After decades of world leaders trying to set global goals to address climate change and extreme poverty, city mayors gathered at the Vatican last week to promise that they will take concerted action. The Pope spoke briefly to them in Spanish on Tuesday 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 and held in the Vatican’s synod hall. The academies invited the leaders to share “best practices”, to sign a declaration asserting that climate change and extreme poverty are influenced by human activity, and to promise to pursue low-impact development to make cities “socially inclusive, safe, resilient and sustainable”. The Pope told his audience that “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”. “I really do hope that a fundamental, basic agreement is reached” at the UN climate summit in Paris in December, he added.