Boston Mayor Walsh will meet with several well known municipal leaders who are addressing an unprecedented assembly of mayors from around the world at the Vatican, including eight from the U.S.

We've heard so far from the mayors of Paris, Sao Paulo, Madrid, Vancouver and San Francisco. Earlier, the assembled leaders heard from survivors of forced prostitution around the world. In the background, New York Mayor Bill DiBlasio — formerly of Cambridge — is discussing how his city has been dealing with the twin scourges of climate change and human trafficking.

One of the principal questions arising from this symposium is why Pope Francis chose to convene a forum on modern slavery and climate change with leaders of cities rather than nations. It’s both strategic and tactical, says one of the organizers: Because mayors can move quicker on these issues than federal governments. In addition, I was told, "They're better equipped to engage citizens and local communities that tend to know their mayor over and above their Congressperson."

It’s also a question I posed to Walsh.

"Whatever the issue might be, we are the body of government that can make the most change," Walsh said. "What's happening here in the Vatican is just following that same theme, and I think you're seeing a shift in the world from legislative bodies carrying out different initiatives shifting over to mayors."

Later today Walsh and fellow municipal leaders will meet with Pope Francis, who has linked the massive flow of migrants into this country and across Europe to global warming, which has exacerbated poverty and the movement of millions across borders.