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 pollution, the Keystone XL oil pipeline, new controls on coal-fired plants and several other hot-button issues that affect the nation’s economy and people’s health.

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.