The budget is almost closed: The individual contributors to global sea-level rise match-up

A simple sum of all the contributors to sea level rise reinforces confidence in Earth observation data

 

Recent findings from a European Space Agency (ESA) funded study reinforces confidence in our understanding of global mean sea-level rise by calculating precise numbers and accuracy measures for individual contributors to sea-level rise. The study led by Prof. Martin Horwath at Technical University of Dresden (Germany, see press release) with participation of Nansen Center scientists (Dr. Roshin P. Raj and Prof. Johnny A. Johannessen) was published in Earth System Science Data (vol. 14, 411-447, 2022) on 7 February 2022 and documents how the sum of sea-level contributions assessed on a month-to-month basis matches the total sea-level change observed by satellites.

 

The study has largely benefitted from the Climate Change Initiative (CCI) program of ESA which has generated high-quality and continuous space-based records of Essential Climate Variables (ECV), including a number of variables contributing to sea level change.

Global mean sea level has risen by more than 3 centimetres per decade (3 mm/year) since precise satellite altimetry measurements began in the 1990s primarily resulting from ocean warming and water mass increase (see figure). The thermal expansion effect amounts to 1.15 mm/year (38% of the total rise). In comparison the addition of water masses is assessed at 1.73 mm/year (57% of the total rise) mainly through melting of glaciers and the two ice sheets in Antarctica and Greenland as well ground water depletion.

The thermal expansion of the ocean was assessed by a new combination of direct measurements of the Argo network of ocean profiling floats with sea surface temperature records derived from the ESA CCI program. The ocean mass change, on the other hand, was determined both from the sum of the individual contributions derived from the ESA CCI program and by direct satellite-based measurements of tiny changes of the Earth’s gravity field entailed by regional changes of ice and water masses.

Time series of Sea Level Budget elements including the thermal expansion (steric) and individual contributions to ocean-mass change. See legend for attribution of graphs.  Seasonal variations are subtracted. Each graph shows deviations with respect to a mean value over 2006–2015. Graphs are shifted arbitrarily along the ordinate axis. Standard uncertainties are shown by transparent bands (except for the sum-of-contribution graphs). Credit: Horwath et al. 2022Time series of Sea Level Budget elements including the thermal expansion (steric) and individual contributions to ocean-mass change. See legend for attribution of graphs. Seasonal variations are subtracted. Each graph shows deviations with respect to a mean value over 2006–2015. Graphs are shifted arbitrarily along the ordinate axis. Standard uncertainties are shown by transparent bands (except for the sum-of-contribution graphs). Credit: Horwath et al. 2022

The results are in line with previous studies. However, the advancements include the novel and consistent approach to specifying the accuracy limits of all contributing sources to the sea-level budget. These add up to about 10% of the total observed sea-level rise.  This accuracy is the margin within which a mismatch between the sum of the parts and the whole may be expected. 

The results still call for further improvements in the quantitative understanding of the satellite measurements related to sea level change and the corresponding physical processes in question. For example, slow deformations of the solid Earth beneath the ocean affect satellite observations, and these effects need to be quantified and separated from changes within the ocean itself.

 

Read ESA's news story on this publication!

 

Reference: 

Horwath, M., Gutknecht, B. D., Cazenave, A., Palanisamy, H. K., Marti, F., Marzeion, B., Paul, F., Le Bris, R., Hogg, A. E., Otosaka, I., Shepherd, A., Döll, P., Cáceres, D., Müller Schmied, H., Johannessen, J. A., Nilsen, J. E. Ø., Raj, R. P., Forsberg, R., Sandberg Sørensen, L., Barletta, V. R., Simonsen, S. B., Knudsen, P., Andersen, O. B., Ranndal, H., Rose, S. K., Merchant, C. J., Macintosh, C. R., von Schuckmann, K., Novotny, K., Groh, A., Restano, M., and Benveniste, J.: Global sea-level budget and ocean-mass budget, with a focus on advanced data products and uncertainty characterisation, Earth System Science Data, 14, 411–447, https://doi.org/10.5194/essd-14-411-2022, 2022.

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