When Malcolm Bowman presents his scheme for barriers for New York Harbor, he uses a PowerPoint presentation that begins with a cheeky question:

Q: When do you plan for a flood?

A: Too late.

From his study of the history of flood barriers, Bowman knows that it takes a disaster, plus years of planning, to marshal the political will and wherewithal to get such projects off the drawing board. Katrina’s legacy is being felt far beyond the confines of New Orleans, as coastal cities around the world begin to look closely at their own long-term exposure. A handful have recognized rising seas as a serious threat to national security, and are protecting themselves with barriers and other flood protection infrastructure — e.g., retractable barriers around the Venice lagoon, in Italy, and St. Petersburg, Russia, both of which are taking years to build; and the honeycomb of giant stormwater holding tanks beneath Tokyo, which sits directly in the path of tsunamis and cyclones.

Such huge projects are the latest chapters in the long-running story of man’s compulsion to engineer his way past immense natural impediments. But if Jacob is right, this impulse may no longer be useful in the face of human- engineered climate change. Indeed, coastal cities like New York may have arrived at a pivotal moment, where they must begin to reverse the tide of relentless urban expansion and beat a strategic retreat from the water’s edge.

With or without barriers or other such mechanisms, one point is certain: failure to act is no longer an option. Jacob cites Alexandria, the cultural hub of the ancient world. Situated on the Nile Delta, the original city was gradually overwhelmed by silt. “By the time Napoleon got there,” Jacob says, “it was a village.”