Dozens of environmentally-friendly mayors from around the world are set to sign a Vatican declaration urging their national leaders to approve a "bold climate agreement" to tackle global warming.
A total of 60 mayors are attending a two-day climate conference at the Vatican featuring an audience with Pope Francis, whose recent environment encyclical is aimed at keeping up the pressure on world leaders ahead of Paris climate negotiations in December.
The final declaration states that "human-induced climate change is a scientific reality and its effective control is a moral imperative for humanity".
The document calls for financial incentives to transition to low-carbon and renewable energy and to shift public financing away from the military to "urgent investments" in sustainable development, with wealthy countries helping poorer ones.
And it says political leaders have a "special responsibility" at the Paris talks to approve a "bold climate agreement that confines global warming to a limit safe for humanity, while protecting the poor and the vulnerable from ongoing climate change that gravely endangers their lives".
In one of the opening speeches, California governor Jerry Brown denounced global warming deniers who he said are "bamboozling" the public and politicians with false information to persuade them that the world is not getting warmer.
Mr Brown urged the mayors not to be complacent in opposing climate deniers. California has enacted the toughest greenhouse gas emissions standards in North America.
"We have a very powerful opposition that, at least in my country, spends billions on trying to keep from office people such as yourselves and elect troglodytes and other deniers of the obvious science," he said to applause.
Other mayors attending hail from Boston; Boulder, Colorado; New York City; Oslo, Norway; San Francisco and Vancouver. Many belong to the new Carbon Neutral Cities Alliance, whose members have committed to reducing greenhouse gas emissions by at least 80% by 2050 or sooner.
Other mayors hail from the developing world, including Libreville, Gabon; Siquirres, Costa Rica; and Kochi, India.
Experts have long said that cities are key to reducing global warming since urban areas account for nearly three-quarters of human emissions.
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.
But many conservatives have rejected or dismissed the encyclical as flawed and irresponsible.
In addition to the climate declaration, mayors will be asked to sign a statement against human trafficking.