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The agency's biggest achievement in Obama's first term was the passage of fuel economy standards for passenger vehicles through model year 2025, which are expected to slash 6 billion metric tons, greenhouse gas emissions from the atmosphere.

But two other significant rules saw delays, including long-awaited regulations that would limit greenhouse gas emissions from new power plants. The draft rule was released in March 2012.

In 2011, the administration delayed implementation of a rule limiting ground-level smog until 2013, a move that irked environmentalists - and reportedly Jackson herself - who say the rule is necessary for protecting public health. Most expect the review of the National Ambient Air Quality Standards for Ozone to be revived early in the second term, despite concerns about its high cost.

Greenhouse gas regulations for existing large power plants are also likely to be released, even though they will face even more opposition from industry.

Other EPA regulations that are set to be finalized include rules that would limit emissions from industrial boilers, reduce particulate matter and regulate cement makers. A report on the impacts of hydraulic fracturing on water, due in 2013, could lead to new regulations for the controversial process amid a national boom in natural gas production.

These regulations are more likely to be the crux of the government's environmental work than any broad legislative proposals, said Daniel Weiss, a senior fellow for energy and environment at the Center for American Progress.

"When asked about global warming, the president has said we have to work on the economy first," Weiss said. "That reflects this idea that they'll use non-legislative tools and focus on existing authority through the EPA. They've got a lot of unfinished business."

Many of the green groups would like the agency to be more aggressive now that it has been freed from election-year pressures.

That worries Nicolas Loris, an energy economist with the Heritage Foundation, a conservative think tank that has railed against the EPA rules.

"The incentives to cater to the economy and industry are gone," Loris said. "That could mean stronger regulations ... that are economically damaging. Some of the ones they've proposed are already the most stringent and unprecedented."

Last week the Natural Resources Defense Council issued a plan for EPA to use the Clean Air Act to impose tougher greenhouse gas emission standards on existing polluters that it said could cut carbon pollution by 26 percent by 2020. In a news release, NRDC executive director Peter Lehner said the plan - which would set state-specific regulations rather than a national target - shows "how the United States can make big reductions in carbon pollution that drive climate change."

With the agency's path largely set, most agree that Jackson's replacement would likely come from within the agency, so the agency can continue its work with minimal disruption.

One of the most commonly mentioned names was Perciasepe, the EPA's current deputy administrator. As Jackson's second-in-command, Perciasepe was deeply involved in the agency's work during Obama's first term, including representing the agency several times in Congressional hearings. He also headed up the EPA's water and air quality departments under President Clinton and previously served as chief operating officer for the Audubon Society.