His 350.org aims to speed up the worldwide effort to tackle climate change. It's prodding people to get involved, and to involve their leaders, in time for the December 2009 Copenhagen talks on climate change.

Americans have no time to waste, he told a gathering at the First Unitarian Church in Salt Lake City on Sunday. A concentration of 350 parts per million carbon dioxide in the atmosphere represents “some margin of safety,” McKibben told the packed sanctuary.

“Either we get it right now or we might as well give up,” he said, adding that addressing climate change is not a technological problem but a political one.

“The answer to the question is more or less entirely in your hands, you and people like you.”

McKibben last spoke in Salt Lake City in March, shortly after unveiling 350.org.

He talked then, and on Sunday, about Salt Lake City's important role in his previous campaign on climate change, Step It Up. Salt Lake City hosted what is thought to be the largest of 1,400 rallies worldwide in Step It Up's day of action in April 2007.

McKibben urged his audience to look out for a letter-writing campaign that 350.org will launch in the next few weeks and to alert friends in other parts of the world about it.

With so many returned missionaries in the state who are in a unique position to grasp the impacts that climate change might have in poor and remote parts of the world, “Salt Lake is key,” McKibben said.

He emphasized, though, that people must act fast. Climate change is advancing faster than expected, and many scientists fear that ecological systems will swing so far out of whack, that Earth cannot recover.

That means tackling climate change is urgent and requires far tougher goals, he said. Under one of those goals, the Western Climate Initiative, of which Utah is a part, greenhouse gasses would be cut 15 percent by 2020.

In his daylong Salt Lake City stopover, he met with community members, including businesspeople at the Outdoor Retailer convention and civic activists who met for a private breakfast, as well as those at the First Unitarian talk.

Joan Gregory, coordinator of the church's environmental ministry, said McKibben's talk hit home.

“This message is so important to get out to people,” she said. “And the number gets it to people in a way that they can understand it, they can take away.”

Jan Gillilan said she expects to be involved in future 350.org events, in part, because she worries about her children's and her grandchildren's future.

“I'm concerned about the kind of world they are going to inherit.”

fahys@sltrib.com