PHOENIX (AP) — A forum in Arizona aimed at gathering grass-roots voices to help shape the Democratic Party platform drew calls Friday for a tax on carbon emissions to curb climate change and for an end to the practice of fracking in natural gas production.

The gathering offered a contrast in the priorities of the progressive supporters of Bernie Sanders and the more mainstream backers of Hillary Clinton, though no outward bitterness surfaced at the meeting. Clinton is the presumptive Democratic nominee, but Sanders has not conceded the race.

Dr. Michael Mann, director of the Earth System Science Center at Pennsylvania State University, urged party officials to take climate change seriously, citing heavy floods in Texas and South Carolina over the last year, rising sea levels and other effects. "If we are going to tackle this problem, we will need a price on carbon," Mann said.

Sanders supports a carbon tax, while Clinton has promised to protect regulations put in place by the Obama administration that set limits on carbon pollution from existing and future power plants.

The Democratic National Convention Committee's forum in Phoenix Friday and Saturday marked the second of four such meetings. Similar forums will be held on June 24 and 25 in St. Louis and on July 8 and 9 in Orlando, Florida. The Democratic Party will hold its convention in late July in Philadelphia.

About 50 people sat in the audience as committee members heard suggestions on environmental and energy policies.

Roman Feher, a representative for the Laborers' International Union of North America, which represents about 500,000 construction workers, warned against endorsing energy policies that disproportionally affect workers from one particular industry.

Feher said the United States will continue to rely on coal, natural gas and other sources. He said the party's 2016 platform should recognize the important role that natural gas plays in meeting America's energy needs.

Actor Mark Ruffalo, who spoke to committee members through video conferencing, said the country needs to turn away from fossil fuels and instead rely entirely on wind, solar and other alternative energy sources.

Sanders' environmental plan sets goals of creating a 100 percent clean-energy system sustained by wind and solar power.

Ruffalo said political leaders need to be made to feel uncomfortable for relying on fossil fuels. "We are going to have to make people uncomfortable. Our leaders are going to have to be strong. They are going to have to fight," Ruffalo said.