by Alessandro Graziadei – Unimondo*
In a study entitled “Coevolution of Extreme Sea Levels and Sea-Level Rise Under Global Warming”, published in the American Geophysical Union’s (AGU) Earth’s Future, Hamed Moftakhari, Georgios Boumis and Hamid Moradkhani of the University of Alabama say that “most coastal communities will experience 100-year floods every year by the end of the century […] even under a moderate scenario in which carbon dioxide emissions peak by 2040”. Apparently, “as early as 2050, regions worldwide could experience major floods every 9 to 15 years on average.” Based on historical data, a 100-year flood is an extreme water level with a 1% chance of being exceeded in a given year. Despite their name, “centennial floods” may affect the same area for several consecutive years or not at all within a century, but “in a warmer climate, the threshold that we expect to be exceeded on average once every hundred years will be exceeded much more frequently until they are no longer considered centennial events.” Moftakhari, Boumis and Moradkhani point out.
Extreme flooding can be caused by water pushed inland by storms, tides and waves, but this study focuses on a component that contributes to flooding on a much longer time scale: sea level rise. “As higher seas approach the coast, coastal infrastructure will be closer to the water, increasing the likelihood that storms, tides and waves will impact communities.” To analyse trends and estimate future extreme sea levels, the researchers used data from more than 300 tide gauges around the world under two carbon emissions scenarios outlined by the International Panel on Climate Change (IPCC): one in which carbon dioxide emissions continue to rise until the end of the century, and one in which carbon dioxide emissions peak by 2040 and then decline. “In both scenarios, sea-level rise will lead to an increase in flood events on a 100-year basis in most of the locations studied.” Realistic projections of future coastal conditions are now essential to activate better spatial planning and implement coastal protection measures that could help coastal communities reduce flooding and avoid disasters.
And what are the prospects for Italy? If, according to the latest report by the Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA), “almost 94% of Italian municipalities are at risk from landslides, floods and coastal erosion, and more than 8 million people live in areas at high risk from this type of event”, the study “Sea level rise projections up to 2150 in the northern Mediterranean coasts”, published in Environmental Research Letters last December by a team of researchers from the National Institute of Geophysics and Volcanology (INGV), and carried out by the LESIA – Observatoire della Ricerca Ambientale (INGV), published last December in Environmental Research Letters by a team of researchers from the National Institute of Geophysics and Volcanology (INGV), LESIA – Observatoire de Paris and the Radboud Radio Lab of Radboud Universiteit in the Netherlands.
According to the researchers involved in this international study, “the projections of sea-level rise for 2021 published by the Intergovernmental Panel on Climate Change (IPCC) in its AR6 report would be underestimated along the coasts.” Subsidence, i.e. the slow downward movement of land due to natural or anthropogenic causes, plays a crucial role in accelerating sea level rise along the coasts, triggered by global warming, and for the study’s lead author, Antonio Vecchio, “our analyses show that precisely because of subsidence, in some areas of the Mediterranean sea level will rise almost three times faster than in more stable areas.”
The research, which was carried out using data from a large number of GNSS satellite geodetic stations located within 5 km of the sea, and from which it is possible to calculate vertical ground displacement velocities with millimetre precision, showed maximum and minimum differences compared to the IPCC report ranging from +109 cm to about -77 cm, with an average value of about 8 cm higher. According to Marco Anzidei, who coordinated the research for INGV, sea level rise and subsidence “mean that about 38,500 km2 of the Mediterranean coastline – of which about 19,000 km2 in the northern sector of the basin alone – will soon be more exposed to the risk of marine flooding, with a consequent greater impact on the environment, human activities and infrastructure.”
In Italy, the most vulnerable coasts are those of Friuli Venezia-Giulia, Veneto, Emilia-Romagna and northern Puglia on the Adriatic side, and those of Tuscany, Lazio and part of Sardinia on the Tyrrhenian side. For Anzidei, “It is therefore necessary to take concrete action to support coastal populations that will be increasingly vulnerable to rising sea levels and the greater risks they pose by the end of this century and beyond.” And it needs to be done better. Today, many of the engineers who design structures such as sea walls and breakwaters to protect communities from these extreme floods still rely on a concept known as ‘stationarity’, whereby the patterns we have observed in the past will remain the same in the future, but there are many factors related to climate change that modulate these patterns.
In the future, the shift in extreme sea levels will not be uniform and the risk of flooding will no longer reflect historical coastal patterns. Therefore, most of the tools, design guidelines and practice manuals that are based on the assumption of stationarity will need to be updated to allow us to keep up with the effects of climate change. In the meantime, it is good to remember that coastal defences, if well-designed, still play an important role in the ability of coastal communities to resist severe flooding. Still, unfortunately, this will not always be the case in the future and to survive they will need new, unique and often radical solutions, such as the abandonment of some coastal areas, even in Italy.
* Unimondo, founded on 10 December 1998 under the auspices of the Fontana Onlus Foundation, is an online news outlet dedicated to providing authoritative content on peace, sustainable human development, human rights and environmental issues. It offers diverse and timely information, amplifying the voices of different facets of Italian and global civil society. As the Italian hub of the OneWorld network, founded in London in 1995, it is part of a global network with 11 centres worldwide and 1,600 partner associations.