The Environmental Protection Agency on Monday made a landmark finding that greenhouse gases endanger public health, paving the way for stringent regulation of power plants, steel mills, autos and other emitters.

"These long-overdue findings cement 2009's place in history as the year when the United States government began addressing the challenge of greenhouse-gas pollution," EPA Administrator Lisa Jackson said at a news conference in Washington, D.C.

If the EPA moves forward with regulations limiting greenhouse gases such as carbon dioxide under the Clean Air Act, it would have big implications for Northwest Indiana. Heavy industry predominates in the region with most of the electricity consumed here derived from coal-fired power plants, which are big emitters of carbon dioxide.

The endangerment finding could eventually lead to regulations that more than double the number of air permits required nationwide, increasing the burden on business, according to Kay Nelson, environmental affairs director for the Northwest Indiana Forum.

"And it's not just industries that would be affected," Nelson said. "The current thresholds would also include office buildings, retail malls and schools."

Environmentalists generally cheered Monday's ruling, saying it will provide the hammer that will finally make business understand climate change has to be addressed.

"I think it's going to help Northwest Indiana catch up with the rest of the world," said Lee Botts, a longtime environmentalist and a founder of the Alliance for the Great Lakes. "I think it will create new job possibilities as well as reduce utility costs and energy costs for existing businesses and improve the environment."

Green jobs, such as those of builders who weatherize homes, will grow once regulations are in place limiting greenhouse gas emissions, Botts said.

The EPA said the scientific evidence surrounding climate change clearly shows that greenhouse gases "threaten the public health and welfare of the American people" and that the emissions -- mainly carbon dioxide from burning fossil fuels -- should be regulated under the Clean Air Act.

In reaction to the EPA ruling, U.S. industries renewed calls for "responsible" climate change legislation in Congress, as opposed to letting the EPA regulate greenhouse gases.

"We are not opposed to addressing climate change," said NIPSCO spokesman Nick Meyer. "We simply are advocating for legislation that is fair and is achievable for everyone."

Sen. John Kerry, D-Mass., lead author of the Senate bill, has argued that if Congress doesn't act, the EPA will regulate greenhouse gas emissions. He has called EPA regulation a "blunt instrument" that would pose a bigger problem for industry than legislation crafted to mitigate some of the costs of shifting away from carbon-emitting fossil fuels.

NIPSCO officials say the Waxman-Markey bill passed by the House and the Kerry-Boxer bill proposed for the Senate both set unrealizable standards for reducing carbon emissions.

"Whatever change does happen in terms of new legislation cannot happen overnight," Meyer said. "We can't just shut down our coal-powered plants."

NIPSCO predicts electricity prices would soar 50 percent to 130 percent over the next 25 years under the Kerry-Boxer bill.

U.S. Steel referred media inquiries Monday to the American Iron and Steel Institute, which has made it known it wants to make sure energy-intensive imports such as steel bear the same climate-related costs as domestic products.

"Unilateral U.S. regulation through the Clean Air Act cannot solve this global problem, but will only reduce the competitiveness of energy-intensive U.S. industries like steel and cost Americans good-paying jobs," said Thomas J. Gibson, president and CEO of the institute.

Environmental groups were just as strenuous in saying the EPA finding that greenhouse gases imperil human health was long overdue -- and just might save the Earth

"This is a concrete sign that we need to address the rising impact of carbon pollution, which threatens our health -- and is causing glaciers to melt, sea levels to rise and more extreme weather to occur," said David Doniger, a policy director at the Natural Resources Defense Council.

"Today’s announcement shows that the U.S. government is serious about tackling this problem and putting limits on the largest sources of carbon pollution, including vehicles and coal-fired plants."

The EPA announcement came as the Obama administration looked to boost its arguments at an international climate conference that the United States is aggressively taking actions to combat global warming, even though Congress has yet to pass legislation. The conference opened Monday in Copenhagen.

The action by the EPA, which had been anticipated for months, clearly was timed to add to the momentum toward some sort of agreement on climate change at the Copenhagen conference and to try to prod Congress to approve climate legislation.

Under a U.S. Supreme Court ruling, the endangerment finding is needed before the EPA can regulate carbon dioxide and five other greenhouse gases released from automobiles, power plants and factories.

The EPA signaled last April that it was inclined to view heat-trapping pollution as a threat to public health and welfare and began to take public comments under a formal rulemaking. The action marked a reversal from the Bush administration, which had refused to issue such a finding, despite a conclusion by EPA scientists that it was warranted.

Times staff writers Sarah Tompkins and Bowdeya Tweh and The Associated Press contributed to this story.