Emissions of two air pollutants from the U.S.'s largest electricity generators have fallen since 1990, while emissions of carbon dioxide, the most significant human-caused greenhouse gas, have been steadily rising, according to new research. A report issued by the environmental nonprofit NRDC, the sustainable investment group Ceres, and the utility Public Service Enterprise Group (PSEG), found that government regulation of nitrogen oxides (NOx) and sulfur dioxide (SO2) helped significantly reduce emissions of the pollutants. CO2 is not subject to government regulation.