The coastline is receding in the face of climate change

The coastline or coastline is the boundary between land and sea. It can naturally recede when the coast is subject to erosion or thrive when sediment accumulates. But faced with climate change, the expected changes should be more pronounced. The study appeared July 7, 2021 in the journal Scientific reports is interested in 41 estuaries (estuaries) in the world, including the mouths of the Gironde and Loire in France.

The authors, a multidisciplinary team of researchers, used three radiation coercion scenarios called RCP (Representative Path of Concentration) to model the fate of these estuaries. Forced radiation is a disturbance in the radiation balance of the Earth’s climate system, that is, between incoming solar radiation and the emission of infrared radiation that leaves the atmosphere. It is no longer a secret, this radiation is stimulated by climate change of anthropogenic origin. Developed by the Intergovernmental Panel on Climate Change (IPCC), the RCPs are scenarios that represent likely climate variants resulting from an increase in this level of greenhouse gas emissions in the coming years (2000-2100).

According to researchers ’models, regardless of the RCP scenario, climate change will reject the shores of almost all (90%) of the 41 estuaries studied during the 21st century.

According to the most optimistic scenario, therefore low CO emissions2, 46% of the studied estuaries would retreat by more than 100 meters, compared to 68% of the estuaries according to the most pessimistic scenario. The African estuary would be hardest hit, for example, the coast of Beira in Mozambique would retreat by 485 meters under the most pessimistic scenario. In France, the coast of the Gironde estuary should retreat by 224 meters in the event of low emissions, while that of the Loire would retreat by 133 meters. Finally, the most pessimistic scenario predicts a loss of 426 meters and 268 meters!

Mouths: delicate balance

Estuaries are ecotones, ie areas of transition between two ecosystems, in this case between rivers and oceans. “Estuaries continuously exchange sediments between these two ecosystems in order to remain in a certain balance”, says Janaka Bamunawala, the first author of this work. “Climate change illustrated by rising temperatures and sea levels, changing precipitation and land use is causing an imbalance in these sediments due to rising water levels“, she specifies. Consequently, rivers have to feed these sandy areas more sediment, otherwise the coast is eroded and the sea slows down on the banks. in the United States, the Gambia River in Senegal, St. Paul in Liberia, and Kalutara in Sri Lanka), even under the RCP worst-case scenario.

Innovative model

The added value of this work results in the model used by the researchers taking into account changes in sediment volume between estuaries and adjacent shores and the Bruun effect, an estimate between shoreline erosion and sea level rise. “So far, many forecasting models have been interested in historical coastal variations as he studied them in 2018 Arjen Luijendijk, researcher at the Deltares Institute, testifies Janaka Banuwala. Using their previous works, the authors are the first to make extensive, long-term probabilistic predictions of the fate of the estuary.

If the results of the model are probabilistic and represent the uncertainties associated with the long period studied, one thing is certain: we are in the same boat when it comes to climate change. Coastlines will be receded with certainty regardless of the scenario if CO emissions occur2 anthropogenic does not decrease.

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