Reuters: U.S. power plants released less carbon dioxide into the atmosphere last year as the economy slowed down and temperatures were milder than usual, a report from an environmental group said on Monday. Climate warming carbon emissions from U.S. power plants fell 3.1 percent in 2008, the Environmental Integrity Project said. The drop in emissions coincided with a 3.3 percent decline in gross electric output in 2008 from the prior year. U.S. President Barack Obama has pledged to cut …