VATICAN CITY -- Dozens of mayors from around the world, including Vancouver, demanded Tuesday that their national leaders take bold steps at the Paris climate talks this year, saying that could be the last chance to keep the Earth's warming to levels still safe for humanity.

One by one, some of the 60 mayors invited to a two-day Vatican conference lined up to sign a final declaration stating that "human-induced climate change is a scientific reality and its effective control is a moral imperative for humanity."

Vancouver Mayor Gregor Robertson, the only Canadian mayor who is part of the gathering, signed the declaration.

The mayors were aiming to keep the pressure on world leaders ahead of the Paris negotiations in December. Pope Francis last month released an environmental encyclical that denounced what he calls a fossil fuel-based world economy that exploits the poor and destroys the Earth.

Francis told the gathering Tuesday that he had "a lot of hope" that the Paris negotiations would succeed, but also told the mayors: "You are the conscience of humanity."

Experts have long said that cities are key to reducing global warming since urban areas account for nearly three-quarters of human emissions. Mayor after mayor made an individual plea Tuesday at the conference for the world to change its ways.

Drawing rousing applause, California Gov. Jerry Brown denounced global warming deniers who he said are "bamboozling" the public and politicians with false information to persuade them that the world isn't getting warmer.

Brown, a former Jesuit seminarian, urged the mayors to not 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.

New York City Mayor Bill de Blasio announced new greenhouse gas emissions targets for the Big Apple -- committing the city to reducing its emissions 40 per cent by 2030 -- and urged other cities to follow suit.

"The Paris summit is just months away," De Blasio said. "We need to see it as the finish line of a sprint, and take every local action we can in the coming months to maximize the chance that our national governments will act boldly."

De Blasio is a founding member of an alliance of world cities that have committed to reducing emissions 80 per cent by 2050 or sooner.

Robertson, the Vancouver mayor, called Blasio and Brown's speeches "powerful addresses" in a tweet sent early Tuesday, adding that California and New York City were demonstrating "crucial climate leadership."

He also tweeted: "Honoured to sign joint declaration for climate action with ΓåòPontifex + global mayors."

San Francisco Mayor Edwin Lee also announced new measures, saying the city that takes its name from the pope's nature-loving namesake, St. Francis of Assisi, would transition its municipal fleet of fire trucks, buses and trucks from petroleum diesel to renewable energy sources by the end of the year.

Stockholm Mayor Karin Wanngard said the Paris climate talks must take fossil fuels off the table and focus instead on renewable energy sources.

"Climate negotiators must dare to push boundaries and exclude fossil fuels as an option and reward solutions that are long-term sustainable and renewable," she said.

Stockholm is one of the world's leaders in using renewable energy sources, with 75 per cent of the city's public transport network running on renewable energy. Wanngard's goal is to make the Swedish capital fossil fuel-free by 2040.

The climax of Tuesday's inaugural session was the afternoon audience with Francis, who has become a hero to the environmental movement and has used his moral authority and enormous popularity to focus world attention on climate change and its effects on the poor.

Francis' other main priority has been to raise awareness about human trafficking. The Vatican conference is aimed at showing how both are related: The exploitation of the Earth and its most vulnerable people, with global warming often responsible for creating "environmental refugees" forced to flee homes because of drought or other climate-induced natural disasters.

Francis told the gathering that while he had high hopes about the Paris climate negotiations, he also wanted the United Nations to focus more on human trafficking.

"The United Nations has to deal with this," he said.

The Vatican is angling for the U.N.'s new Sustainable Development Goals, to be finalized in September, to make a solid reference to the problems of human trafficking and modern-day slavery.

"Addressing both these phenomenon, climate change and modern slavery, is a herculean task for us as city administrators," said Tony Chammany, the mayor of Kochi, India.

Chammany detailed how repeated years of global warming-induced drought in India was pushing impoverished farmers into cities, making them ripe for the "dark dungeons of slavery" and exploitation.

The conference got off to a sobering start by hearing from two Mexican women who were victims of modern-day slavery.

Ana Laura Peres Jaimes showed the mayors photos of some of the 600 scars that she suffered as an indentured servant, forced to iron for hours a day without food, water or even a bathroom. She said she had to urinate in a plastic bag.

Karla Jacinto, a 22-year-old mother of two, told how she was forced into prostitution at the age of 12, servicing more than 30 men a day for the next four years until she was rescued.

"I didn't think I was worth anything. I thought I was just an object that was used and thrown away," she told the hushed audience hall.

The conference's final declaration calls for financial incentives to transition economies from using fossil fuels to low-carbon and renewable energies 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."