Environmental Protection Agency Administrator Lisa Jackson announced Thursday morning that she will be stepping down after four years with the Obama administration, leaving the position open to a wide array of potential candidates.

Jackson's departure, though not a surprise, will coincide with the beginning of Obama's second term, leaving his approach to environmental issues—especially how he intends to combat the urgency of climate change—much in question.

With highly contested environmental issues likely for the coming year—including calls for dramatic reductions of green-house-gas emissions, an approval question for the Keystone XL tar sands pipeline, restrictions on hydraulic fracturing and others—Senate Republicans are expected to make it difficult to replace Jackson with someone who would seriously challenge the fossil fuel industry.

As the Fuel Fix Blog notes:

During Jackson’s tenure, congressional aides sometimes joked that the EPA head spent more time on Capitol Hill than the lawmakers who frequently called her to testify.

The next EPA administrator is likely to oversee high-profile policy governing greenhouse gas emissions from power plants and refineries and low-sulfur transportation fuel. The agency could play a key role in regulating the use of hydraulic fracturing technology being used to unlock natural gas and oil previously trapped in dense rock formations.

Any potential replacement — no matter how well liked — could be in for a tough confirmation battle, as [GOP] senators use debate over the nominee to highlight their opposition to the Obama administration’s environmental policies.

Responding to the announcement, Center for American Progress Action Fund fellow Dan Weiss stated:

Senate Republicans could wage pitched confirmation battles to bully the administration into weakening public health safeguards....The Republicans may use this opportunity to squeeze commitments from the president to weaken public health protections he put in place during the first term.

Obama's choice for Jackson's replacement could be indicative of his approach to the environment in the next four years. Possible choices include Bob Perciasepe, deputy EPA administrator; Mary Nichols, the head of California’s Air Resources Board; Gina McCarthy, assistant administrator for EPA’s office of Air and Radiation; and Kathleen McGinty, former secretary of Pennsylvania’s Department of Environmental Protection.

The New York Times adds:

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.

Perciasepe, Jackson's current deputy, will temporarily run the agency until the replacement is confirmed.