A couple poses for a portrait on cement blocks near the citadel in Alexandria, Egypt. AP

Egypt's coastal city of Alexandria has survived invasions, fires and earthquakes since it was founded by Alexander the Great more than 2,000 years ago.

But the fabled port city now faces a new menace from climate change.

Rising sea levels threaten to inundate poorer neighborhoods and archaeological sites, prompting authorities to erect concrete barriers out at sea to hold back the surging waves.

Stanley Beach in Alexandria, Egypt, at the same site of a 1933 photograph, foreground. AP

A severe storm in 2015 flooded large parts of the city, killing at least six deaths as two dozen homes collapsed, exposing weaknesses in the local infrastructure.

The city, Egypt's second-largest, is surrounded on three sides by the Mediterranean and backs up to a lake, making it uniquely susceptible to the rise in sea levels caused by global warming and the melting of the polar ice caps.

A fisherman and his cat stand beside a cement barrier.AP

Back in the late 1940s and 1950s, it was a haven for writers and artists that drew both Egypt's well-heeled and foreign tourists for its beauty and charm. Today, more than 60 kilometers (40 miles) of waterfront make it a prime summer destination for Egyptians, but many of its most famous beaches already show signs of erosion.

cement barriers reinforce the sea wall near the citadel in Alexandria, Egypt. AP

Experts acknowledge that regional variations in sea level rise and its effects are still not well understood. But in Alexandria, a port city home to more than 5 million people and 40% of Egypt's industrial capacity, there are already signs of change.

Egypt's Ministry of Water Resources and Irrigation says the sea level rose by an average of 1.8 millimeters annually until 1993. Over the following two decades that rose to 2.1 millimeters a year, and since 2012 it has reached as high as 3.2 millimeters per year, enough to threaten building foundations.

Associated Press