Such an "endangerment" decision is necessary for the EPA to move ahead early next year with new emission standards for cars. EPA Administrator Lisa Jackson has said it could also mean large emitters such as power stations, cement kilns, crude-oil refineries and chemical plants would have to curb their greenhouse gas output.
The announcement would also give President Barack Obama and his climate envoy negotiating leverage at a global climate summit starting next week in Copenhagen, Denmark and increase pressure on Congress to pass a climate bill that would modify the price of polluting.
Joe Mendelson, Global Warming Policy Director for National Wildlife Federation, said the endangerment decision, would happen at "absolutely the right time."
"With House legislation passed, a bipartisan Senate bill in the works, and strong EPA action a virtual certainty, the president goes to Copenhagen with a very strong hand to play," Mr. Mendelson said.
The EPA's Ms. Jackson and President Obama's energy and climate czar Carol Browner have said they would prefer Congress to take action but are prepared to move ahead in the absence of lawmakers crafting their own law.