Sixty mayors from around the world gathered at the Vatican on Tuesday at a two-day conference where they pledged to fight climate change and to help the poor deal with its impacts. The meeting, which focused on the role of cities in addressing the global problems of climate change, sustainable development, and human trafficking, was the first time that the Vatican brought together mayors to mobilize grassroots action and urge world leaders to action ahead of the COP21 in Paris.

In a declaration released earlier this week, mayors pledged to urge world leaders to pass “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.”

It’s not a green encyclical; it’s a social encyclical.

Pope Francis addressed the meeting of the mayors on Tuesday, thanking them for coming together, and emphasizing the impact that people can have on the planet, and vice versa:

“We can’t say that the person is here, and the care for the environment is there,” he explained in Spanish. “This is what I was trying to express in the encyclical Laudato Si’. We can’t separate man from all else. There is a mutual impact. It’s not a green encyclical; it’s a social encyclical.”

The mayors responded in kind to Francis’s inspiring words. Bill de Blasio, mayor of New York, told his peers in a 10-minute speech, “His Holiness did not convene us here to ratify the status quo, but in fact to upend it. It’s increasingly clear that we, the local leaders of the world, have many tools, more than we may have in fact realized, and we must use them boldly even as our national governments hesitate.”

“Political and business leaders are not taking climat echange seriosuly enough,” California Governor Jerry Brown urged in an interview before the conference. The hope is that local government leaders can create pressure on the national leaders.