One city banned Styrofoam. Another has the highest percentage of "clean" cars in Europe. Still another has cut greenhouse gas emissions by 29 percent since 1990 — while seeing its GDP grow 19 percent.

Dozens of environmentally friendly mayors from around the world are meeting at the Vatican this week to bask in the star power of eco-Pope Francis and commit to reducing global warming and helping the urban poor deal with its effects.

It's the latest — and perhaps most important — initiative the Vatican has taken to keep the momentum alive after Francis released his landmark environment encyclical and as governments head into crucial climate negotiations in Paris in December.

Already, the Vatican has engaged Nobel science laureates, global faith leaders, the U.N. leadership, eco-friendly businesses and even the self-described "secular Jewish feminist" and environmental advocate Naomi Klein to promote Francis' message that caring for the Earth — and humanity — is an urgent moral imperative.

Now, the pope is turning to mayors, some 60 of whom have signed up to attend the two-day meeting at the Vatican that starts Tuesday. Several belong to the Carbon Neutral Cities Alliance, a new association of cities that have committed to reducing greenhouse gas emissions by at least 80 percent by 2050 or sooner.

The mayors of New York City, Oslo, Vancouver and Boulder, Colorado, are attending. Also represented is San Francisco, which has banned plastic bags and Styrofoam; Stockholm, which has the highest percentage of clean vehicles in Europe; and Berlin, which cut its emissions by 29 percent in the past 25 years.

Other mayors hail from the developing world: Libreville, Gabon; Soroti, Uganda; and Siquirres, Costa Rica.

Experts have long said that cities are key to reducing global warming since urban areas account for nearly three-quarters of human emissions. Mayors are also on the front lines in responding to the effects of climate change — especially when seawater enters coastal cities' freshwater systems or when floods hit as a result of global warming-induced severe weather.

"They can't afford the luxury of a false debate over whether the problem is real," said Alden Meyer of the Union of Concerned Scientists, which has been active for decades in international climate negotiations. "They're dealing with the consequences already."

In his sweeping manifesto last month, Francis blamed global warming on an unfair, fossil fuel-based industrial economic model that harms the poor the most.

The broad coalition that the Vatican has put together to promote the message is part of Francis' belief that grass-roots movements and non-traditional players are key to changing the global system since traditional political institutions and unions have often sold out to corporate interests.

In addition to climate, the two-day meeting is addressing another of Francis' pet priorities: human trafficking. Organizers say climate issues and trafficking are related: both involve the exploitation of the Earth and its people, with the poor hit the hardest.

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