Even as lawmakers continue to debate the science behind global warming, the Environmental Protection Agency intends to begin regulating greenhouse gas emissions from American businesses next year.

In a letter to lawmakers issued Monday, EPA Administrator Lisa Jackson said the agency intends to begin regulating greenhouse gas emissions from large polluters such as power plants in 2011, exercising its authority under the Clean Air Act. EPA will phase in the permitting process for emitters over several years, and small businesses won't be affected until 2016 at the earliest, she wrote.

Republican lawmakers hammered Jackson on the science behind climate change during a hearing Tuesday before the Senate Environment and Public Works Committee.

Ranking member James Inhofe, R-Okla., issued a report detailing questions that have been raised about climate change findings made by the Intergovernmental Panel on Climate Change. EPA relied in part on reports from that panel to decide last year that greenhouse gas emissions post a danger to the public and should be regulated.

"EPA needs to scrap the endangerment finding and start over again," Inhofe said.

But Jackson said EPA based its findings on reports from numerous scientific bodies and she has not seen any evidence to contradict the ruling that greenhouse gas emissions are a danger to the public.

"The science behind climate change is settled, and human activity is responsible for global warming," Jackson said.

The agency is seeking an additional $43 million in next year's budget to begin regulating greenhouse gas emissions. The bulk of the funds — $25 million — would be provided to states to help them develop the technical capacity to tackle greenhouse gas emissions. Another $13.5 million would help EPA develop new standards for reducing greenhouse gas emissions from vehicles.