Coastal erosion threatens this ancient city — and others much closer to home – USC Today

A new USC study reveals a dramatic surge in building collapses in the ancient Egyptian port city of Alexandria, directly linked to rising sea levels and seawater intrusion.

Once a rare occurrence, building collapses in Alexandria — one of the world’s oldest cities, often called the “bride of the Mediterranean” for its beauty — have accelerated from approximately one per year to an alarming 40 per year over the past decade, the researchers found.

“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 Essam Heggy, a water scientist at the USC Viterbi School of Engineering and the study’s corresponding author.

“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 small sea level increases — just a few centimeters — can have devastating effects, Heggy said, threatening even cities as historically resilient as Alexandria, which has withstood centuries of earthquakes, invasions and fires, and even a modern metropolis like Los Angeles, where flash floods and mudslides are now complicating recovery from the recent wildfires.

Published in Earth’s Future, an AGU journal, the study coincides with troubling findings from NASA and NOAA showing that parts of California — including the San Francisco Bay Area, Central Valley and coastal Southern California — are sinking. These minor elevation changes can significantly heighten flood risks and saltwater intrusion, scientists warn.

Like Alexandria, California’s coastal cities face growing threats from saltwater intrusion, which weakens infrastructure, degrades water supplies and drives up the cost of living.

“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…”