We remember Xynthia in February 2010 in France, but also Katrina in August 2005 in Louisiana (USA). Sea submarines are particularly striking disasters because their size exceeds the memory and imagination of humans. The sea suddenly occupies land areas that we believed were out of its reach. The phenomenon has been known for a long time. “The combination of large waves, the astronomical tide of spring water with the atmospheric tide, that is, the island of the sea due to low pressures and wind“, sums up Rafael Almar, a researcher at Laboratory for Geophysical and Spatial Studies of Oceanography (LEGOS, CNRS / IRD / CNES / University of Toulouse) and co-author of the article Communications of nature published on June 18, 2021, which provides evidence of a worrying phenomenon: these events increased by 50% between 1993 and 2015.
Different elements of sinking: sea level, tides, atmospheric pressure and waves. Copyright Raphal Almar / Nature Communications
Why 1993? Since from that year satellite images began to provide sea level monitoring, then it remained to assess the responsibility of an important actor in underwater waters: waves. “Scientists had at their disposal very accurate images of the coast, especially the heights of natural protections such as dunes or artificial ones such as embankments, as well as the topographic configuration of coastal marine areas that affects the strength of the waves.“Rafael Almar specifies. The researchers were therefore able to construct a model that includes the local sea level provided by satellites, tides, depth of atmospheric depression and wind force that characterize the storm, coincidence at high tide with the arrival of the storm, coastline configuration (beaches, slopes) mountains, dunes, etc.) and its protection, and in particular the frequency and intensity of the waves which are indeed more or less strong, more or less high according to the relief on which the waves will break.These high-resolution images were provided by a Japanese satellite ALOS World 3D. It has allowed researchers to consider this element, which often causes protection failure. At 1.5 million kilometers of the planet’s coast, 1/3 is exposed to wave violence.
A phenomenon that mainly affects tropical countries
The model was applied to 14,000 points on all coasts of the planet to quantify the duration of sea floods in the last years of the research period until 2015, i.e. more than 15,000 hours per year on average where the sea temporarily took the place of land. In the case of temperate zones (especially in the northwestern United States, Scandinavia, and the far east of Russia), the shores of the intertropical zone are most commonly affected, indicating in passing that the phenomenon is underestimated because it affects either sparsely populated areas or shelters with little infrastructure and economic value. .
“We then looked back to see if our model gave results that matched the observations made and to get the trend.“Rafael Almar continues. The result? Marine submarines flooded the earth an average of 10,000 hours a year during the first years of the study after 1993, a 50% increase in two decades entirely attributed to rising water levels. Alternatively, the team compared the results of its model with data recorded during Storm Xynthia to find a good deal.
A new tool for predicting extreme events
After the overhead projector, it’s time to predict. As with IPCC climate models, the researchers applied theirs to scenarios of increased greenhouse gases in the atmosphere and ocean, resulting in rising sea levels. Under the worst-case scenario, where no effort is made to limit emissions, submarines at sea would be 50 times more numerous. “Acceleration is even inevitable and the population will feel it clearly from 2050, Rafael Almar warns. In the second half of the century, the intensity and number of events will depend on efforts to reduce greenhouse gases.“.
It is a newly designed tool for adapting to climate change. Because the idea is to improve this model at the local level to assess the effectiveness of the protection structure, but also the feasibility of building infrastructure or buildings in an area that risks being regularly flooded. By the sea. “Alos World 3D is the most accurate satellite today, but in a few years, especially with French missions like Co3D, we will make further progress and then we will be able to provide a decision-making tool locally for public authorities and businessesRafael Almar hopes.