Pope Francis greets Rome Mayor Ignazio Marino as he meets mayors gathered in the Synod Hall during a conference on Modern Slavery and Climate Change at the Vatican, Tuesday, July 21, 2015. Dozens of environmentally friendly mayors from around the world are meeting at the Vatican this week to bask in the star power of eco-Pope Francis and commit to reducing global warming and helping the urban poor deal with its effects. Next to Marino, at left, is Paris Mayor Anne Hidalgo, at his right is New York Mayor Bill de Blasio and between the two, one row back is Boston Mayor Marty Walsh. (AP Photo/L'Ossservatore Romano, Pool) Photo: Associated Press