The sea level budget closure project

Sea level is one of the most prominent indicators of climate change. It integrates changes of several components of the climate system in response to anthropogenic forcing as well as natural forcing factors related to natural sources and internal climate variability.

Present-day global mean sea level rise primarily reflects ocean warming (through thermal expansion of sea waters) and land ice melt. Changes in land storage constitute an additional contribution. As a primary objective of the ESA CCI programme was to reduce current uncertainties of sea level change and its individual components, better closure of the sea level budget should now be possible. This will improve our understanding of processes involved in causing global mean sea level rise and its regional variability, and further help to improve models used for projections of future climate changes.

Closure of the sea level budget implies that we have:
ΔSL(t) = ΔMOcean(t) + ΔSSL(t)
where ΔSL(t) is time-variable sea level, ΔMOcean(t) is time variable ocean mass component, ΔSSL(t) is steric sea level components (effect of the depth integrated change in sea water density due to ocean temperature and salinity variations).

Water mass conservation in the climate system implies closure of the ocean mass budget:
ΔMOcean (t) = [ ΔMGlaciers(t) + ΔMice sheets(t) + ΔMLWS(t) + ΔMAtm(t) + missing mass terms ]
where the ΔM(t) terms in the right hand side refer to glaciers and ice sheets mass balances, changes in land water storage (LWS, including seasonal snow cover), and atmospheric water vapour.

Studying the sea level budget and the ocean mass budget with accurate climate records will bring major scientific benefits – adding confidence to satellite-based assessments of climate change, shedding light on missing or poorly known processes such as land hydrology that remain a challenge for Earth Observation, or heat uptake by the deep ocean. This will allow the key processes affecting sea level rise to be precisely quantified, leading to more informed model projections. Together with extreme events like storm surges and tropical cyclones, regional/local sea level changes represent a major threat in low-lying, highly populated coastal regions of the world, each of which affects coastal planning. The project is therefore a timely opportunity to add societal value and visibility to the CCI, allowing it to form the backbone of a milestone contribution to climate science.

This project intends to analyse in an integrative context recent results obtained by the ESA CCI programme for the sea level, glaciers and ice sheets Essential Climate Variables (ECV), with additional account for assessments of the ocean thermal expansion component which utilise the CCI Sea Surface Temperature ECV. Among other objectives, an important task will consist of assessing the quality of the CCI products involved in the sea level budget.

The first phase of this project covered the precise altimetry era (starting in 1993) with a special focus given to the period 2003/2005 to 2015, coinciding with the availability of GRACE space gravimetry data and Argo drifter data.

The new project phase (SBLC_cci+) aims to improve the closure of the global mean sea level budget by: 1) lengthening the time series, 2) assessing budget closure at global and regional scales, 3) providing an explanation of temporal and spatial variability at global and local scales. Depending on the availability of the various elements, the global sea level budget will be updated up to 2022/2023. In addition, the project will address the regional variability of sea level and sea surface temperature, and investigate the contributions of natural/internal climate variability and anthropogenic forcing (detection/attribution) to the associated spatial trends. By extending to regional spatial scales, we can pinpoint areas where the sea level budget does not close, resulting in a regional breakdown of the assessment of the items that accounts for a significant portion of the individual components used.