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These 20 cities have the most to lose from rising sea levels

The world's coastal cities will change a lot between now and 2050. They'll get wealthier, for one. Many of them will add millions more people. And some, like New Orleans, will continue sinking into the ground, thanks to things like erosion or groundwater pumping.

On top of all that, there's sea-level rise to consider. As the world gets warmer, climate scientists expect the oceans to rise somewhere around one foot by 2050.

Tally it all up, and flooding is going to become a bigger, more expensive problem for the world's coastal cities. How much bigger? A new study in Nature Climate Change estimates that average annual losses from flooding in the world's biggest coastal cities could rise from about $6 billion per year today to $1 trillion per year by 2050.

But that's only if cities took no preventative action at all. By contrast, if coastal cities worked to bolster their defenses — from levees to pumps to movable barriers — and improved flood monitoring, then the average annual losses go down to $63 billion per year. That's still a big increase, but it's not nearly as steep. (That said, the authors estimate those defenses could cost around $50 billion per year between now and 2050.)

Tim McDonnell of Climate Desk has put together a helpful map from the data in the study showing the 20 cities facing the highest annual average flooding costs by mid-century. This study builds on previous estimates by taking coastal defenses into account:

The United States is well-represented here, not least because its cities are so wealthy and have a lot of stuff to damage: New York City faces about $2 billion in annual losses from flooding in 2050, Miami faces $2.5 billion per year, and New Orleans faces $1.8 billion per year. Boston and Tampa Bay are also looking at considerable expenses. Again, this is all assuming these cities improve their flood defenses to maintain "relative risk levels."

Meanwhile, notice that the city facing the highest absolute costs, by far, is Guangzhou — and that 5 of the 20 cities facing the steepest flooding costs are located in China. Studies like these may help explain why the Chinese government has placed an increased focus on climate change in recent years.*

It's worth noting, however, that climate change is only part of the reason for the increase in flooding risks. The authors estimate that average flood damage would increase to about $52 billion annually even if climate change wasn't a factor at all — just based on population and economic growth. (As more and more Americans move to the coast, the cost of flooding goes up.) Rising sea levels then add another $11 billion per year on top of that.

Even that could be a low-end estimate, though: This study is factoring in relatively mild sea-level rise scenarios of just 7.9 inches by mid-century. If the seas rise more, the costs go up. And then there are the years after 2050 to consider. A recent leaked draft of the forthcoming IPCC report suggested that sea levels could rise more than three feet by the end of the century if emissions continue unchecked.

It's also worth taking a look at the map of cities that will see the biggest relative increase in average annual losses between now and 2050 (these aren't absolute costs, but they'll see the biggest change). Many of these cities, like Houston and Istanbul and Athens and Alexandria, don't usually feature prominently in sea-level-rise conversations:

Note that the map above is assuming a relatively mild sea-level rise (7.9 inches by mid-century) and a steady investment in flood-control technology.

* Note: Another option is to look at which cities are expected to face the most damage as a percentage of their overall GDP. In that case, the top-ten list looks a bit different, composed more heavily of cities in poorer countries:

1) Guangzhou, China
2) New Orleans, United States
3) Guayaquil, Ecuador
4) Ho Chi Minh City, Vietnam
5) Abidjan, Cote d'Ivoire
6) Zhanjing, China
7) Mumbai, India
8) Khulna, Bangladesh
9) Palembang, Indonesia
10) Shenzen, China.

Further reading:

--The full study, led by World Bank economist Stephane Hallegatte, is here. Many of the results can be found here in table form.

--Can we stop the seas from rising? Yes, but less than you think.