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The major U.N. summit on climate change opened Monday in Copenhagen, but the big environmental news was made across the Atlantic in Washington. In an afternoon press conference, Environmental Protection Agency (EPA) Administrator Lisa Jackson announced that the agency had finalized its finding that greenhouse gases, including carbon dioxide, pose a threat to human health and welfare.

The ruling allows the EPA to begin regulating greenhouse-gas emissions from power plants, factories and major industrial polluters, although the precise details of that regulation have yet to be worked out. "The threat is real," said Jackson. "If we don't act to reduce greenhouse-gas emissions, the planet we will leave to the future will be very different than the one we know today."

Jackson's announcement was the final step in a response that has been nearly three years in the making — since April 2007, when the Supreme Court ruled in Massachusetts v. EPA that the Clean Air Act gives the EPA the authority to regulate emissions of greenhouse gases, if they are indeed a threat to human health and welfare. At the time, the court directed the agency to review the latest science on climate change in order to make a determination.

Under former President George W. Bush, the EPA largely punted on the question, even burying analysis from its own scientists in the waning months of that Administration. When President Barack Obama took office, he directed the new EPA to kick-start the regulation process — nearly 11 months and 380,000 public comments later, the agency is now poised to regulate CO₂ as a pollutant. "This cements 2009's place in history as the year the U.S. government began seriously addressing the challenge of greenhouse-gas pollution," said Jackson.

Although the announcement had been in the pipeline for a while, the timing was appropriate, with Copenhagen getting under way. The EPA ruling is a signal to other countries that the U.S. is prepared to contribute to a climate treaty, and it is a useful tool for Obama, who will participate in the Copenhagen summit on Dec. 18, its final day. "In light of the EPA endangerment finding, the President's appearance in Copenhagen will carry even more weight, because it shows that America is taking this issue very seriously and is moving forward," said Senator Barbara Boxer, head of the Senate Environment and Public Works Committee, in a statement.

It is not clear what the EPA's decision will mean practically for major emitters. Jackson said she did not have a timetable for when the agency would publish a detailed plan to reduce greenhouse-gas emissions, and both Jackson and Obama have said repeatedly that they would prefer legislative action — in the form of a carbon cap-and-trade bill — over top-down regulation. The House has already passed a bill that would cap U.S. carbon emissions at 17% below 2005 levels by 2020, and the Senate is considering similar legislation. The threat of EPA regulation might be enough to nudge the Senate in the right direction. "We still need to pass a Senate bill because we need a long-term, clear signal about the effort we will undertake to cut carbon," said Jack Schmidt, international climate policy director for the Natural Resources Defense Council.

For their part, business groups and many conservatives remain even more opposed to the possibility of EPA regulation of greenhouse gases than they do to a cap-and-trade bill. They say CO₂ is far more prevalent than any other pollutant the EPA has ever attempted to regulate under the Clean Air Act and that top-down regulation would lay a heavy burden on U.S. business. "An endangerment finding from the EPA could result in a top-down command-and-control regime that will choke off growth by adding new mandates to virtually every major construction and renovation project," said U.S. Chamber of Commerce President and CEO Thomas J. Donohue.

Such a command-and-control regime is a long way off, and may never happen — especially if cap and trade is finally passed. But with the most important environmental summit in history kicking off, the EPA's news couldn't come at a better time for greens. "Every positive announcement will improve our chances of staying below the 2°C target," said Danish Minister Connie Hedegaard, who is presiding over the Copenhagen summit. "But we all know only too well, we are not there yet."

• Bryan Walsh
• The EPA finalized its finding that greenhouse gases, including CO2, pose a threat to human health and welfare -- a signal that the U.S. is ready to regulate carbon emissions