The dramatic increase in using natural gas to produce electricity in the United States has led to an equally dramatic decline in the amount of pollutants and carbon dioxide emitted from the nation’s power plants, according to a new study. The study, conducted by researchers at the National Oceanic and Atmospheric Administration, showed that the switch from coal-fired to natural gas-fired power plants has reduced CO2 emissions by 23 percent and emissions of nitrogen oxides and sulfur dioxide by 40 and 44 percent, respectively. Examining power plant emissions from 1997 to 2012, the scientists found that new combined-cycle natural gas-fired power plants — which use two heat engines in tandem to convert a higher fraction of heat into electrical energy — emit less than half the amount of CO2 as coal-fired power plants. The study, to be published in the journal Earth’s Future, said that the fraction of electricity produced in the U.S. from coal fell from 83 percent in 1997 to 59 percent in 2012. Meanwhile, the fraction of electricity generated by combined-cycle natural gas plants rose from zero to 34 percent.