With the baby boomer generation also came a vast increase of infrastructure - roads, bridges and dams. With this infrastructure came substantial environmental changes: dams and levees cut off fish migration; roads fragmented forests and facilitated the spread of invasive plant species; oil and gas platforms discharged waste and released atmospheric pollution.

Many of these structures are now badly in need of repair, at a price tag of more than $1.6 trillion. But a substantial number are abandoned or are no longer used for their original purpose, and government policies on decommissioning, if they exist, are often vague.

Doyle and his colleagues, from other universities and non-profit organizations, propose that some of this aging infrastructure could be viewed as assets rather than liabilities.

"There are ways to combine the decommissioning of some of our infrastructure with ecosystem restoration that would result in multiple benefits," said Doyle, an associate professor of geography in UNC's College of Arts and Sciences and the director of the Center for Landscape Change and Health at UNC's Institute for the Environment.

"Removing aging infrastructure can be a significant opportunity for ecosystem restoration, and can also remove a safety liability, as well as reduce long-term economic costs of constant repairs," he said.

Among the country's inventory of infrastructure listed in the paper are: 3,500 dams that have been rated unsafe; more than 15,000 miles of levees, many with unknown structural integrity; 1,300 offshore oil and gas platforms sitting idle; and a maintenance backlog of over 42,000 miles for U.S. Forest Service roads.

But as these structures decay, there is a growing science, and industry, in ecosystem restoration.

The National Inventory of Dams lists more than 79,000 dams, most of which were built in the early- to mid-20th century. In North Carolina, more than 1,200 dams are "functionally obsolete," Doyle said. Many of these dams pose significant safety hazards, and require constant inspections and repairs even if they no long produce benefits.

"They are disintegrating, they are often not producing power, but their environmental effects are still there," Doyle said.

But, Doyle said, dam removal often leads to a rapid recovery of fish and other flora and fauna. In North Carolina, dams have been included in "mitigation banking," which allows "environmental credits" from dam removal to be applied to other areas.

"Until the 1990s dams were repaired, then the practice gradually started to change," Doyle said. "People realized in some cases, what's the point? Now, with smaller dams, removal is increasingly on the table as an option."

In addition to dam removal, Doyle and his colleagues also reviewed the environmental benefits of decommissioning offshore oil platforms, military installations, forest roads and levees. While in its infancy, Doyle says infrastructure decommissioning programs hold great promise for large-scale environmental restoration.

The U.S. Senate passed the National Infrastructure Improvement Act in August 2007, and it now rests in the House of Representatives. This law proposes to create the National Commission on the Infrastructure of the U.S., which would address some of the issues of infrastructure decommissioning.

But if federal policies are established, they should include "exit strategies, with decommissioning that results in ecological restoration as a viable option," Doyle said.

Co-authors on the paper are Emily Stanley of the University of Wisconsin-Madison, David Havlick of the University of Colorado at Colorado Springs, Mark Kaiser of Louisiana State University, George Steinbach of the California Artificial Reef Enhancement Program, William Graf of the University of South Carolina, Gerald Galloway of the University of Maryland and J. Adam Riggsbee of Restoration Systems.

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