooding that will become even more challenging with sea level rise; the researchers describe the city as a poster child for New Zealand communities responding and adapting to climate change and rising seas.

The findings are published in the journal Earth’s Future.

The researchers used 2019–2023 data from a network of 35 groundwater sensors installed across Dunedin’s low-lying coastal land, where much of the city’s infrastructure is located. They compared the sensor data with data on tides, rainfall, and other factors to forecast rising sea level’s future influence on groundwater.

The findings suggest that sea level rise will first drive a rise in groundwater level that will reduce the land’s ability to absorb rainfall. With further sea level rise, groundwater may rise even more and begin causing problems while still belowground, such as overwhelming wastewater systems, infiltrating basements, and destabilizing building foundations. Eventually, the groundwater may rise high enough to emerge as springs and cause flooding.

The researchers conclude that flood hazards resulting from rising groundwater can extend much farther inland than many people expect. In addition, assuming that the protective topography of Dunedin’s sand dune barrier does not undergo significant change, these groundwater effects will occur sooner than any direct flooding from the rising sea.

The researchers note that their approach contains key assumptions and uncertainties—for instance, that groundwater and sea level will rise at the same rate and the water table will maintain approximately the same shape, but that conservative predictions are valuable for planning and managing hazards in Dunedin. Because the method is relatively simple and inexpensive, it could also be applied in similar coastal regions around the world, they say.

This story is republished courtesy of Eos, hosted by the American Geophysical Union. Read the original story here.