The US Supreme Court on Monday agreed to examine a major environmental lawsuit that seeks to force six electric power companies to cap and reduce their carbon-dioxide emissions to fight global warming...The lawsuit...targets what it claims are the largest emitters of carbon dioxide in the United States and among the largest in the world...What makes the lawsuit unusual is that it is an attempt to fill a vacuum in US environmental policy on how best to regulate greenhouse gas emissions...A federal judge dismissed the lawsuit on grounds that it raised sensitive policy questions best left to the political branches. But a federal appeals court...reversed that decision, allowing the lawsuit to move forward. The power companies are now asking the US Supreme Court to reverse the appeals court...The companies named...include American Electric Power..., Southern Company, the Tennessee Valley Authority, Xcel Energy, and Cinergy Corp. [now Duke Energy].