Contemporary sea level changes from satellite altimetry: What have we learned? What are the new challenges?

Since the early 1990s, high-precision multi-mission satellite altimetry has provided a 25-year-long sea level record from which global mean sea level rise and superimposed interannual and regional variability can be derived. Most recent results show that the global mean sea level is rising at a mean rate of 3.1 ± 0.3 mm/yr since January 1993. A clear acceleration is also visible on this 25-year time span, estimated to 0.10 mm/yr2. Mapping of spatial trend patterns continue to show deviation from the global mean rise in a number of regions. However, as the altimetry record lengthens, the ratio of regional trends to the global mean rise tends to decrease, with a factor of amplification of only 2, compared to 3 to 4 some years ago. Estimates of thermal expansion from Argo and ocean mass change from GRACE show that over the GRACE and Argo time span (since 2005) the sea level budget is almost closed. Assessment of the sea level budget over the entire altimetry era (since 1993) based on estimates of individual mass components for the glaciers and the ice sheets provides some upper bound for the still poorly known contribution from water storage on land. At regional scale, ocean thermal expansion is still the main cause of the spatial trend patterns observed by satellite altimetry. However, removing the steric component reveals residual signal that still needs interpretation. In the remaining of this review, we briefly discuss future sea level changes and associated coastal impacts. Finally, we address the issue of remaining gaps in sea level studies, in particular the need for producing coastal sea level products from dedicated satellite altimetry processing of sea level data in global coastal zones.