Ancient Egyptian city of Alexandria — the birthplace of Cleopatra — is crumbling into the sea at an unprecedented rate

The historic Egyptian city of Alexandria is crumbling due to rising sea levels, scientists have found.

Over the past decade, the rate at which buildings have collapsed along the ancient city's seafront has increased from around one per year to as many as 40 per year, according to a study published Feb. 12 in the journal Earth's Future.

In the past 20 years, 280 buildings in this 2,300-year-old port city — known for being the birthplace of Cleopatra and the ancient home of the famed Library of Alexandria — have collapsed due to coastal erosion, and 7,000 more are at risk of collapsing in the future, according to the paper. Between 2014 and 2020 alone, 86 buildings completely crumbled, and 201 partially collapsed across the city, resulting in 85 deaths.

"The true cost of this loss extends far beyond bricks and mortar," study co-author Essam Heggy, a water scientist at the University of Southern California's Viterbi School of Engineering, said in a statement. "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."

The crumbling of coastal buildings results from sea level rise and the subsequent intrusion of seawater into the ground under the city. As saltwater creeps farther inland due to rising sea levels, it increases groundwater levels beneath buildings and other infrastructure and erodes the soil. This can lead the ground to sink, which makes buildings unstable and at risk of collapse. Additionally, saltwater corrodes the steel reinforcements of building foundations, further weakening the structures.

Average global sea levels have risen by between 8 and 9 inches (20 to 23 centimeters) since 1880, with a 4-inch (10 cm) rise since 1993 alone, according to the National Oceanic and Atmospheric Administration (NOAA). If nothing is done to curb climate change, U.S. sea levels could rise by as much as 7.2 feet (2.2 meters) by 2100 compared with the levels seen in 2000.

Low-lying cites face the highest risk of erosion and flooding due to sea level rise, especially those along the U.S. East Coast, West Coast and Gulf Coast, according to NASA.

"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."

In the study, the researchers mapped collapsed buildings around Alexandria between 2001 and 2021, and compared satellite images from between 1974 and 2021 with maps of the city from 1887, 1959 and 2001 to determine sea level rise. Using this data, they determined that Alexandria has seen its coastline move inland by tens of meters over the past few decades, with some regions experiencing up to 79 to 118 feet (24 to 36 m) of soil erosion per year.

The researchers also analyzed chemical isotopes in the soils to determine the degree of soil erosion.

"Our isotope analysis revealed that buildings are collapsing from the bottom up, as seawater intrusion erodes foundations and weakens the soil," study co-author Ibrahim Saleh, a soil radiation scientist at Alexandria University, said in the statement. "It isn't the buildings themselves, but the ground underneath them that's being affected."

The researchers suggest several ways that Alexandria could prepare for the oncoming onslaught of seawater. These steps include building sand dunes and other barriers along the coast, elevating buildings, and relocating people who live in the highest-risk areas.

"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."