Michael Regan, who has led the Environmental Protection Agency throughout President Joe Biden‘s four-year term, said Friday he will leave the agency Dec. 31, about three weeks before Biden’ leaves office.

In a letter to agency employees, Regan said he was proud of the EPA‘s work to confront climate change, restrict air and water pollution and spend tens of billions of dollars under the administration’s landmark climate law to spur clean energy development.

Those efforts slashed harmful greenhouse gas emissions and other air pollutants that endanger communities, “delivering significant economic and public health benefits in areas long overburdened by pollution,″ he said. The agency also created thousands of jobs and lowered costs for families, he said.

Regan, the first Black man to head the EPA, noted that the agency elevated the role of environmental justice under his watch and “placed it at the center of our decision-making.” Regan went on a “Journey to Justice” tour from Mississippi to Texas in 2021. The five-day trip from Jackson, Miss., to New Orleans and Houston highlighted low-income, mostly minority communities adversely affected by decades of industrial pollution.

Regan, 48, a North Carolina native who headed the state Department of Environmental Quality before taking over at EPA in early 2021, said he will return to the state with his family.

Jane Nishida, EPA’s assistant administrator for the Office of International and Tribal Affairs, will serve as acting administrator for the final weeks of the term, Regan said. Regan’s chief of staff, Dan Utech, will serve as acting deputy administrator.

Regan, who worked at EPA earlier in his career, thanked Biden for naming him to the post, adding: “Ten years ago, I could have never imagined coming back to lead EPA alongside such committed and dedicated staff.”

In the past four years, the agency has “made huge strides to remove lead pipes from communities and protect drinking water sources for millions of people in America,” Regan wrote. “We have acted on forever chemicals like PFAS to protect families from pollution in the products we use, the water we drink, and in the backyards where our children play.”

EPA also “reinvigorated enforcement efforts” after four years of environmental rollbacks under former President Donald Trump, “holding polluters accountable and making sure they pay for cleanups of legacy pollution sites across the country,” Regan said.

EPA has banned dangerous chemicals such as asbestos and trichloroethylene, also known as TCE, and responded to environmental and public health emergencies across the country, including in Asheville, North Carolina, and Tampa, Florida, in just the last year, Regan said.

The agency also hired thousands of new employees, including hundreds of scientists who left the agency during the first Trump administration. President-elect Donald Trump, who has named former New York Rep. Lee Zeldin to be EPA administrator, has said he will again slash environmental regulations when he returns to office for a second term next month. Trump also has said he will target what he calls onerous rules on power plants, factories and oil and natural gas production imposed under Biden.

While not mentioning Trump or Zeldin, Regan told staff that “the work continues. I have nothing but optimism and faith in your commitment to continue delivering public health and environmental protections for every person in this great country.”