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A New Method of Assessing the Rate of Global Sea Level Rise

Paper Reviewed
Woppelmann, G., Marcos, M., Santamaria-Gomez, A., Martin-Miguez, B., Bouin, M.-N. and Gravelle, M. 2014. Evidence for a differential sea level rise between hemispheres over the twentieth century. Geophysical Research Letters 41: 1639-1643.

As a preface to describing their new approach to the subject, Woppelmann et al. (2014) write that "whichever data analysis strategy is employed, the evidence for sea level rise primarily comes from the information provided by long tide gauge records," which "are mainly located along the coasts of northeast America or western Europe." And given this uneven distribution, they say that "information on long-term spatial variability is limited," citing Woodworth (2006), while additionally noting that "in the majority of studies the tide gauge records have only been corrected for the vertical land motion associated with the glacial isostatic adjustment (GIA)," citing Peltier (2004). And irrespective of the accuracy of the GIA models involved, they state that "other geophysical processes can cause vertical displacements of the land upon which the tide gauges are grounded," citing as examples the facts that (1) "delta regions are prone to subsidence processes, which are often caused by sediment compaction and removal of underground water," as noted by Kolker et al. (2011) and Woppelmann et al. (2013), and that (2) "tectonically active areas are likely to display abrupt vertical land movements," citing Ballu et al. (2011).

The way in which the six European researchers overcame this latter problem was to accurately determine the vertical motion of the land upon which each of the tide gauges employed in their study was located. This they did, based on data they obtained from the Global Positioning System (GPS) that the University of La Rochelle consortium (Santamaria-Gomez et al., 2012) used to produce the final gauge-site vertical velocities. And what did their results reveal?

Woppelmann et al. (2014) report that their work revealed the existence of "a clearly distinct behavior between the Northern and the Southern Hemispheres," with mean sea level rates of rise of 2.0 mm/year and 1.1 mm/year, respectively. And given the coherent spatial patterns they observed, they go on to say that a mean global sea level rate-of-rise value of 1.5 ± 0.5 mm/year is inferred from "a weighted average of the hemispheric trends according to the area they represent." And they note that these findings "challenge the widely accepted value of global sea level rise for the twentieth century."

Ballu, V., Bouin, M.-N., Simeoni, P., Crawford, W.C., Calmant, S., Bore, J., Kanas, T. and Pelletier, B. 2011. Comparing the role of absolute sea-level rise and vertical tectonic motions in coastal flooding, Tores Islands (Vanuatu). Proceedings of the National Academy of Sciences USA 108: 13,019-13,022.

Kolker, A.S., Allison, M.A. and Hameed, S. 2011. An evaluation of subsidence rates and sea-level variability in the northern Gulf of Mexico. Geophysical Research Letters 38: 10.1029/2011GL049458.

Peltier, W.R. 2004. Global glacial isostasy and the surface of the Ice-Age Earth: The ICE-5G (VM2) model and GRACE. Annual Review of Earth and Planetary Sciences 32: 111-149.

Santamaria-Gomez, A., Gravelle, M., Collilieux, X., Guichard, M., Martin-Miguez, B., Tiphaneau, P. and Woppelmann, G. 2012. Mitigating the effects of vertical land motion in tide gauge records using state-of-the-art GPS velocity field. Global and Planetary Change 98-99: 6-17.

Woodworth, P.L. 2006. Some important issues to do with long-term sea level change. Philosophical Transactions of the Royal Society A 364: 787-803.

Woppelmann, G., Le Cozannet, G., de Michele, M., Raucoules, D., Cazenave, A., Garcin, M., Hanson, S., Marcos, M. and Santamaria-Gomez, A. 2013. Is land subsidence increasing the exposure to sea level rise in Alexandria, Egypt? Geophysical Research Letters 40: 1-5.

Posted 8 December 2014