"The Holy See can make nice speeches," Pope Francis said in remarks to participants, "but the most important work goes from the peripheries to the conscience of human kind." The Pope told the mayors they were the ones most able to affect those in the peripheries.
Francis also touched on the problems of addiction, unemployment, health and human trafficking experienced by those in urban areas as well as deforestation of the Amazon and excess technology affecting forced migration.
"Ecology is human," he said. "We cannot separate man from everything else."
The two-day conference, titled "Modern Slavery and Climate Change: The Commitment of the Cities" and "Prosperity, People and Planet: Achieving Sustainable Development in Our Cities," is the latest in a series of public efforts on the part of the Vatican to influence the debate on climate change and human trafficking
The summit follows the Pope's landmark letter on the environment, "Laudato Si,"
which slammed a slew of modern trends -- the heedless worship of technology, our addiction to fossil fuels and compulsive consumerism -- and warned that humanity's "reckless" behavior has pushed the planet to a perilous "breaking point."
In September, the United Nations will vote on sustainable development goals, and member nations will submit plans to combat climate change this winter in Paris.
"I have a great hope in the Paris summit in December," Pope Francis said to conference participants.
"I really do hope that a fundamental basic agreement is reached. The U.N. (needs) to take a very strong position on this issue, in particular trafficking of human beings."
New York Mayor Bill de Blasio announced that his city, the largest in the world to commit to reducing greenhouse gases by 80% by 2050, is now committing to a 40% reduction by 2030, following a similar initiative taken by California.
"I hope the scale of our efforts," de Blasio said, "inspires others to go even farther."
California Gov. Jerry Brown quoted the founder of the Italian Communist Party, Antonio Gramsci, "pessimism of the intellect, optimism of the will," to encourage mayors to not be, "in any way confident or complacent," in the fight for change.
"We have very powerful opposition," Brown said, "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."
Kochi, India, Mayor Tony Chammany said his country has the most slaves in the world, but that it has enacted laws to protect children from forced labor and to ensure them the right to schooling as well as a 2005 law, named after Gandhi, that aims to guarantee Indians the right to work.
The mayor and town council members in Madrid walk and use public transport, according to Manuela Carmena, the mayor of Spain's capital.
"We can't entrust everything to laws," she said. "We must set a good example."
The Vatican conference was also an opportunity for local leaders to meet each other.
Vincent N'Cho Kouaoh, deputy governor of Abidjan, Ivory Coast, marveled at the coincidence that he was speaking just after the mayor of Lampedusa, the small southern Italian island that has struggled to cope with mass immigration from Africa in recent years.
"God always has a plan," the Ivory Coast official said on the coincidence of the lineup.
"I wish to take this opportunity to express my solidarity to the mayor and to congratulate her on all she is doing."
Conference participants closed the day by signing, together with the Pope, a "Joint Declaration Against Modern Slavery."