Vertical land movement key to understanding future sea-level rise

GNS Science InSAR Scientist Ian Hamling played a key role in producing the vertical land movement estimates that sit at the heart of these new projections. 

“With around 15,000 kilometres of coastline, we knew traditional approaches to measuring vertical land movement were impossible. To overcome this, we combined observations from interferometric Synthetic Aperture Radar (InSAR) and GPS stations to increase the breadth and density of vertical land movement estimates”. 

Using radar images captured by satellites, InSAR measures the change in distance between the land surface and the satellite. By differencing radar images acquired at different times, we can generate deformation maps – called interferograms – covering large areas but at high resolutions. 

The team used SAR images between 2003 and 2011 for their estimates. This minimised the influence of large earthquakes that have struck New Zealand since 2011 and altered regular patterns of uplift and subsidence. 

“Using all of the available images, we generated more than 2500 individual interferograms to estimate the vertical displacement rate over an eight-year period. We then used GPS data to check these InSAR observations and put them in a consistent reference frame” says Ian. 

The result is the highest resolution and most systematic map of countrywide vertical land movement that has ever been produced. We now have bespoke sea-level rise projections for every two-kilometre stretch of our coastline.

GNS Science Environment and Climate Theme Leader Richard Levy is one of the co-leaders of SeaRise. He says that the new projections will provide invaluable insight for decisionmakers as they plan to adapt to a changing climate.

“Understanding the impact of vertical land movement on sea level rise significantly changes the time we have to plan for its impacts. The amount of sea level rise we might expect to occur in 50 to 60 years could actually happen in two or three decades. 

“In Wellington, the amount of sea-level rise that local infrastructure can handle is relatively small at 30 centimetres. That amount of sea level rise is already unavoidable, no matter how much we curb emissions. This will drastically change the frequency of inundation events – turning once in a century flooding into an annual event. 

“At the same time, land in much of Wellington is sinking at nearly the same rate that global sea levels are rising. So, we can expect that 30 centimetres to happen twice as quickly, and planners need to be fast-forwarding adaptation plans.”