They're a thousand miles apart, but New York and Miami share a common reality: The Atlantic Ocean laps at their front doors. And as that ocean's water level rises in the decades to come, floods that used to be a once-in-a-lifetime event will become commonplace, while the new once-in-a-lifetime flood will be far more severe than in the last century.

It's a reality both cities are trying to face, in different ways. Climate Central took an extended look at the situation this month, in a long-form article by Rob Motta and James White ( "A Tale of Two Cities: Miami, New York, and Life on the Edge").

"New York City was in the process of planning for sea level rise when Sandy hit in October 2012," Climate Central points out. "If the city did not have reason to prepare for sea level rise before Sandy, it sure did afterwards. Sandy, a '1-in-70 year storm' with its 9-foot storm surge, flooded 17% of the city, impacting 443,000 people and 88,700 buildings. It caused $19 billion in damage to the city alone. Swiss RE, a re-insurance company, estimated that if nothing is done to protect against sea level rise, a similar 1-in-70 year storm would cause $90 billion in damage to the city by the 2050s."

It's the combined risk of sea level rise and storm surge that threatens the city with catastrophe. "Subway entrances in New York are less than 6 feet above high tide," Climate Central observes. "It would take a while for the seas to rise by this amount. But with Sandy's 9-foot storm surge on top of a high tide, the entranceways to subways and tunnels were easily breached, causing extensive flooding and damage. Damage to the region's transportation system alone topped $8 billion."

So what's the implication of the projected coming rise in sea levels? "[Sandy's] level of flooding in New York City may be unprecedented, but consider what a 3-foot sea level rise would do," writes Climate Central. "With these higher seas all major storms since 1950 would have flooded the subways." That would be ten Sandy-like disasters in a span of sixty years.

Miami is a similar but different story, the report observes. "The Miami metropolitan area also faces the challenge of rising seas, but with a number of interesting differences both physically and politically. Most of the area is within a few feet of sea level, meaning it will not take much of a storm surge to flood the city. Even now, when a high tide hits, water bubbles up through the grates in the streets and floods parts of Miami and Fort Lauderdale.  The area also relies on the Biscayne aquifer for much of its water supply, and as sea level rises, the fresh-salt water interface is moving inland—meaning wells that are close to the coast must be abandoned and moved inland to continue to get fresh water. This can only go on for so long before securing enough fresh water from underground sources becomes a problem. But perhaps the biggest issue for the area is that the land is made up primarily of porous limestone through which groundwater moves easily, which means that levees are not a viable option—water will simply go underneath."

If levee protection is unfeasible, the authors write, Miami is left with two less attractive options: Adapt the city's structures and landscapes to the rising water, or head for the hills.

"So what can the people of South Florida do? If protection is not viable because of the porous ground, that leaves adaptation or retreat," the authors write. "Adaptation may work for a while, but ultimately it is hard to imagine high-rise buildings with water lapping at their bases.
That leaves retreat. Many people think that a relocation of people is the only viable option for South Florida. And if that retreat is not carefully managed, then the hit to the area's economy and tax base could be substantial."

To attack the problem, local authorities in the Miami region have formed a consortium called the Southeast Florida Regional Climate Change Compact. The group's Regional Climate Action Plan contains 110 recommendations for improving the region's resilience in the face of climate change.

New York City's draft plan to address climate change and sea level rise, titled "A Stronger, More Resilient New York," is hosted at the website of the city's newly created Office of Recovery and Resiliency (ORR).