Coal-hungry Indiana ranks fourth in the nation for carbon dioxide emissions from mostly aging power plants that contribute to global warming, an environmental group said Tuesday.

The report released by Washington, D.C.-based Environment America ranked Indiana behind Texas, Ohio and Florida. Pennsylvania was fifth. The study was based on U.S. Environmental Protection Agency data for 2007, the latest year for which final data were available.

Power plants in Texas put out nearly twice the amount of carbon dioxide as those in Ohio and Florida, the group said.

Nationwide, power plants released 2.6 billion tons of carbon dioxide, or about the amount produced by 449 million cars, the group said. That accounted for 42 percent of carbon dioxide emissions in the U.S. in 2007. In Indiana, power plants released about 132 million tons of carbon dioxide.

The study found 73 percent of emissions came from older coal-burning power plants built before 1980, like Duke Energy's Gibson Power Station in Owensville that began operation in 1975, which the group said is the fourth dirtiest power plant in the United States. Nearly half of the nation's power plants were built before 1980, the report said.

"It's time for the oldest and dirtiest power plants to clean up their act," said Megan Severson, the group's Midwest field organizer. "In order to stop global warming and reap all the benefits of clean energy, we must require old clunker power plants to meet modern standards for global warming pollution."

But Duke spokesman Lew Middleton said it was "misleading" to characterize the Gibson plant as dirty due to carbon dioxide emissions since no technology is available to control that form of pollution. Technology that would allow coal-burning plants to capture their carbon dioxide is still being developed.

Middleton said Duke had invested more than $1 billion to outfit the Gibson plant with scrubbers and other devices to limit other types of pollution such as mercury.

Indiana gets about 95 percent of its electricity from coal-fired power plants.

Environment America said cleaning up aging power plants was critical to stopping global warming, which Indiana University-Purdue University at Indianapolis scientist Gabriel Filippelli said is already affecting rainfall patterns in Indiana.

A global temperature rise of 2 degrees Fahrenheit since pre-industrial times has caused a shift to heavier springtime rains, causing flooding and complicating agriculture, said Filippelli.