Treating wastewater is expensive: to get rid of organic matter, phosphates, and nitrogen compounds that farms and cities release into rivers, you need to pump the water through costly complexes of pumps, filters, and tanks. A tidal marsh, on the other hand, does the same job free of charge. The polluted water fertilizes plants and microbes, which in turn support a huge food web, and by the time the water makes its way out of the marsh, it’s scrubbed clean. One acre of tidal marsh performs about $2,800 worth of water purification every year. Multiply that by the 165 million acres of coastal wetlands on the planet, and you get a tidy annual bill of $462 billion.
That sort of figure doesn’t often make its way into environmental decisions, but Robert Costanza thinks it should. Every time we do something that affects these ecosystems, we’re making a trade-off, but we ...
