Researchers who combined satellite data and digital models showed that the risk of floods as a result of coastal inundation may increase in an accelerated manner during the 21st century, up to 50 times under the scenario of global warming without stopping, especially in the tropics. This increase is mainly caused by a combination of sea level rise and ocean waves.
Researchers who combined satellite data and digital models showed that the risk of floods as a result of coastal inundation may increase in an accelerated manner during the 21st century, up to 50 times under the scenario of global warming without stopping, especially in the tropics. This increase is mainly caused by a combination of sea level rise and ocean waves.
Almost 10% of the world's population lives in low-lying coastal areas. In addition to ongoing erosion and sea level rise, these areas and their unique ecosystems face devastating hazards, including flood events due to the displacement of natural and man-made defenses. We saw examples of this in the case of Hurricane Katrina, which hit the United States in 2005, Cyclone Xinthia in Europe in 2010, and Typhoon Haiyan in Asia in 2013 (the largest tropical cyclone ever recorded). These one-off events are expected to become more severe and frequent due to global warming, while the consequences will also increase due to increased human pressure, such as coastal and infrastructure development and rapid urbanization.
Despite the significant role of ocean waves in determining coastal sea level, in the past their contribution to coastal flooding was largely ignored, mainly due to the lack of accurate topographic information on the coast.
Measuring past events to assess future risks
French researchers from IRD, CNES, Mercator Océan – together with colleagues from the Netherlands, Brazil, Portugal, Italy and Nigeria, have created an unprecedented global integrated digital surface elevation model with new estimates of sea level elevation in the extreme scenario. These extreme water levels are influenced by tides, wave height analysis exacerbated by wind as well as relying on existing measurements of natural and artificial coastal defenses.
The study began by quantifying the increase in global flooding events that occurred between 1993 and 2015. To this end, satellite data was used to define two key parameters for the topography of the coast: the local coastal slope and the areas near the beaches that are below sea level. The extreme level of the coastal waters was calculated in hourly increments in order to identify the potential annual number of hours during which coastal defenses could be removed in each area.
"The combination of tides and large wave events is the main contributor to coastal inundation episodes," says Rafael Elmer, a coastal dynamics researcher at the IRD and Research Center. "We have identified hot spots, where the increase in the risk of evacuation of residents is higher, such as in the Gulf of Mexico, the southern Mediterranean, West Africa, Madagascar and the Baltic Sea."
Acceleration during the 21st century
The scientists also made a preliminary global assessment of the potential for overshooting the existing coastline during the 21st century, by considering different scenarios of sea level rise. The findings show that the number of median hours could increase at a faster rate than the average rate of sea level rise. "The frequency of the free jump is accelerating exponentially and will be clearly felt as early as 2050, regardless of the climate scenario. By the end of the century, the intensity of the acceleration will depend on the future trajectories of greenhouse gas emissions and therefore on sea level rise. In the case of a high emissions scenario, the number of hours of global excess may increase by Fifty compared to current levels," warns Rafael Elmer. As we progress through the 21st century, additional studies at the local and regional level will be needed to test these global predictions, which provide a solid basis for proposing effective adaptation measures in the identified hotspots.
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