Dozens of environmentally friendly mayors from around the world will be signing off on a Vatican declaration Tuesday calling for their national leaders to approve a "bold climate agreement" that keeps global warming at a safe limit for humanity, The Associated Press has learned.

Some 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 pressure on world leaders in the run-up to Paris climate negotiations in December.

The final declaration, a copy of which was seen by the AP, 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."

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Pope Francis recently released an encyclical letter, Laudato Si, the first papal document dealing with the environment. (Max Rossi/Reuters)

Attending the meeting is Gov. Jerry Brown of California, which has enacted the toughest greenhouse gas emissions standards in North America, as well as the mayors of Boston; Boulder, Colorado; New York City; Oslo, Norway; San Francisco; Stockholm; and Vancouver. Many belong to the new Carbon Neutral Cities Alliance, whose members have committed to reducing greenhouse gas emissions by at least 80 per cent by 2050 or sooner.

Other mayors hail from the developing world, including Libreville, Gabon; Siquirres, Costa Rica; and Kochi, India.

Cities key to climate change fight

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.

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.