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 don’t realize it, says Costanza, an ecological economist from the University of Maryland. Costanza is part of a growing movement of researchers who are trying to find the value of ecosystems—including not only their commercial products, like forest timber, but also the less visible ways in which they make the planet habitable. Forests prevent soil erosion and mud slides that would be expensive to stop artificially; coral reefs not only act as nurseries for young fish but also help protect shorelines.
This year Costanza and a group of like-minded colleagues put together a rough estimate of the total value of the world’s ecosystems. This is a back-of-the-envelope calculation, but nobody had done it before, says Costanza. It’s far from perfect—the researchers used values calculated from studies of a handful of swamps, for example, and applied them to all swamps worldwide. But when faced with uncertainty, they tried to stick to low-end figures. And still the total price was astounding: ecosystems, they estimate, provide $33 trillion dollars of services every year—which is $8 trillion more than the gross national product of all the countries on Earth. The assumption of a lot of economists was that, yeah, there are these services, but they are probably pretty minor—maybe 1 or 2 percent of gnp, and so they don’t have to be a main focus, says Costanza.
The map Costanza’s group created reveals that not all ecosystems are created equal. The services provided by forests and grasslands that make up much of the interior of the United States are worth about $100 an acre. But coastal ecosystems—such as estuaries, tidal marshes, mangroves, sea-grass beds, and coral reefs—are much more pricey, running up to as much as $8,000 or $9,000 an acre a year. Although they make up only 6.3 percent of the planet’s surface, they make up 43 percent of the value of its ecological services. In ecology, the edges are where a lot of important things happen, says Costanza. But sadly, these are some of the most endangered ecosystems of all, in part because over half the world’s population lives in coastal regions. We can’t count on tidal marshes to take care of our sewage anymore. At least now we have a better idea of the value of what we’re losing.