Rising seas are already toppling coastal cities

Building collapses in one of the world’s oldest port cities have dramatically increased in recent years, and researchers are pointing to sea-level rise and seawater intrusion as key drivers.

The alarming trend highlights the vulnerability of historic coastal hubs to even small changes in ocean levels.

In the port city of Alexandria, located on Egypt’s Mediterranean coast, the rate of building collapses has jumped from approximately one per year to around 40 per year. 

Study co-author Essam Heggy is a water scientist at the University of Southern California (USC) Viterbi School of Engineering. 

“The true cost of this loss extends far beyond bricks and mortar. We are witnessing the gradual disappearance of historic coastal cities, with Alexandria sounding the alarm. What once seemed like distant climate risks are now a present reality,” said Heggy.

Rising seas and intensifying storms 

Alexandria has a history stretching back more than two millennia and has survived earthquakes, tsunamis, and conquests. Yet modern challenges appear to be outpacing the city’s traditional resilience. 

“For centuries, Alexandria’s structures stood as marvels of resilient engineering, enduring earthquakes, storm surges, tsunamis and more. But now, rising seas and intensifying storms – fueled by climate change – are undoing in decades what took millennia of human ingenuity to create,” said Sara Fouad, a landscape architect at the Technical University of Munich (TUM) and the study’s first author.

Even slight sea-level increments – just a few centimeters – can prove destructive for coastal cities, weakening the ground beneath buildings and eroding essential infrastructure. 

These dangers are not confined to Alexandria. Along many shorelines worldwide, scientists have identified the gradual sinking of coastal terrain, coupled with rising seas, which exacerbates the risk of flooding and saltwater infiltration into structures.

Alarm bells for other coastal regions

The latest research on Alexandria parallels findings from NASA and NOAA, which have documented areas of California – such as the San Francisco Bay Area, Central Valley, and parts of Southern California – also sinking. 

Though these changes may be measured in mere centimeters, they heighten flood risks significantly when combined with coastal erosion. 

“Our study challenges the common misconception that we’ll only need to worry when sea levels rise by a meter,” Heggy said. “However, what we’re showing here is that coastlines globally, especially Mediterranean coastlines similar to California’s, are already changing and causing building collapses at an unprecedented rate.”

Such phenomena demonstrate how cities on the Mediterranean – long admired for their coastal beauty – face problems that echo those now emerging in other shoreline communities, from Los Angeles to Naples. 

Rising seas and shifting shorelines threaten everything from ancient monuments to modern housing developments.

The collapse of Alexandria’s coastline

To quantify the growing crisis, the research team employed a comprehensive three-part strategy. First, they produced a detailed digital map of collapsed buildings in six districts of Alexandria’s historic core, among the most densely populated areas of the city. 

The map includes location, size, construction materials, foundation depth, and floor count. Information was compiled via site visits, governmental records, news stories, and private company reports spanning 2001 to 2021.

Second, the group examined satellite images and historical maps from 1887, 1959, and 2001 to chart how parts of the city’s 50-mile shoreline have receded by tens of meters in recent decades. 

By calculating the annual rate of coastal retreat, they found that the diminished coastline is raising groundwater levels, allowing saltwater to permeate deeper into the ground beneath buildings.

Third, they conducted a chemical isotope analysis of soil samples to pinpoint where seawater intrusion is undermining structural foundations. 

“Our isotope analysis revealed that buildings are collapsing from the bottom up, as seawater intrusion erodes foundations and weakens the soil. It isn’t the buildings themselves, but the ground underneath them that’s being affected,” said Ibrahim H. Saleh, a soil radiation scientist at Alexandria University and one of the study’s co-authors.

A nature-based response

Among the proposed solutions is a nature-based coastal defense strategy, which involves creating sand dunes and planting vegetation along the shoreline. 

This approach, the researchers argue, could help slow incoming seawater and keep groundwater levels from rising to the point where they compromise building footings. Such methods have proven effective in other regions where climate-driven coastal erosion is intensifying.

“Preserving the diverse architectural attributes of Mediterranean historic cities is a powerful reminder of how landscape transformation has played a crucial role in creating climate-resilient societies,” noted Udo Weilacher, a landscape architect at TUM and study co-author. 

The idea is not only to protect roads, infrastructure, and buildings, but also to maintain the cultural identity of heritage sites under siege from a rapidly changing environment.

Protecting historic cities from rising seas

Alexandria has long been celebrated as a crossroads of civilizations, highlighted by iconic structures such as the Great Lighthouse (Pharos) in ancient times. Today’s unraveling coastline, however, represents a pressing threat to that cultural tapestry. 

“Historic cities like Alexandria, which represent the cradle of cultural exchange, innovation and history, are crucial for safeguarding our shared human heritage,” Heggy said. “As climate change accelerates sea level rise and coastal erosion, protecting them isn’t just about saving buildings; it’s about preserving who we are.”

While the study’s authors urge immediate action in Alexandria, they also view the city as a bellwether for many other coastal urban centers. With targeted mitigation measures and renewed focus on nature-based solutions, it may still be possible to safeguard these locations – even as global warming ushers in an era of volatile seas. 

The researchers hope that policy initiatives and international collaboration can help retain the architectural marvels of Alexandria and other ancient coastal sites for generations to come.

The study is published in Earth’s Future, an American Geophysical Union (AGU) journal.

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