Reuters

The geography of where rising sea levels will likely cost us the most.

There are a few different ways to think about which cities will suffer the most from flooding caused by future sea-level rise. A developing capital like Abidjan in the Ivory Coast is located on a particularly susceptible coastline of Africa. But a wealthy city like Miami has some of its most expensive real estate already sitting in the way of the rising tide.

So where do these cities really stand relative to each other, as they face a shared threat from climate change? Or, put another way, who needs to start pouring the most money into flood protection right now?

The Organisation for Economic Co-operation and Development has been trying to put numbers to these questions, and a new paper from the effort published in the journal Nature Climate Change suggests the cumulative costs are already rising and likely to get ridiculous by 2050. Modeling population increases and economic growth, the researchers estimate that the global cost of flooding in the world's 136 largest coastal cities could rise to $52 billion a year in the next few decades, up from about $6 billion in 2005.

Many of the metro areas at greatest risk are in China and the United States. But given their high wealth and meager investments in flood protection, three U.S. cities in particular – Miami, New York City, and New Orleans – could be responsible for 31 percent of all of the financial cost of flooding from these 136 cities, based on the 2005 benchmark.

Given current flood protection, those three U.S. cities ranked No. 2, 3, and 4 in the OECD's calculation of the places most vulnerable to the largest average annual losses from flooding (with a potential tab of $672 million in Miami alone). The below map plots the 20 global cities where flood risks, as of 2005, were the largest as measured in average annual losses. Cities are ranked in parentheses.

20 Global Cities with the Largest Annual Flood Risk in U.S. Dollars: This second map looks at the same question in a slightly different way, ranking the 20 cities where the annual cost of flooding represents the largest share of city GDP. These are the places where the wealth gobbled up by encroaching water might be felt the most, and they include a lot of developing coastal Asian cities... and New Orleans. "This value," the researchers write, "can be understood as the share of the city’s economic output that should be saved annually to pay for future flood losses."

20 Global Cities with the Largest Flood Risk Compared to Local GDP: Top image of a man wading through flood waters this week in Manila: John Javellana/Reuters   

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