Abstract
Present-day global mean sea level rise is caused by ocean thermal expansion, ice mass loss from glaciers and ice sheets, as well as changes in terrestrial water storage. For that reason, sea level is one of the best indicators of climate change as it integrates the response of several components of the climate system to internal and external forcing factors. Monitoring the global mean sea level allows detecting changes (e.g., in trend or acceleration) in one or more components. Besides, assessing closure of the sea level budget allows us to check whether observed sea level change is indeed explained by the sum of changes affecting each component. If not, this would reflect errors in some of the components or missing contributions not accounted for in the budget. Since the launch of TOPEX/Poseidon in 1992, a precise 27-year continuous record of sea level change is available. It has allowed major advances in our understanding of how the Earth is responding to climate change. The last two decades are also marked by the launch of the GRACE satellite gravity mission and the development of the Argo network of profiling floats. GRACE space gravimetry allows the monitoring of mass redistributions inside the Earth system, in particular land ice mass variations as well as changes in terrestrial water storage and in ocean mass, while Argo floats allow monitoring sea water thermal expansion due to the warming of the oceans. Together, satellite altimetry, space gravity, and Argo measurements provide unprecedented insight into the magnitude, spatial variability, and causes of present-day sea level change. With this observational network, we are now in a position to address many outstanding questions that are important to planning for future sea level rise. Here, we detail the network for observing sea level and its components, underscore the importance of these observations, and emphasize the need to maintain current systems, improve their sensors, and supplement the observational network where gaps in our knowledge remain.
Highlights
Sea level varies over a broad range of spatial and temporal scales in response to a large variety of physical processes
At the time of writing, 6 altimetry missions are in orbit: Jason-2, Jason-3, SARAL/AltiKa, CryoSat-2, Sentinel-3A, and Sentinel-3B
Such changes would be reflected in the total sea level observations from satellite altimeters, which creates a gap in sea level budget calculations that rely on Argo for the steric contribution
Summary
Sea level varies over a broad range of spatial and temporal scales in response to a large variety of physical processes. Natural variability inside the climate system, for example related to coupled atmosphere-ocean perturbations such as El Nino-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), or Pacific Decadal Oscillation (PDO) cause interannual to multidecadal sea level variations at regional and global spatial scales. On these time scales, deformations of the solid Earth and changes in the gravity field caused by mass redistributions occurring inside the Earth or at its surface produce regional sea level variations. Tide gauge instruments located along coastlines have for more than a century provided invaluable information on historical sea level evolution. Regional and coastal sea level are the topic of other white papers (Benveniste et al, 2019; Ponte et al, 2019), they are not addressed here
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