Environmental Protection Agency Administrator Lisa Jackson, the first-ever African-American to hold that post, announced Thursday she is leaving after a four-year tenure marked by strong confrontations with congressional Republicans over her pushes to reduce pollution and carbon emissions.

“Under her leadership, the EPA has taken sensible and important steps to protect the air we breathe and the water we drink,” Obama said in a statement praising Jackson.

Jackson, a chemical engineer, had run the New Jersey Department of Environmental Protection before coming to Washington. It’s unclear what her resignation means for climate change legislation, which liberal Democrats are urging President Obama to address in a second term. Jackson’s departure also reduces the diversity of Obama’s Cabinet for now, although administration officials are closely looking for both minority and female candidates as Obama overhauls his top advisers in a second term. (United Nations Ambassador Susan Rice, who withdrew her name as a candidate for Secretary of State, remains a Cabinet member, and Attorney General Eric Holder is expected to remain in his post.)