Lisa P. Jackson is stepping down as administrator of the Environmental Protection Agency after a four-year tenure that began with high hopes of sweeping action to address climate change and other environmental ills but ended with a series of rear-guard actions to defend the agency against challenges from industry, Republicans in Congress and, at times, the Obama White House.
Ms. Jackson, 50, told President Obama shortly after his re-election in November that she wanted to leave the administration early next year. She informed the E.P.A. staff of her decision on Thursday morning and issued a brief statement saying that she was confident "the ship is sailing in the right direction."
She has not said what she intends to do after leaving government, and no sucessor was immediately named, although it is expected that Robert Perciasepe, the E.P.A. deputy administrator, will take over at least temporarily.
Ms. Jackson's departure comes as many in the environmental movement are questioning Mr. Obama's commitment to dealing with climate change and other environmental problems. After his re-election, and a campaign in which global warming was barely mentioned by either candidate, Mr. Obama said that his first priority would be jobs and the economy and that he intended only to foster a "conversation" on climate change in the coming months.
That ambivalence is a far cry from the hopes that accompanied his early months in office, when he identified climate change as one of humanity's defining challenges. Mr. Obama put the White House's full lobbying power behind a House cap-and-trade bill that would have limited climate-altering emissions and brought profound changes in how the nation produces and consumes energy.
But after the effort stalled in the Senate, the administration abandoned broad-scale climate change efforts, instead focusing on smaller regulatory actions largely though the Clean Air Act.
White House and E.P.A. officials said Ms. Jackson's decision to leave government was her own and the timing had been negotiated with the White House.
Mr. Obama praised her in a statement, calling her "an important part of my team."
"Over the last four years, Lisa Jackson has shown an unwavering commitment to the health of our families and our children," the president said. "Under her leadership, the E.P.A. has taken sensible and important steps to protect the air we breathe and the water we drink, including implementing the first national standard for harmful mercury pollution, taking important action to combat climate change under the Clean Air Act, and playing a key role in establishing historic fuel economy standards that will save the average American family thousands of dollars at the pump, while also slashing carbon pollution."
After Republicans seized control of the House in 2010, Ms. Jackson became a favored target of the new Republican majority's aversion to what it termed "job-killing regulations." One coal industry official accused her of waging "regulatory jihad," and she was summoned to testify before hostile House committees dozens of times in 2011. She was frequently subjected to harsh questioning that at times bordered on the disrespectful.
Ms. Jackson, the first African-American to head the E.P.A., brushed off that treatment as part of the territory and a reflection of the new partisan reality in Washington. More difficult for her was the lack of support she received at times from environmental groups, who saw every compromise as a betrayal, and from the White House, which was trying to balance worries about the economy and the president's re-election campaign against the perceived costs of tough environmental policies.
The White House rejected or scaled back a number of proposed new regulations from the environmental agency, most notably the withdrawal of a proposed new standard for ozone pollution that Ms. Jackson sought in the summer of 2011. President Obama rejected the proposal on the grounds that it would be too costly for industry and local government to comply with at a time of continuing economic distress. Other new rules, including those for emissions from industrial boilers and cement factories, were either watered down or their introduction delayed after complaints from lawmakers, lobbyists and businesses.
Despite a number of disappointments, however, Ms. Jackson has achieved some notable firsts, including the finding that carbon dioxide and five other gases that contribute to global warming meet the definition of pollutants under the Clean Air Act. That so-called endangerment finding, which has survived federal court challenges from industry, allowed the agency to negotiate strict new emissions standards for cars and light trucks, the first time the federal government has limited global warming pollution.
The new vehicle standards will eliminate billions of tons of carbon dioxide emissions and double the fuel efficiency of the American light-duty transportation fleet over the next decade.
The finding also formed the basis of the first steps toward regulating greenhouse gas emissions from new power plants, and, possibly, toward requiring existing ones to reduce global warming pollution. The rule governing new power plants in effect bans the construction of new coal-fired power plants unless they capture carbon dioxide emissions, a technology so far unproven on a commercial scale.
The E.P.A. under Ms. Jackson also established the first standards for emissions of mercury, arsenic and other airborne toxins from power plants, and finalized a rule reducing industrial pollution that crosses state borders. The latter rule was struck down by a federal court and is under appeal.
Ms. Jackson, a native of New Orleans who holds chemical engineering degrees from Tulane University and Princeton, has spent most of her professional career at the E.P.A. She led the Department of Environmental Protection in New Jersey from 2006 to 2008 under Jon S. Corzine, then governor, who named her his chief of staff in late 2008, shortly before Mr. Obama chose her to head the federal environmental agency.
This month, the E.P.A.'s inspector general, prodded by Republicans in Congress, announced that he was opening an inquiry into Ms. Jackson's use of a secondary e-mail account to conduct business inside the agency. Ms. Jackson has said that she used the second account because her public e-mail address was widely known and that her e-mail alias -- "Richard Windsor" -- derived from the name of her dog and her former home in Windsor Township, N.J..
It is not known when the inquiry will be completed.
In a brief interview on Wednesday evening, Ms. Jackson said that she hoped to decompress after four intense years running the E.P.A., which has 17,000 employees and an $8 billion annual budget. She said she would probably do some consulting and public speaking but has not begun looking for a new job. She is thought to be a candidate for the presidency of Princeton.
Asked what she considered most important in her tenure, Ms. Jackson mentioned the endangerment finding, because it was the first time that the federal government began to address climate change. She also said that, although it received little notice during her tenure, she was proud of her role in expanding the environmental agenda to include voices that have been little heard, including low-income communities, native Alaskans and American Indian tribes.
"Before me," she said, "some people said that African-Americans don't care about the environment. I don't think that will ever be the case again."
This article originally appeared in The New York Times.