"It's hard to argue that all the asphalt that humans have put down in the city of Phoenix hasn't done something to increase the heat here,'' Tobin said. And the same is true, he said, of other activities, including greenhouse gases.

"We're burning these things so I do think it's impactful,'' he said. "The question is, to what extent?''

The views of commissioners are significant since they have purview over regulations that now require utilities to obtain at least 15 percent of their power from alternate "clean'' sources like solar, wind and geothermal by 2025. The commission can not only increase or decrease that mandate but also is deciding what costs utilities can impose on homeowners who install rooftop solar.

Only three of the contenders will advance to the general election where they will face Democrats Bill Mundell and Tom Chabin.

As anticipated, the debate provoked some fireworks over the ongoing contention by Burns that Arizona Public Service secretly helped finance independent campaigns that spent $3.2 million in 2014 to elect Republicans Tom Forese and Doug Little.

Among this year's candidates, Burns is alone in wanting new rules to require utilities to disclose their political spending. He said he's the only one looking for "transparency.''