Pope Francis told the mayors that they were important because they were at the "grass roots" and could make concrete changes and put pressure on leaders above them.
The Pope spoke briefly off-the-cuff in Spanish July 21 after more than 60 mayors attended a day-long workshop on modern slavery and climate change, sponsored by the pontifical academies of Sciences and Social Sciences in the Vatican's synod hall. The academies invited the leaders to share best practices, to sign a declaration recognizing that climate change and extreme poverty are influenced by human activity, and to pledge to pursue low-impact development to make cities "socially inclusive, safe, resilient and sustainable."
The Pope told his audience, "We really have to involve the United Nations in these things" and make sure it takes "a very strong position on this issue, especially the trafficking of human beings that is caused by this environmental situation and the exploitation of people."
"I really do hope that a fundamental, basic agreement is reached" at the UN climate summit in Paris in December, he added.
Mayor Mitchell Landrieu of New Orleans told Catholic News Service, "There is a vivid recognition that mayors are key players in changing how policies that have before now been spoken about across nations are actually applied on the streets of the cities. Mayors are actually responsible for getting things done."
Mayor Bill de Blasio of New York City said in his speech, "It's increasingly clear that we local leaders of the world have many tools and that we must use them boldly even as our national governments hesitate."
Landrieu told CNS that he was looking forward to hearing what other mayors were doing to make cities more resilient and "get a practical guide on climate change." When Hurricane Katrina hit 10 years ago, he said, his city became "the canary in the coal mine" showing the world how extreme weather associated with climate change can devastate a major city.
A number of mayors said that their countries and cities are still learning about the actual extent and seriousness of modern-day slavery and human trafficking and therefore found it helpful the Vatican invited two former victims to tell their stories.
Mayor Tony Chammany of Kochi, India, told CNS his region faces huge problems with forced organ donations. Even though there are strict laws against such crimes, the problem "is corruption at the level of governance. We have laws, but who is going to enforce them?" he said.
Another problem, he said, is that mitigating climate change and curbing poverty and exploitation cost money and demand investments, and so today's financial crisis is hindering greater efforts.
De Blasio said making an impact will not be easy; Pope Francis' recent encyclical on creation "holds individuals accountable for the fate of our planet, but it rightly asks the most of governments."
People are being "pushed by the highest moral authority to take the next step, no matter how challenging it appears to be," he said.
Mayor Gregor Robertson of Vancouver told CNS that governments are not accustomed to hearing spiritual leaders speak so courageously and effectively about social and ecological ills. It's not out of place for the Pope to add his voice to the political debate because "we all have a moral responsibility to speak out when we see injustice and a planet in peril" and take real action, he said.
Mayor Kagiso Thutlwe of Gaborone, Botswana, told CNS that he disagrees with claims that radically reducing greenhouse gas emissions will hurt development.
"There was a period when our forefathers didn't know anything about development," he said.
That changed when outside assistance brought in modern methods "and this development is now making us be engaged with the world" and getting his people access to new technologies, he said.
In fact, Thutlwe said he sees public-private partnerships as being key to growing a more "green" development, for example by partnering with solar-panel companies to exploit his country's abundance of sunshine and get needed electricity to more people.