SUNDANCE — San Francisco Mayor Gavin Newsom has charted an ambitious course to reduce his city's emissions of climate-changing greenhouse gases, and so has Mark Begich, mayor of the considerably more conservative city of Anchorage, Alaska.

It's evidence, they believe, that you don't have to be liberal to make a difference in the fight against global warming.

Begich and Newsom were among the 30 mayors from across the United States who gathered at Robert Redford's Sundance Resort on Monday for the first full day of the second annual Sundance Summit.

The summit, the brainchild of Redford and Salt Lake City Mayor Rocky Anderson, aims to spur action at the local level to reduce global warming. The summit encourages mayors especially to take steps to reduce the emission of greenhouse gases in their cities.

Begich said that over the past year, he has made a number of changes in Anchorage that are palatable to his conservative community. Those include hiring a full-time employee to focus on renewable energy and conservation and boosting recycling efforts.

Newsom, meanwhile, touts a long list of San Francisco's achievements. They include the largest solar power plant in North America and plans for an underwater turbine in the San Francisco Bay to harness the power of the ebb and flow of tides. It is expected to supply enough energy to power 12.5 percent of San Francisco's homes.

"The environment should be apolitical," Newsom said. "It's not about left or right. It's about right or wrong."

He said climate-change prevention programs need not be onerous to taxpayers. Rather than replacing the city's entire fleet with fuel-efficient vehicles, for example, San Francisco is buying alternative-fuel vehicles as gas vehicles are retired. Newsom said his city's environmental initiatives illustrate that economic growth can work hand in hand with environmental protection.

Amid all the idea-sharing, a common theme emerged: Global warming is a serious threat, but there is reason for hope.

"I'd like to see this gathering, as we did last year, as an example of real optimism," Redford said.

Anderson agreed, saying there has been "unbelievable progress in so many cities" since last year's summit.

But Rosina Bierbaum, dean of natural resources and environmental policy at the University of Michigan, said the science suggests there is still plenty of reason for concern.

The atmosphere today, she said, has 33 percent more carbon dioxide than it did in 1850. Carbon dioxide is the most common "greenhouse gas" — blamed for trapping heat and raising the earth's average temperature by 1 degree Fahrenheit over the past century. That temperature rise has been uneven around the world, she said, with parts of Alaska now 5 degrees warmer than they were 100 years ago.