The SunZia transmission line that would link sun and wind power from central New Mexico with cities in Arizona is just the sort of energy project an environmentalist could love -- or hate. And it is just the sort of line the Interior Department has been tasked with promoting -- or guarding against.

If built, the 460-mile line would carry about 3,000 megawatts of power, enough to avoid the need for a handful of coal-fired plants and to help utilities meet mandated targets for use of renewable fuel. "We have to connect the sun of the deserts and the winds of the plains to places where people live," Interior Secretary Ken Salazar said recently.

But the line would also cross grasslands, skirt two national wildlife refuges and traverse the Rio Grande, all habitat areas rich in wildlife. The graceful sandhill crane, for example, makes its winter home in the wetlands of New Mexico's Bosque del Apache National Wildlife Refuge, right next to the path of the proposed power line. And much of the area falls under the protection of the Interior Department's Bureau of Land Management (BLM).

Renewable-energy development, which the Obama administration has made a priority, is posing conflicts between economic interests and environmental concerns, not entirely unlike the way offshore oil and gas development pits economics against environment. But because of concerns about climate, many environmentalists and government agencies could find themselves straddling both sides, especially in Western states where the federal government is a major landowner.

"Everybody in New Mexico loves the sandhill cranes," said Ned Farquhar, a former aide to New Mexico Gov. Bill Richardson (D). "We also love our renewable energy. So we have to figure this out."

Farquhar made that comment a month ago when he was working for the Natural Resources Defense Council. Since then, he has been appointed head of the BLM -- in charge of figuring it out.

As the push for renewable-energy development intensifies across the United States, scientists and activists have begun to voice concern that policymakers have underestimated the environmental impact of projects that are otherwise "green."

"There is no free lunch when it comes to meeting our energy needs," said Johanna Wald, a senior lawyer at the Natural Resources Defense Council. She added, however, that the renewables boom "offers a chance to do it right."

"We want to do it differently compared to how we did oil and gas development," she said.

There is no question that permit applications for renewable-energy projects are on the rise, especially on federal land in the West. According to Ray Brady, leader of the BLM's energy policy team, the bureau has received 199 applications for solar projects encompassing 1.7 million acres of land, though only two of them have undergone environmental assessments.

The agency has already authorized 206 wind projects -- 28 of them to generate power, the rest primarily to test a region's wind-generation capacity -- and at least 200 more are awaiting approval.

The fact that eight Western states have established "renewable portfolio standards" has accelerated the push for new projects, Brady said, because those policies are forcing utilities to find additional renewable sources of electricity.

"For all of these reasons, BLM does have a challenge because of the additional work involved," said Brady, who predicted that the agency may hire as many as 100 people just to work on renewable-energy permits. "Clearly there's an interest in expediting and streamlining the process. However, we need to make the right decisions that are based on the best science."

One of the biggest challenges renewable-energy projects pose is that they often take up much more land than conventional sources, such as coal-fired power plants. A team of scientists, several of whom work for the Nature Conservancy, has written a paper that will appear in the journal PLoS One showing that it can take 300 times as much land to produce a given amount of energy from soy biodiesel as from a nuclear power plant. Regardless of the climate policy the nation adopts, the paper predicts that by 2030, energy production will occupy an additional 79,537 square miles of land.

The impact will be "substantial," said Jimmie Powell, the Nature Conservancy's national energy leader and one of the paper's co-authors. "It's important to know where the footprint is going to be."

In some cases, scientists are just beginning to discover the unintended effect of projects such as wind turbines. Grassland birds such as the lesser prairie chicken and the greater sage grouse, both of which are candidates for listing under the Endangered Species Act, appear to avoid vertical structures such as wind turbines and transmission-line towers. This is proving to be a problem in states such as Kansas, an ideal site for wind power, because as more turbines are built, lesser prairie chickens will confine themselves to narrow ranges, fragmenting a population that must be connected to survive.

"Nobody knows what's in the bird's head, but presumably there's an inherited behavior that allows the birds to avoid avian predators who could perch overhead," said Michael Bean, wildlife director for the Environmental Defense Fund, an advocacy group.

The U.S. Fish and Wildlife Service has proposed requiring that developers keep wind turbines at least five miles away from a prairie grouse lek, or mating area, but the wind industry has resisted this idea.

Ditlev Engel, president and chief executive of the Danish wind-energy company Vestas, said anecdotal evidence about birds being caught in turbine blades and other environmental horror stories do not usually hold up under scrutiny.

"Do people think it's better all those birds are breathing CO2? I'm not a scientist, but I doubt it," said Engel, whose company is expanding its U.S. manufacturing and distribution operations. "Let's get the facts on the table and not the feelings. The fact is, these are not issues."

In many instances, producers of renewable energy are coordinating with environmental groups and federal agencies to try to map out the best locations for energy production, whether in the West or offshore. The Natural Resources Defense Council and the National Audubon Society have created an online mapping project, using Google Earth, of 13 Western states to show where renewable projects would have the most impact. Out of the 860 million acres in those states, for example, there are 10,000 conservation areas, and 128 million acres are off limits to energy development.

In the case of SunZia, the company has been working to minimize the impact of its proposed transmission line. Tom Wray, manager of generation and transmission projects, said that as much as 80 percent of the line's path would parallel existing lines. He said that it would cross the Rio Grande north of the sandhill crane's flyway and that it would zig and zag to skirt environmentally sensitive areas. Every mile added to the length of the line, however, would add about $1 million to the project.

"We're not aware of any threatened or endangered species habitat or impact issues that we can't mitigate or deal with," Wray said.

Lawrence A. Selzer, president of the Conservation Fund, said the new administration is eager to advance these projects without alienating environmentalists. "The answer from President Obama can't be no," he said. "They've got to find a way to say yes."