Published online on February 13, 2013, in Nature.

In the wake of Hurricane Sandy, scientists and officials are trying to protect the largest US city from future floods.

Joe Leader’s heart sank as he descended into the South Ferry subway station at the southern tip of Manhattan in New York. It was 8 p.m. on 29 October, and Hurricane Sandy had just made landfall some 150 kilometres south in New Jersey. As chief maintenance officer for the New York city subway system, Leader was out on patrol. He had hoped that the South Ferry station would be a refuge from the storm. Instead, he was greeted by wailing smoke alarms and the roar of gushing water. Three-quarters of the way down the final set of stairs, he pointed his flashlight into the darkness: seawater had already submerged the train platform and was rising a step every minute or two.

“Up until that moment,” Leader recalls, standing on the very same steps, “I thought we were going to be fine.”

Opened in 2009 at a cost of US$545 million, the South Ferry station is now a mess of peeling paint, broken escalators and corroded electrical equipment. Much of Manhattan has returned to normal, but this station, just blocks from one of the world’s main financial hubs, could be out of service for 2–3 years. It is just one remnant of a coastal catastrophe wrought by the largest storm in New York’s recorded history.

Sandy represents the most significant test yet of the city’s claim to be an international leader on the climate front. Working with scientists over the past decade, New York has sought to gird itself against extreme weather and swelling seas and to curb emissions of greenhouse gases — a long-term planning process that few other cities have attempted. But Sandy laid bare the city’s vulnerabilities, killing 43 people, leaving thousands homeless, causing an estimated $19 billion in public and private losses and paralysing the financial district. The New York Stock Exchange closed for the first time since 1888, when it was shut down by a massive blizzard.

As the humbled city begins to rebuild, scientists and engineers are trying to assess what happened during Sandy and what problems New York is likely to face in a warmer future. But in a dilemma that echoes wider debates about climate change, there is no consensus about the magnitude of the potential threats — and no agreement about how much the city should spend on coastal defences to reduce them.

On 6 December, during his first major public address after the storm, New York mayor Michael Bloomberg promised to reinvest wisely and to pursue long-term sustainability. But he warned: “We have to live in the real world and make tough decisions based on the costs and benefits.” And he noted that climate change poses threats not just from flooding but also from drought and heat waves. The city must be mindful, he said, “not to fight the last war and miss the new one ahead”.

Calculated risks

In the immediate aftermath of Sandy, lower Manhattan looked like a war zone. Each night, streams of refugees wielding flashlights wandered north out of the blackout zone, where flood waters had knocked out an electrical substation.

The storm devastated several other parts of the city as well. In Staten Island, pounding waves destroyed hundreds of homes, and one neighbourhood in Queens burned to ashes after water sparked an electrical fire. Power outages lasted for more than two weeks in parts of the city. Chastened by the flooding and acutely aware that Hurricane Irene, in 2011, was a near miss, the city is now wondering what comes next.

To continue reading, visit Nature.