Each year, government economists around the world calculate the value of goods produced and services provided by all of the activity within the borders of their nations. The value of all those human-produced services is the Gross National Product.

What those figures don't include is all the things we get for free - air cleaning by forests, water filtering from wetlands, and pollination by bees, just to name a few. Those services, not factored into traditional economic estimates, nonetheless have values: If wetlands weren't cleaning water, humans would have to pay for more water pollution controls. Recently, scientists from around the world teamed up to estimate what might be called the world's Gross Natural Product.

They concluded that the value of these natural ecological services, often taken for granted, was in the neighborhood of $33 trillion - about twice that of the world's gross national product.

"Our point at this global level is to say, 'Hey, these systems are providing a big chunk of our total welfare, so you had better start worrying about them, and not just presume that they are small potatoes,'" said Robert Costanza, of the Institute for Ecological Economics, a joint program of the University of Maryland's Center for Environmental Science and its College Park campus.

Costanza wrote the paper, which appeared last year in the scientific journal Nature, with a group of natural and social scientists from universities across the United States, the Netherlands, Sweden and Argentina.

In their analysis, Costanza and his colleagues reviewed available literature about the natural processes performed by the Earth's major ecosystems: Forests, wetlands, oceans, estuaries, grasslands and so on.

These ecosystems offer a wide range of services. Forests provide raw material such as lumber, but also provide recreational opportunities. Waste treatment is a major function of many ecosystems. (Imagine a world where nothing decomposed.) Regulating atmospheric gases such as carbon dioxide, soil creation and the purification of water are all natural services that
people take for granted.

In some cases, the services are essential to life - if nature wasn't providing them, people would have to find a way to perform them, sometimes at great cost. In other cases, the value is largely aesthetic, such as wildlife watching or the appreciation of a mountain landscape.

The scientists used a variety of techniques to attach price tags to all of those functions. In some cases, they used information about how much it costs to replace a particular function - such as the flood control capacity of a wetland. In other cases, they used studies that estimated people's willingness to pay for things, such as recreational opportunities, or the chance to view scenery.

Then the scientists added up the total services for each ecosystem. They found that coastal environments, including estuaries, sea grass beds, continental shelves and coral reefs had disproportionately high values. While covering only 6.3 percent of the world's surface, they are responsible for 43 percent of the world's ecosystem services.

Estuaries, on a per-acre basis, are the most valuable systems anywhere, according to the study, which put their worth at $10,378 an acre. Most of that value is in nutrient cycling - turning nitrogen and phosphorus into food that fuels the aquatic food web.

Using that figure, the Chesapeake Bay and the tidal portions of its major tributaries perform services worth about $29 billion a year. Grass beds and wetlands associated with the Bay would push that number even higher.

Costanza, though, cautioned against reading too much into such estimates. The figures used for the paper were worldwide averages based on scattered studies - the exact value of services in a particular place could vary quite a bit. Polluted areas would likely be less valuable than more pristine sites.

The value of the Bay may be reduced, Costanza noted, because of its degraded conditions. So many nutrients come into the Chesapeake that they produce more algae than can be consumed by predators. Instead, algae sink to the bottom and fuel the growth of less-valuable bacteria which, in turn, deplete the water of oxygen.

"If you start overfertilizing the Bay, the service it provides is going to be damaged," Costanza said. "The Bay can probably handle a little bit of extra nutrients and continue to play that role in nutrient cycling. But once you get beyond a certain point, it starts damaging the Bay's ability to cycle nutrients. We don't know exactly where that point is, but I think we've probably passed it."

Not only is the value of the Bay open to question, but Costanza and his colleagues admit to a wide margin of error for their global estimate. Their $33 trillion estimate actually was the middle of a range that fell between $16 trillion and $54 trillion a year.  

If anything, though, Costanza said the $33 trillion estimate was probably low. "We would be very surprised if the number was less than that, but we wouldn't be surprised at all if the number was more than that."

After all, the scientists could come up with no estimates of services provided by huge areas of the Earth's surface - deserts, tundra, ice caps and urban areas - because no one had done the work. Better estimates will be possible in the future as people begin to more seriously study ecosystem services.

"Our point was, even if you take the limited studies that are already out there, you still come up with quite a large estimate," Costanza said. "So it was compelling enough to put that out."

Though valuable, these services are not factored into market decisions. No one pays for the services lost when a wetlands is drained, a forest burned or an estuary polluted. Eventually, Costanza said such studies will help decision makers "internalize" ecological values in their policies. For example, higher taxes could be assessed on actions such as pollution, wetland draining and other activities that reduce ecosystem functions.

"By assessing a tax, charging people for the damages that they do, that would force them to consider those services in their decision making," Costanza said. "The government has a role to play there in terms of improving the market, making the market work the way that it is supposed to." 

Some people are already putting this to work. Bob Orth, a scientist at the Virginia Institute of Marine Science, using numbers generated in Costanza's paper showing that seagrass beds produce, on average, economic services worth $7,700 an acre, has calculated that the nearly 64,000 acres of grass beds in the Bay are worth nearly $500 million per year.

