By Tim Radford, Climate News NetworkThis piece first appeared at Climate News Network.

LONDON — The best thing to protect your property from the sea is a sand dune – or a mangrove swamp, or a coral reef, kelp forest or sea grass meadow. Nature, which has been doing the job for three billion years, has had time to work out the surest and most enduring sea defences, according to US researchers.

Katie Arkema of the Natural Capital Project at Stanford University in California and colleagues found that the conservation of natural habitat could reduce by half the people and property at risk from coastal storms.

As planetary temperatures rise and the ice caps retreat, sea levels are expected to rise: more frequent or more intense storms and flooding are also forecast.

In total, 23 of the 25 most populous counties in the US are on the coasts. Coastal engineering is expensive, and anyway may not be the best solution.

The researchers mapped the US coastline and calculated exposure to risk by 2100 to a resolution of one square kilometre. They factored in a number of scenarios for sea level rise and climate change. They also considered the densities of populations of the poor and the elderly and families with small children – those people most vulnerable to coastal storms.

They report in Nature Climate Change that they looked at natural habitat and began to assess its value in protecting coasts: in many cases, of course, development has removed this natural protection. Notoriously, Louisiana lost vast tracts of wetlands in the 20th century, and with it the natural protection for New Orleans against storm surges of the kind that arrived in 2005 with Hurricane Katrina.

But they found that 67% of the US coastline was protected by natural habitat. Inevitably, much natural protection had also been lost.

“Today, 16% of the US coastline comprises `high hazard’ areas, harbouring 1.3 million people, 250,000 elderly, 30,000 families below the poverty line and US$300 billion in residential property value”, the researchers say.

There clearly is a case for coastal engineering. Urban areas and ports need breakwaters, and shoreline communities need some kind of protection.

After Superstorm Sandy battered and flooded New York in 2012, two beach communities in New Jersey discovered to their surprise that they had been protected by a sea wall built in the 19th century, and long since buried and forgotten.

But the Stanford-led researchers argue that such structures are costly, difficult to maintain and may not be as effective as a natural buffer.

“The natural environment plays a key role in protecting our nation’s coasts”, said Dr Arkema. “If we lose these defences, we will have to have either massive investment in engineered defences or risk greater damage to people and billions in property.”

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