WASHINGTON — The Obama administration’s chief environmental watchdog, EPA Administrator Lisa Jackson, is stepping down after a nearly four-year tenure marked by high-profile brawls over global warming, the Keystone XL oil pipeline, and new controls on coal-fired plants.

Jackson, the agency’s first black administrator, constantly found herself caught between administration pledges to solve controversial environmental problems and steady resistance from Republicans and industrial groups who complained that the agency’s rules destroyed jobs and made it harder for American companies to compete internationally.

The GOP chairman of the House Energy and Commerce Committee, Rep. Fred Upton, said last year that Jackson would need her own parking spot at the Capitol because he planned to bring her in so frequently for questioning. Republican presidential nominee Mitt Romney called for her firing.

Jackson, 50, a chemical engineer, did not point to any particular reason for her departure. Historically, Cabinet members looking to move on will leave at the beginning of a president’s second term.

She gave no exact date for her departure, but will leave after Obama’s State of the Union address in late January.