The results of several scientific studies conducted since 1993 have confirmed a 3.2 cm sea level rise. Although this variation might appear negligible, it has in fact turned out to be twice as high as that recorded over the whole of the previous century. This increase in sea level is a consequence of global warming. When sea temperature rises, the sea expands and therefore occupies a greater volume. This phenomenon is now well known to scientists, but other processes that have received less research attention, such as the tidal cycle, seem to contribute at global scale just as much to changes in sea level.
A team coordinated by IRD scientists compared a series of satellite images collected at regular intervals over 20 years to measure the contribution of the bidecennial tidal cycle on global sea-level variations. In the first phase of the study, the scientists focused on the 350 km of French Guianan coastline found to be highly suitable for observation of the phenomenon. This is a virgin region completely unaffected by any human activity and bears the certainty that the slightest change observed in the geomorphology of that coast is natural in origin. The geographical zone is moreover covered by an ecosystem of mangroves whose coastal fringe reacts almost immediately to fluctuations in marine conditions.
The study used 60 images taken by Spot, Landsat, ASAR and JERS satellites to follow-up the changes and developments of the mangrove areas over the 20-year period from 1986 to 2006, in other words a complete bidecennial tidal cycle. In parallel and over the same period, altimetric satellites (Ssalto data produced by Aviso) gave a measure of the change in the sea level. By comparing and contrasting the data resulting from these two types of satellite device, the scientists arrived at a measure of the process’s contribution of the physical features of the coastline.
Their analysis indicated that a 3% increase in tidal amplitude on the French Guiana coast, and along the whole of the 1500 km stretch of coastline of the Guiana Plateau, induced more than 100 m of coastal erosion and shoreline retreat during the first ten years of the cycle. A subsequent 3% fall in the course of the second half of the cycle then allowed regeneration of the mangrove colonies, a sure sign of coastal advance. The results also suggested that 75% of the rise of the open sea level recorded for this coastal zone during the first ten years of the cycle was attributable to the tidal cycle.
On the Guiana Plateau coast, the tidal range –the difference between the high-tide and low-tide water levels– is quite low as it settles at around two metres on average. In this context, it is predicted that between 2006 and 2015 the rise in open sea level, directly linked to the bidecennial cycle, will not exceed a few centimetres. It should therefore be about the same order of magnitude as the sea level increase linked to thermal expansion of the ocean.
Extrapolation of the results obtained for the Guiana Plateau coast led to an estimate of the impact of the tidal cycle on the sea level rise at global scale (see Map). Coastal zones exist where the tidal range is much more spectacular in size than on the Guianan coasts. At Mont Saint-Michel in France, for example, it can be more than 12 m. And in Ungava Bay, on the East coast of Canada, where the world’s largest tidal amplitudes are recorded, it reaches as high as 20 m. In these regions, from the present day (2008) to 2015, the bidecennial tidal cycle could cause a rise in the open sea level of more than 50 cm, or 25 times greater than the rise linked to global-warming induced oceanic thermal expansion.
Over the period 2015-2025, the second phase of this cycle is predicted to contribute to a regular fall in the open sea level. At planetary scale, it could thus partly compensate for the effects of the global-warming related rise in the sea water. Thanks to a better awareness of the cyclic nature of the tides, probably one of the most predictable cyclic systems in the world, this research should, over the next 20 years, lead to a better understanding of coastal geomorphology and in particular the processes of coastal erosion.
- N. Gratiot, E. J. Anthony, A. Gardel, C. Gaucherel, C. Proisy, J. T. Wells, Significant contribution of the 18.6 year tidal cycle to regional coastal changes. Nature Geoscience. volume 1, March 2008. Doi: 10.1038/ngeo127
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