PhD candidate Fabio Mangini (UiB/NERSC) recently published his second article, and his findings confirm that satellites can be used to reliably determine regional variations in the sea level in coastal zones of Norway. This is relevant for predicting future changes along coastlines, helping society to mitigate effects of climate change.

Why is the sea level important?

Norwegian fishing villages are exposed to sea-level variations. Photo: Stein Egil Liland, pexels.com

All coastal communities, ranging from small fishing villages in Norway, over island nations like Tuvalu and the Maldives, to megacities like Shanghai in China, are threatened by sea-level variations. The global mean sea-level change consistently hits headlines worldwide since it is one of the clearest manifestations of our changing climate. Sea-level changes mostly reflect variations in ocean temperature and land ice mass. Both these factors are crucial: Warming ocean water expands, causing the sea level to rise. And glaciers and ice sheets on land are melting, adding freshwater to the ocean, also causing the sea level to rise. Looking at the global mean sea-level change is relevant for assessing Earth’s climate, but it is not useful for trying to adapt. The sea level experiences large regional variations which are not captured when studying the global changes. Therefore, local studies are necessary to properly assess how vulnerable coastal areas are to future changes, and to implement suitable adaptation policies. This requires a good understanding and good tools for monitoring current and predicting future variations. Both tasks are extremely relevant, because coastal areas are generally densely populated – worldwide!

Measuring sea-level height from the coast

One way to collect data on the sea level in a region is to use tide gauges. They are instruments that measure the sea surface height in one spot continuously. Not every coastline has a dense net of such sensors to monitor sea-level variations, many stretches of the African coastline are for example not well monitored regarding the sea level. But Norway on the contrary has many sensors along its over 100.000 km long coastline. This is why Fabio Mangini, together with his co-authors and supervisors from Bergen and the UK, chose Norway as study area. Antonio Bonaduce and Laurent Bertino from the Nansen Center are both co-authors and supervisors in the PhD project. The team set out to compare tide gauge data from along the Norwegian coast to satellite altimetry data to see if the satellite data shows the same as the tide gauges which measure the sea level directly in the water.

Measuring sea-level height from space

Satellite altimetry is a way to measure the height of a satellite above the Earth's surface, and it can be used to measure the sea level from space. It is a tool that has previously been assumed to be not very reliable for making observations of sea-level variability close to the coast. But scientists and technical personnel have put a lot of effort into increasing its reliability in the past years. Fabio and his co-authors demonstrate that the latest satellite altimetry and tide gauges show the comparable sea-level changes along the Norwegian coast during the 16-year-long period they studied, 2003 to 2018. For example, the difference between the sea-level trend from satellite altimetry and tide gauges ranges only between -0.85 to +1.15 mm per year, which is very little. The two datasets are in good agreement along the entire coastline of Norway, and even north of the Arctic Circle, where only 3 of 9 satellites provided data for the study. Since the two datasets are so similar, the paper also shows that the temperature- and salinity-contributions to the Norwegian sea level (and, therefore, our interpretation of the causes behind the observed sea-level variability) remain the same: Using tide gauges or satellite altimetry data to observe the sea-level variability are equally good methods. 

Importance of the study

These findings are good news: satellite altimetry can be used to monitor sea-level variations not only over the open ocean, but also in coastal areas. Because satellite altimetry covers almost the entire globe, the article indicates that satellite altimetry can be used to measure sea-level change along coastal regions where tide gauges are not available. In the long run, this satellite data can be used to produce more reliable sea-level predictions and projections for the future. Policymakers might also find these findings interesting in light of climate adaptation policies, as satellite altimetry data ought to be used in the upcoming sea-level change report for Norway.

Reference:

Mangini, F, Chafik, LM, Bonaduce, A, Bertino, L, and JEØ Nilsen. Sea-level variability and change along the Norwegian coast between 2003 and 2018 from satellite altimetry, tide gauges, and hydrography. Ocean Science. 2022;18(2). doi: https://doi.org/10.5194/os-18-331-2022