The figures became a standard part of the presentation he made to lawmakers and government officials as he pressed for stronger measures to protect the Bay's grass beds. "These are probably one of the most valuable habitats in the world," Orth said.

Still, the idea of putting a price tag on nature has long been controversial. Some environmentalists have rejected the idea that a price can be placed on nature: Because we depend on clean air and water, their value is infinite.At the other extreme, some economists have argued that natural systems have little value and can be quickly replaced: New wetlands can be built or forests planted.

"What we're saying is that it's somewhere in between," Costanza said. "They have economic value, and it's not infinite and it's not zero. And when you make these tradeoffs, you have to be cognizant of what it is that you are giving up."

The value of ecosystems would not necessarily remain stagnant over time. If some resources are depleted, it could dramatically push up the value of those that remain. "The value could jump to infinity," Costanza said. "If we destroy the ozone layer altogether, that's a major problem."

The study does not factor in such critical thresholds, Costanza said. What's important, he said, is that people begin recognizing the value of such services before the resource becomes so rare it is, literally, priceless.

"You want to make sure that you stay away from those thresholds," he said. "We're best off with a more precautionary approach. We don't know what is going to happen, but it could be really bad so we had better not push it too hard."

Pricing the Planet

Scientists from around the world teamed up to estimate what might be called the world's Gross Natural Product, which is based on services provided by nature in 17 categories (See below) that are often taken for granted. The figures listed here are a summary of the average global value for these ecosystem goods and services on an annual basis, by biome.

Open Ocean
Total Global Value: $8.381 trillion
Value by Hectare: $252

Estuaries
Global Value: $4.110 trillion
Value by Hectare: $22,832

Seagrass/Algae Beds
Global Value: $3.801 trillion
Value by Hectare: $19,004

Coral Reefs
Global Value: $375 billion
Value by Hectare: $6,075

Shelf
Global Value: $4.283 trillion
Value by Hectare: $1,610

Total Coastal
Includes estuaries, coral reefs, shelf
& seagrass/algae beds

Global Value: $12.568 trillion
Value by Hectare: $4,052

Tropical Forests
Global Value: $3.813 trillion
Value by Hectare: $2,007

Temperate/Boreal Forests
Global Value: $894 billion
Value by Hectare: $302

Total Forests
Includes tropical & temperate/boreal forests
Global Value: $4.706 trillion
Value by Hectare: $969

Grass/Rangelands
Global Value: $906 billion
Value by Hectare: $232

Lakes/Rivers
Global Value: $1.7 trillion
Value by Hectare: $8,498

Tidal Marsh/Mangroves
Global Value: $1.648 trillion
Value by Hectare: $9,990

Swamps/Floodplains
Global Value: $3.231 trillion
Value by Hectare: $19,580

Total Wetlands
Includes tidal marsh/mangroves
& swamps/floodplains

Global Value: $4.879 trillion
Value by Hectare: $14,785

Cropland
Includes only the "natural" part of the land, not standard agricultural crops
Global Value: $128 billion
Value by Hectare: $92

Values for Desert, Tundra, Ice/Rock and Urban biomes are unknown untilfurther study is done in these areas. 

A hectare equals 2.47 acres.

Ecosystem Services

Gas Regulation: Regulates the chemical composition of the atmosphere, such as the oxygen/carbon dioxide balance; ozone for ultraviolet radiation protection.
Climate Regulation: Regulates global temperature, precipitation and other biologically mediated climatic processes at global or local levels, including greenhouse gas regulation.
Disturbance Regulation: The ecosystem's response to environmental fluctuations, including storm protection, flood control, drought recovery and other aspects of habitat response to environmental variablity mainly controlled by vegetation structure.
Water Regulation: Provides water for transportation, agriculture, or industry.
Water Supply: Provides water for reservoirs and aquifers.
Erosion Control & Sediment Retention: Prevents the loss of soil by wind, runoff, etc. and the storage of silt in lakes and wetlands.
Soil Formation: The weathering of rock and the accummulation of organic material.
Nutrient Cycling: The aquisition, storage and processing that turns nitrogen and phosphorus into food.
Waste Treatment: The recovery of mobile nutrients and removal or breakdown of excess or xenic nutrients or compounds, including waste treatment, pollution control and detoxicfication.
Pollination: Provides pollinators for plants.
Biological Control: Includes the predator control of prey species.
Habitat: Provides habitat and nurseries for resident and migratory populations.
Food Production: Features fish, game, crops, nuts and fruits obtained by hunting, gathering, subsistence farming or fishing.
Raw Materials: Raw materials such as lumber, fuel or fodder (excluding fossil fuels and minerals).
Genetic Resources: Sources of unique biological materials and products such as: medicine; products for materials science; ornamental species including pets and horticultural varieties of plants; and genes for resistance to plant pathogens and crop pests.
Recreation: Offers a venue for ecotourism, sportfishing and other recreational activities.
Culture: The aesthetic, educational, spiritual and/or scientific values of ecosystems.

Sources Robert Costanza et al.