Global mean sea level is a key indicator of climate change. In other words, it helps monitor the state of the Earth's climate as it responds to variations in different components of the climate system, such as the warming of the oceans and the melting of glaciers and ice sheets. Moreover, global mean sea level is expected to rise, and even accelerate, over the remaining part of the 21st century. As a result, future changes in mean sea level will impact coastal communities and infrastructures, causing loss of lifes and substantial economic damage. However, the variability and impact will be different across the Earth.
Local sea-level variations can significantly differ from the global average. Therefore, regional sea-level studies are needed to properly assess the vulnerability of coastal areas to future sea-level change and to implement suitable adaptation policies. A carefully planned adaptation strategy can help reduce, for example, the likelihood of coastal floodings, coastal erosion and salt-water intrusion on land.
Regional sea-level studies aim towards more reliable estimates of how sea level will evolve in the future. Indeed, they are necessary as they help provide a better understanding of the mechanisms responsible for regional sea-level variations.
As a common practice, to understand the drivers of sea-level variability, the full sea-level signal is decomposed into its steric and manometric components. Steric sea-level variations result from changes in the ocean temperature and salinity. For example, if an area of the ocean experiences warming or cooling, the sea level in the region rises or drops due to thermal expansion and contraction. Manometric sea-level variations result from global and local changes in the mass of the ocean which, in turn, follow from the redistribution of water within the ocean or from the exchange of water among the ocean, the atmosphere and land ice.
The institutional doctoral study initiated by the Nansen Center focuses on sea-level variations in northern Europe and, in particular, along the coast of Norway. At first sight, Norway appears little vulnerable to sea-level variations due to its mountainous coastline which are not strongly affected by erosion. However, most of Norway's major cites are located at the sea level and have undergone significant urban developments in recent years. Therefore, local planning in Norway can benefit from a better understanding of the sea-level variability along its coast.
This study has assessed the ability of satellite altimetry and satellite gravimetry to provide insight into the sea-level variability along the Norwegian coast. Satellite altimetry allows us to measure the total sea level from space, whereas the satellite gravimetry measures the manometric sea-level. Both techniques have revolutionized the study of oceanography as they provide continuous observations over large parts of the global oceans during more than two decades, in areas previously only little accessible. However, while an understanding of coastal sea-level variations is crucial due to their impact on the coastal population, the quality of satellite observations decreases in coastal areas due to the interference of land.
Our work on satellite altimetry and satellite gravimetry return encouraging results. The estimates provided by the satellite measurements show an overall good agreement with those obtained from the sea level monitoring stations installed along the coast of Norway. On the one hand, this result shows the good quality of satellite altimetry and gravimetry in the region and, therefore, highlights the progress made by the scientific community to correct both the satellite altimetry and satellite gravimetry datasets for coastal effects. On the other hand, it confirms the good standard of the existing Norwegian observing system and the consequent validity of previous studies.
The results appear encouraging because it opens up to future research opportunities on sea level. At first, our results indicate that satellite altimetry and in-situ instruments return different sea-level trends along western Norway. Secondly, our results show that satellite gravimetry captures the year-to-year variations of the manometric sea level along the coast of Norway better than the existing in-situ instruments. Therefore, satellite gravimetry might be used to understand the processes responsible for local sea-level variations along the Norwegian coast. Last but not least, our results suggest that satellite altimetry and gravimetry can be employed to study sea-level variations in coastal regions other than the Norwegian coast. This has important implications for the numerous areas of the coastal ocean, such as the coast of Africa and southern Asia, where in-situ instruments are either sparse or non-existent and, therefore, a proper analysis and understanding of local sea-level variations becomes possible.
Prosjektet kan gi bedre kunnskap om fremtidig havnivåstigning langs Norske kyster. Den bidrar til NERSC strategi å være en konkurransedyktig aktør i fjernmåling av havnivå i Europa. Prosjektet blir fulgt opp av en strategisk prosjekt fra Bjerkessenteret for klimadynamikk.