Sea level is a key climate indicator as it represents the integrated response to changes in the atmosphere, ocean and cryosphere [1]. Sea-level rise (SLR), together with vertical land movements and changes in sediment supply, threatens coastal population and infrastructure worldwide and without adaptation, the resulting damages are expected to amount up to 10% of the global gross domestic product (GDP) annually by 2100 [2]. SLR during the 21st century is expected to be larger than during the 20th century, and is projected to continue for several thousands of years even if greenhouse gas emissions are stopped now [3].
The latest IPCC report [4] points to a likely global mean sea-level rise of 28–55 cm and 63-101 cm for a very low and a very high emission scenario, respectively, by the end of the 21st century. Also, for the first time it puts forward estimates for a high-end scenario (albeit with low confidence) with global SLR approaching 2 m by 2100 and exceeding 15 m by 2300.
As sea level continues to rise, more regions around the world are projected to become exposed to permanent or episodic flooding [5]. Hence, it is mandatory to continuously assess and monitor sea level changes using all possible tools that the climate science community has developed, such as in-situ and remote sensing-based observational products, model-based products, and products fusing models and observations [6], [7], while carefully considering the associated uncertainties [8].
We address major gaps in contemporary sea-level research with a primary focus on the northern high latitudes, e.g. the non-closure of sea-level budget in the Arctic region [9] and the relationship between coastal sea-level variability and large-scale ocean/atmospheric circulation [10], [11]. The Arctic Ocean is frequently excluded from global sea-level studies as monitoring the Arctic environment is non-trivial: the Arctic observing network is notably lacking the capability to provide a full picture of the changing ocean due to technological and economical limitations to sample the sea surface and seawater properties beneath the ice or in the marginal ice zones.
New observing capabilities from remote sensing will allow us to observe key parameters in the areas of the ocean covered by sea-ice [12] as well as constrain the ocean mesoscale variability which strongly affects mean sea-level projections, e.g. through modification of heat fluxes towards the Antarctic ice shelves [13]. Recently proven methods enhancing the predictability skills of climate variability [14] will offer an optimal playground to assess the sea-level predictability. Furthermore, the suite of satellite, in-situ, reanalysis and climate model datasets are also expected to provide break through information on the physical mechanisms associated with the redistribution of sea water towards shallow continental shelves associated with remote deep ocean warming.
While the so-called “shelf mass loading” (SML) has been acknowledged as the dominant oceanic contribution to coastal sea-level rise [15], it has not been investigated in detail. Commonly, oceanic sea-level rise is represented as the combination of the local steric effect and SML without quantifying their relative importance. It has also not yet been investigated whether the sea-water redistribution is already detectable in the instrumental record.
A new hybrid sea-level reconstructions during the 20th century, based both on models and collection satellite and in-situ observations, developed will be a valuable source of information in addition to the climate models for the high-latitudes sea-level research, e.g., to detect the change of sea-level rates over different decades which can be used to attribute observed changes to natural and anthropogenic drivers of variability.
A novel approach focusing on process-based sea-level projections with coupled climate ice-sheet models [16], including fingerprinting, will be used to project sea level with increased confidence. The above sea-level projections will be integrated with historical observations of shoreline positions (1984-present) as a response to SLR, vertical land movements, sediment supply and distribution [17] to assess future coastal vulnerability.
References
[1] T. Frederikse et al., “The causes of sea-level rise since 1900,” Nature, vol. 584, no. 7821, Art. no. 7821, Aug. 2020, doi: 10.1038/s41586-020-2591-3.
[2] J. Hinkel et al., “Coastal flood damage and adaptation costs under 21st century sea-level rise,” Proc. Natl. Acad. Sci., vol. 111, no. 9, pp. 3292–3297, Mar. 2014, doi: 10.1073/pnas.1222469111.
[3] A. Hu and S. C. Bates, “Internal climate variability and projected future regional steric and dynamic sea level rise,” Nat. Commun., vol. 9, no. 1, Art. no. 1, Mar. 2018, doi: 10.1038/s41467-018-03474-8.
[4] Fox-Kemper, B., H. T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S. S. Drijfhout, T. L. Edwards, N. R. Golledge, M. Hemer, and R. E. Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, I. S. Nurhati, L. Ruiz, J-B. Sallée, A. B. A. Slangen, Y. Yu, “Ocean, Cryosphere and Sea Level Change. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [MassonDelmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.,” 2021.
