With Interior Secretary Ken Salazar’s announcement Wednesday that he intends to resign, the Obama administration finds itself in exactly the same place it was four years ago: looking to fill the three most important environmental posts in the federal government.

The Interior Department, the Environmental Protection Agency and the National Oceanic and Atmospheric Administration all have openings at the top, while the administration has a limited amount of time to press a second-term environmental agenda before the next election cycle begins.

Activists say they are optimistic that President Obama can pursue some ambitious environmental policy goals. “This clearly doesn’t reach the level of gun control or immigration, but I feel it’s on the table in a way it wasn’t in the first term,” said William Meadows, counselor to The Wilderness Society.

By contrast, American Petroleum Institute President Jack Gerard said a new slate of agency heads could “adjust some of the more extreme positions taken early on” during the administration. “The potential is there for a reset button, to turn the focus to job growth and economic recovery,” he said.

Obama’s environmental legacy is likely to be defined by how he handles climate change issues — including whether to permit the Keystone XL pipeline extension or impose carbon limits on existing power plants.

His choices to head the three agencies also will affect decisions ranging from what kind of offshore drilling will take place in the Arctic to whether gold mining can proceed in Alaska’s Bristol Bay watershed.

The top contenders to replace Salazar include former Washington governor Chris Gregoire (D); David Hayes, deputy secretary at Interior; former senator Byron L. Dorgan (D-N.D.); and Office of Personnel Management Director John Berry, who was an assistant secretary at Interior in the Clinton administration and led the National Fish and Wildlife Foundation and the National Zoo. Several green groups are backing Rep. Raúl M. Grijalva (D-Ariz.) for the job.

Gregoire is also under consideration for the EPA slot being vacated by Lisa P. Jackson, along with the agency’s deputy secretary Bob Perciasepe, and the head of its air and radiation office, Gina McCarthy. Rohit “Rit” Aggarwala, a former top aide to New York Mayor Michael R. Bloomberg (I), is another potential candidate.

While NOAA does not rank as a regulatory powerhouse beyond fisheries issues, the science it conducts influences policymaking on offshore drilling as well as public attitudes about climate. Donald Boesch, who heads the University of Maryland’s Center for Environmental Science and served on Obama’s 2010 oil spill commission, has the support of several key Maryland politicians to replace Jane Lubchenco. Kathryn Sullivan, NOAA’s deputy secretary, is also a contender.

Some environmentalists are pushing for Obama to include a senior environmental adviser in the White House. “The administration has great opportunities to make some historic advances on climate and lands protection, and will really only be effective if someone in his inner circle takes this on,” said Michael Brune, executive director of the Sierra Club.

Heather Zichal, deputy assistant to the president for energy and climate change, is the highest ranking adviser. She was deputy to Carol Browner, who was not replaced.

Many environmentalists want Obama to use his executive authority to create new national monuments in his second term. The last Congress was the first since 1966 not to designate a new wilderness area.

“We’re hoping he can leave a legacy for conservation of public lands and have a real vision for it,” said The Wilderness Society President Jamie Williams. “He’s got the ability to respond to America’s desire to protect these places in bolder ways.”

But House Natural Resources Committee Chairman Doc Hastings (R-Wash.) issued a statement Wednesday suggesting he would resist that approach.

“President Obama must now nominate a secretary of the interior who will realize the job- and revenue-creating potential of all-of-the-above energy production, will use sound science to guide decision making, and will protect public access to public lands for recreation and economic development,” Hastings said.

Thomas L. Strickland, who served as Salazar’s assistant secretary and now is a partner at the law firm WilmerHale, said global warming remains “the dominant issue of our time” when it comes to the environment.

“How we come to terms with that and how we deal with it in a way that doesn’t disrupt lives and the economy of the planet, that’s a major challenge,” Strickland said, “but I think it’s doable.”

White House spokesman Clark Stevens reiterated Obama’s intention to tackle climate change, calling it “among his top priorities in his second term” along with “enhancing energy security.”

“While the administration has made historic progress in these areas to date, from improving vehicle fuel efficiency standards to increasing renewable energy production and responsibly expanding development of our domestic oil and gas resources, we know there is more work to do,” he said.