Great 1960s song by John Holt, but little did he knew that scientists are now observing that long-term tidal cycles could complicate sea level rise further by creating positive and negative effects.
Whilst we have quite decent data on sea level rise arising from the melting of the ice sheets and the warming of the ocean, the cumulative impacts of tidal cycles on coastal regions was clear until a team of French scientists used satellite images to look at changes in an untouched region of the French Guiana.
In their quest to measure the 18.6 year tidal cycle on global sea-level variations, they choose an area untouched by man, which would show the slightest changes in the coastline, and minor changes in mangroves whose distribution is affected by seawater. By comparing satellite images over a 20 year period and contrasting it with changes in sea level the researchers found that a 3% increase in tidal amplitude induced more than 100m of coastal erosion along the 1500 km stretch of virgin coastline in the French Guiana. This erosion occurred in the first ten years of the cycle, and after that a fall in the tidal amplitude (by 3%) caused the mangrove forests to regenerate. Whilst this does not discard the impact of sea level rise in the present context, it does show that there are other powerful forces affecting coastal stability and changes.
The team also tried to extrapolate these results at the global scale to establish an estimate of the impact of the tidal cycle on sea level rise (see figure - courtesy of Nature Geoscience).
Adapted from the global map of tidal amplitude proposed by Simon (2007) by considering a modulation of signal of 3%.Grey areas correspond to locations of decrease or negligible rise. The black box (48W-62W-2N-12N) delimits the mud bank system of the Guyanas, South America. Note: Aldabra (Seychelles) region, some regions of Indonesia, and South Pacific which have very high tide amplitudes.
Depending on the change in the tidal amplitude it is estimated that this 18.6 tidal cycle could cause a rise in sea level of more than 50 cm during the period 2008-2015, far greater than the level of sea level rise expected in the next decade. This natural phenomenon reinforces the case of small island states to implement coastal adaptation now and implement wise coastal development practices. Although it is expected that this tidal cycle will cause a reverse of the sea level many coastal infrastructures may be affected by then, and costing nations billions of dollars in protection efforts. In preparing for long-term sea level rise it is important for us to understand that developments taking place today will have to be resilient to these changes, whether it is the tidal cycle or global climate change.
Such research is a wake-up call that even in the context of climate change, integrated coastal management and restoration of coastal areas is relevant and pertinent for this present generation to take action now.
'The tide is high but I'm holding on....' Blondie 1980
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.
Simon, B. La Marée Océanique Côtière (Institut Océanographique, Paris, 2007)