Dozens of mayors from some of the world’s largest cities gathered at the Vatican on Tuesday, urging their national leaders to take bold action to combat climate change. Some of the 60 mayors invited to the 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.” The assembled leaders sought to keep pressure on world leaders in advance of climate negotiations scheduled for this December in Paris, and as new data revealed the first half of 2015 to be the hottest six months on record. Pope Francis told the gathering “You are the conscience of humanity,” saying that he had “a lot of hope” that negotiators in Paris would reach an agreement. Francis last month released an environmental encyclical that denounced what he deemed a fossil fuel-based economy degrading the Earth. Observers agree that cities are essential to reduce global warming since urban areas account for over 75 percent of human emissions. Governor Jerry Brown of the US State of California denounced global warming deniers, accusing them of “bamboozling” the public and politicians with misleading information about climate change. “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. Ambitious goals New York City Mayor Bill de Blasio pledged a 40-percent cut in his city’s emissions by 2030, urging other cities to set similar goals. “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.” Stockholm Mayor Karin Wanngard urged negotiators at the Paris talks to take fossil fuels off the table and focus instead on renewable energy. “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. Harrowing stories of human trafficking The Vatican conference also sought to raise awareness of human trafficking, with those assembled hearing sobering accounts of modern slavery and girls forced into prostitution. Ana Laura Perez James was chained up for five years in Mexico and forced to work twenty hours a day. She told the conference “we remain blind” to the issue of modern-day slavery. The conference also heard from Karla Jacinto, a 22-year-old Mexican mother of two who was forced into prostitution at the age of 12 and was rescued four years later. “I didn’t think I was worth anything. I thought I was just an object that was used and thrown away,” she said. Francis has advocated more action to address human trafficking, and has urged the UN’s new Sustainable Development goals, which will be finalized in September, to highlight the problems of human trafficking and modern-day slavery. “The United Nations has to deal with this,” he said.