Gina McCarthy got an earful Wednesday from Republicans in her first appearance before Congress since she was named head of the Environmental Protection Agency.

In a House committee hearing focused on climate change, McCarthy was grilled by EPA-critical Republicans on the Obama administration's second-term agenda to curb global warming.

McCarthy was baited time and time again into providing the details of a proposal for regulating carbon emissions from existing power plants expected to come out Friday. Republicans are concerned that the new rules will make it impossible for new coal-firing power plants to open.

While she would not divulge the details of the EPA announcement, McCarthy insisted that coal will remain a large piece of the country's energy portfolio.

"I think the rule will provide certainty for new coal," she said.

The proposal is expected to require coal plants to use carbon capture technology to prevent harmful toxins from entering the atmosphere. Republicans, however, warn that technology has not been developed and will be costly to implement.

McCarthy compared the planned regulations with an EPA directive that led to the Clean Air Act of 1977, which required power plants to put "scrubbers" in their stacks to eliminate sulfur dioxide. But Republicans said it wasn't a fair comparison.

"That's the whole difference between the clean air debate and the greenhouse gas debate," said Rep. John Shimkus, R-Ill. "In the clean air debate the technology was available. In the greenhouse gas debate, it's not."

McCarthy rebutted by saying that four coal plants are planned that "would beat anything we would have proposed," though she couldn't name them when asked.

The hearing, which included testimony from Energy Secretary Ernest Moniz, was also a rallying cry for Democrats to push global warming issues into the spotlight and criticize Republicans as "climate change deniers." In the crowd, conservation activists put on tin foil hats whenever a GOP committee members asked questions critical of climate research.

In June, President Obama released a plan to tackle global warming during his final four years in office, appeasing liberal Democrats and environmentalists dismayed by what they saw as a broken promise during the first term.

"His plan is reasonable, it's affordable and it will protect our atmosphere for our children and future generations," said Rep. Henry Waxman, D-Calif. "It will make our country the global leader in the clean energy economy of the future."

Republicans want any efforts to reduce carbon emissions or use of American natural resources to be weighed against the impact on jobs and the economy. While McCarthy and Moniz insisted that a cleaner environment can be achieved without hurting employment, conservatives raised their doubts.

"We hear all the time from small businesses I meet with. Some of the biggest impediments to creating jobs right now are the policies coming out of Washington," said Rep. Steve Scalise, R-La., "and the policies coming out of the EPA are at the top of the list."