U.S. electric utilities' emissions of pollutants that cause acid rain and smog have fallen sharply since the federal government adopted stricter standards in 1990, but greenhouse gas emissions have risen in that time, according to a report released on Wednesday.

The report, produced by environmental investor coalition Ceres, the National Resources Defense Coalition, and Public Service Enterprise Group Inc., analyzed 2004 data on emissions from the top 100 electric power producers in the United States.

It found that between 1990 and 2004, the power producers' emissions of sulfur dioxide decreased about 44 percent, and those of nitrogen oxide fell 36 percent.

However, the report said their emissions of carbon dioxide, the main heat-trapping gas that scientists believe causes global warming, rose about 27 percent in the period and can be expected to increase further as new coal-fired power plants are built.

The amount of electricity generated in the United States rose about 31 percent over that period.

"Voluntary approaches for curbing greenhouse gas emissions are not working," Ceres President Mindy Lubber said in a statement. "Instead of reducing pollution, we now have a spate of new coal plants and inevitable greenhouse gas limits on a collision course that puts companies and shareholders at financial risk."

The federal government does not regulate carbon dioxide and U.S. President George W. Bush opposes forcibly limiting greenhouse gas emissions, preferring voluntary means instead.

Moreover, no technology is commercially available for power producers to use to capture and hold carbon dioxide emitted by their plants.

Daniel Lashof, science director of the Natural Resources Defense Council's Climate Center, said the report proves the efficacy of market-based emissions caps, like those used to regulate SO2 and NOx in the United States.

"Its critical that we establish enforceable limits on carbon dioxide emissions," Lashof said. "That's the most important step that we need to make to drive markets for new technology needed to reduce those emissions."

The U.S. utility industry is split on the subject of how to address climate change, with some companies favoring federal emissions standards or a tax on carbon dioxide, while other favor voluntary reductions.

Southern Co., a top U.S. emitter, argued that a voluntary approach was more appropriate to deal with increases in carbon dioxide emissions.

"Large-scale, cost-effective, low CO2-emitting technologies must be developed if we are to address the climate change issue while we continue to serve a growing economy and meet the projected 30 percent increase in U.S. electricity demand over the next 20 years," said Southern spokeswoman Terri Cohilas.

The report also said the three top power producers in 2004 -- American Electric Power Co. Inc., Southern Co. and publicly owned Tennessee Valley Authority -- were responsible for roughly 20 percent of the emissions by the 100 companies studied.

The three utilities generated about 16 percent of the electricity produced by the power producers analyzed in the report.

Still, those companies are not among the power generators with the worst emission rates, calculated by dividing pounds of pollution by megawatt hours of electricity produced.

By that standard, the worse offenders are much smaller power producers and emitters: Basin Electric Power Coop for carbon dioxide, Buckeye Power for SO2, and Associated Electric Coop for NOx.

The report is intended to serve as a guide for investors looking to assess business risks of future environmental regulations, policymakers and utilities looking to assess their performance.