Unexpected ally against future hurricanes: nature?

"If we do nothing, those habitats are likely to be chipped away," says Dr. Ruckelshaus. If they're lost entirely, due to development pressures or pollution or climate change, the hazards could double, endangering another 1.4 million people currently living near the shore.

On the other hand, if reefs are restored, the shoreline can start to repair itself. Though this wasn't included in their article, which focused on the nation as a whole, Arkema and her team have begun using a more targeted version of their model to predict the effects of habitat restoration. In one project, in Mobile Bay, they helped select the location for a new oyster reef. "In photographs, we've already seen increased shoreline restoration right behind those oyster reefs, over quite a short time period," says Arkema.

How they mapped coastal hazards

First, scientists defined the "hazard risk" for every square kilometer of coast.

Not all shoreline environments are created equal, nor do they provide equal protection. Anything perched on a cliff overlooking the sea is less vulnerable to storm damage than people or property located on mud flats or gentle beaches, say the scientists. No surprise there.

But the offshore environment matters, too. Underwater coral reefs and semi-submerged mangrove or cypress forests can buffer land over a mile inland, they found, while seagrass beds and other underwater plants provide very little protection, and only for the quarter-mile closest to shore. Marshes, oyster reefs, sand dunes, and seaweed forests provide intermediate levels of protection.

Then, they translated "hazard risk" into the actual threat to human life and property by mapping out coastal populations, poor families, elderly people, and residential property values.

After all, the most expensive storms in US history weren't necessarily the biggest, but the ones that made landfall in areas with the highest property values. Similarly, the most deadly storms weren't the strongest, but those that hit places with low-income or elderly populations that couldn't evacuate easily.

And it's not just about property value, the scientists caution. "We know now why the lower Ninth got flooded in hurricane Katrina," says Ruckelshaus. "If they'd known to protect certain marshes, they could have protected the most economically vulnerable populations in New Orleans and Louisiana."

Decision-makers who look at this hazard map shouldn't limit their focus to property values, warn the researchers in their "Nature" letter, lest they "overlook ecosystems that provide disproportionate protection of vulnerable populations." Texas and Virginia, for example, each have more than a thousand low-income families on the coast, though neither state has more than $5 billion in vulnerable property value.

Finally, once they'd mapped out the hazards, the researchers tested their model against reality.

When they compared their model to hurricanes and other coastal disasters between 1995 and 2010, they found that their model predicted the scale of human fatalities and amount of property damage more than 99 percent of the time.

Then they looked into the future, examining likely storm damage over the next century with and without the protection from coral reefs, coastal forests, and other seashore ecosystems.

"Coastal ecosystems are critical to protecting lives and property at a national scale," says Arkema. 

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