U.S. carbon emissions plunged during the 2007-2009 recession, and conservation efforts, a weak recovery and more use of natural gas will help keep those levels down for another 15 years, the Energy Department reported Monday.

Coal-fired power plants will remain the largest source of U.S. electricity throughout that period at nearly 40% of total output, the department projects in its annual report on the future of energy. But the Energy Information Administration's 2012 report finds that emissions of the greenhouse gases blamed for a warming climate are likely to remain below their 2005 levels until 2027.

"These projections reflect increased energy efficiency throughout the economy, updated assessments of energy technologies and domestic energy resources, the influence of evolving consumer preferences and projected slow economic growth," the agency's acting administrator, Howard Gruenspecht, said in a statement accompanying the report.

Overall, the share of fossil fuels as an energy source is expected to drop from 83% to 77% in 2035, the report states.