ANNAPOLIS, Md. -- When Lisa P. Jackson announced at the end of last year that she was stepping down as the administrator of the Environmental Protection Agency, President Obama faced a choice. He could play it safe by appointing her deputy or he could confront Congress head-on and signal a strong commitment to tackling climate change by appointing the agency's head of air quality, Gina McCarthy.
"Why would you want me?" Ms. McCarthy said she asked the president when he offered her the top job. "Do you realize the rules I've done over the past three or four years?"
Ms. McCarthy, an earthy, tough-talking New Englander who drew criticism as the head of the agency's air and radiation office during Mr. Obama's first term, then ticked off a list of controversial air pollution regulations she had helped write: tough greenhouse gas standards for vehicles, a tighter ozone limit that the White House rejected, the first rule on mercury emissions from power plants, and a regulation on smokestack pollution that crosses state lines, which has been blocked by a federal court. She warned that earning confirmation from the Senate might be difficult and that safer choices were available.
The president told Ms. McCarthy that his environmental and presidential legacy would be incomplete without a serious effort to address climate change.
"I'm so glad he said that, because if he hadn't, I wouldn't have wanted this job," she said. "It's an issue I've worked on for so many years, and it just can't wait."
Mr. Obama's decision to nominate Ms. McCarthy, 59, was an act of defiance to Congressional and industry opponents of meaningful action on climate change. It was also a sign of his determination to at least begin to put in place rules to reduce carbon pollution.
Ms. McCarthy was right about her confirmation. She was flooded with more than 1,000 questions from Senate Republicans, who held up a confirmation vote for 136 days, one of the longest delays of any of Mr. Obama's senior nominees. She finally won approval on July 18 on a 59-to-40 vote, as part of a deal reached after Senator Harry Reid of Nevada, the majority leader, threatened to change Senate rules to prevent filibusters on executive branch nominations.
Six Republicans crossed the aisle to vote for her. One Democrat, Joe Manchin III of West Virginia, voted against her, complaining that the administration was waging a "war on coal."
Ms. McCarthy discussed the battles won and the battles yet to be waged on Wednesday, during her first trip outside Washington and her first extended interview as the E.P.A. administrator. Addressing employees at the Chesapeake Bay program office overlooking Annapolis harbor, she said the agency would play a crucial role in dealing with climate change, both in writing the rules to reduce greenhouse gas emissions from new and existing power plants and in helping communities adapt to the inevitable changes wrought by a warming planet.
She also said the agency had to do a better job of explaining its mission to hostile constituencies, including Congress and the agriculture, mining and utility industries.
"We need this agency to reinvent itself, to the extent an agency of 17,000 people can," Ms. McCarthy said in a staff meeting in a waterfront conference room known as the Fish Shack. "I spend a lot of time protecting what we are doing rather than thinking about what we should be doing. The challenges of today are very different from the challenges of 40 years ago. Not every environmental problem deserves a rule."
Ms. McCarthy said Mr. Obama had handed her an epic challenge in his address on climate change at Georgetown University in June. He said that in the face of resistance and inaction in Congress, he would use his executive authority to begin to rein in the emissions that are contributing to global warming. The most meaningful of those powers reside in the E.P.A., which will write regulations governing carbon emissions from power plants, the source of roughly 40 percent of the nation's greenhouse gas pollution.
Under the president's timetable, the first of those rules, covering new fossil fuel plants, is due Sept. 20. The agency must produce draft standards for existing plants, a vastly more complex and controversial undertaking, by next June.
"We worked with him on the schedule," Ms. McCarthy said, referring to the president. "He impressed on us how important it was to get started now. He said to get it done, and get it done right."
Those rules will require a shift in power generation from coal to cleaner-burning natural gas, or development of new cost-effective means of capturing and storing carbon dioxide emissions. The regulations, along with proposed new rules governing coal-mining waste and the disposal of coal ash from power plants, are what Mr. Manchin and others mean when they say the E.P.A. is waging a war on coal.
Ms. McCarthy rejected the charge.
"We don't have a war on coal," she said. "We're doing our business, which is to reduce pollution. We're following the law."
She declined to take a position on the Keystone XL oil pipeline, even though the E.P.A. has submitted two harsh critiques of the environmental impact statements produced by the State Department, which must rule on the pipeline project because it would span the border with Canada.
"That's a matter for the Department of State, and I'm going to leave it there," she said.
Ms. McCarthy is a proud native of the Boston area and a die-hard Red Sox fan. Before going to Washington, she served as a top environmental aide to a half-dozen governors of Massachusetts and Connecticut, including Mitt Romney, the 2012 Republican presidential nominee.
She and her husband, Kenneth McCarey, a wholesale florist, have three adult children, all living in the Boston area. Her eldest, Daniel, 28, married two weeks ago.
In a video message to E.P.A. employees at the beginning of her first full week on the job, Ms. McCarthy looks straight into the camera and says in her thick Hub accent: "Last week was a big week, and I am so pumped. My son got married, our E.P.A. headquarters was renamed after President Clinton, and of course there was this little thing with the U.S. Senate and my confirmation."
She thanked Robert Perciasepe, the E.P.A. deputy who served as acting administrator during the months of vacancy in the top job, and noted that Carol Browner, President Bill Clinton's E.P.A. chief, had held the Bible at her swearing-in ceremony.
Ms. McCarthy also paid tribute to the workers in the E.P.A.'s air office, calling them dedicated, action-oriented and "supah smaht." Those last two words have become something of a catchphrase at the E.P.A. in recent days, but Ms. McCarthy disavowed them.
"Somebody else wrote that," she said later. "It should have been 'wicked smaht.' "
This article originally appeared in The New York Times.