Global warming legislation is simmering in Congress, with a mix of bipartisan bills introduced and bickering over how to regulate greenhouse gas emissions.

While much of the initial debate has tackled broad issues, legislators now are seeking to impose controls on particular sectors. Three new Senate bills focus on powerplants, a prime source of carbon dioxide emissions.

One proposal, introduced on April 19 by Sen. Tom Carper (D-Del.), calls for trimming powerplant emissions of sulfur dioxide by 82%, along with a 68% cut in nitrogen oxides and a 90% reduction in mercury. The bill would require those levels to be met by 2015, as well as implementation of a cap-and-trade program for CO2 emissions.

Also on April 19, Sens. Lamar Alexander (R-Tenn.) and Joseph Lieberman (I-Conn.) introduced a measure calling for the same targets as Carper’s bill.The Alexander-Lieberman proposal, however, creates incentives to make changes more cost-effective, such as auctioning 25% of CO2 allowances and using the proceeds to mitigate increased electricity costs.

Sen. Bernard Sanders (I-Vt.) echoed those efforts on April 24 with his legislation, which also calls for significant reductions in carbon dioxide, nitrogen oxides, sulfur dioxide and mercury emissions.

During an April 24 hearing, the Senate Environment and Public Works Committee shifted the debate to EPA’s authority over greenhouse gases under the Clean Air Act. Chairman Barbara Boxer (D-Calif.) challenged EPA Administrator Stephen Johnson to address global warming in light of author-ity given to EPA in the Supreme Court’s April 2 decision in Massachusetts vs. EPA. “The Supreme Court has left this administration with no excuses for further delay,” Boxer said.

Johnson said EPA would proceed in a “thoughtful, deliberative manner, considering every appropriate option and every appropriate rule” for CO2 emissions. He also announced that EPA has begun to consider a California provision to regulate CO2 emissions from ve-hicles. Eleven other states have adopted California’s standard. Despite pressure from Boxer, Johnson would not commit to a schedule for completing that process.