[5] R. Almar et al., “A global analysis of extreme coastal water levels with implications for potential coastal overtopping,” Nat. Commun., vol. 12, no. 1, p. 3775, Jun. 2021, doi: 10.1038/s41467-021-24008-9.
[6] A. Storto et al., “Ocean Reanalyses: Recent Advances and Unsolved Challenges,” Front. Mar. Sci., vol. 6, 2019, doi: 10.3389/fmars.2019.00418.
[7] S. Dangendorf et al., “Persistent acceleration in global sea-level rise since the 1960s,” Nat. Clim. Change, vol. 9, no. 9, Art. no. 9, Sep. 2019, doi: 10.1038/s41558-019-0531-8.
[8] C. R. MacIntosh, C. J. Merchant, and K. von Schuckmann, “Uncertainties in Steric Sea Level Change Estimation During the Satellite Altimeter Era: Concepts and Practices,” in Integrative Study of the Mean Sea Level and Its Components, A. Cazenave, N. Champollion, F. Paul, and J. Benveniste, Eds. Cham: Springer International Publishing, 2017, pp. 61–89. doi: 10.1007/978-3-319-56490-6_4.
[9] R. P. Raj et al., “Arctic Sea Level Budget Assessment during the GRACE/Argo Time Period,” Remote Sens., vol. 12, no. 17, Art. no. 17, Jan. 2020, doi: 10.3390/rs12172837.
[10] S. Dangendorf, T. Frederikse, L. Chafik, J. M. Klinck, T. Ezer, and B. D. Hamlington, “Data-driven reconstruction reveals large-scale ocean circulation control on coastal sea level,” Nat. Clim. Change, vol. 11, no. 6, pp. 514–520, Jun. 2021, doi: 10.1038/s41558-021-01046-1.
[11] C. W. Hughes et al., “Sea Level and the Role of Coastal Trapped Waves in Mediating the Influence of the Open Ocean on the Coast,” Surv. Geophys., vol. 40, no. 6, pp. 1467–1492, Nov. 2019, doi: 10.1007/s10712-019-09535-x.
[12] P. Prandi, J.-C. Poisson, Y. Faugère, A. Guillot, and G. Dibarboure, “Arctic sea surface height maps from multi-altimeter combination,” Earth Syst. Sci. Data Discuss., pp. 1–29, Apr. 2021, doi: 10.5194/essd-2021-123.
[13] R. M. van Westen and H. A. Dijkstra, “Ocean eddies strongly affect global mean sea-level projections,” Sci. Adv., vol. 7, no. 15, p. eabf1674, Apr. 2021, doi: 10.1126/sciadv.abf1674.
[14] D. M. Smith et al., “North Atlantic climate far more predictable than models
imply,” Nature, vol. 583, no. 7818, Art. no. 7818, Jul. 2020, doi: 10.1038/s41586-020-2525-0.
[15] K. Richter, R. E. M. Riva, and H. Drange, “Impact of self-attraction and loading effects induced by shelf mass loading on projected regional sea level rise,” Geophys. Res. Lett., vol. 40, no. 6, pp. 1144–1148, 2013, doi: 10.1002/grl.50265.
[16] T. L. Edwards et al., “Projected land ice contributions to twenty-first-century sea level rise,” Nature, vol. 593, no. 7857, Art. no. 7857, May 2021, doi: 10.1038/s41586-021-03302-y.
[17] T. Aadland and W. Helland-Hansen, “Progradation Rates Measured at Modern River Outlets: A First-Order Constraint on the Pace of Deltaic Deposition,” J. Geophys. Res. Earth Surf., vol. 124, no. 2, pp. 347–364, 2019, doi: 10.1029/2018JF004750.
[18] P. J. Gleckler, T. M. L. Wigley, B. D. Santer, J. M. Gregory, K. AchutaRao, and K. E. Taylor, “Krakatoa’s signature persists in the ocean,” Nature, vol. 439, no. 7077, pp. 675–675, Feb. 2006, doi: 10.1038/439675a.
[19] D. Swingedouw et al., “Bidecadal North Atlantic ocean circulation variability controlled by timing of volcanic eruptions,” Nat. Commun., vol. 6, no. 1, p. 6545, Mar. 2015, doi: 10.1038/ncomms7545.
[20] C. Deser et al., “Insights from Earth system model initial-condition large ensembles and future prospects,” Nat. Clim. Change, vol. 10, no. 4, pp. 277–286, Apr. 2020, doi: 10.1038/s41558-020-0731-2.