The Vatican is keeping up the pressure on world leaders to listen to Pope Francis’ dual mantras of fighting climate change and human trafficking by inviting mayors to a Vatican meeting next week devoted to battling the exploitation of the Earth and its poorest people.

For the mayors, it’s a photo op of a lifetime: Francis is on vacation but he’s expected to address the July 21 conference, which has attracted 60 or so mayors from Accra (Ghana) to Zagreb (Croatia).

For the Vatican, it’s another move in its unprecedented campaign to keep the momentum alive from Francis’ environment encyclical ahead of U.N. climate change conference at the end of the year.

Francis released his encyclical to great fanfare last month, denouncing the “structurally perverse” global economic system in which the wealthy exploit the poor and destroy the Earth in the process. He sees environmental degradation and human trafficking as related exploitations that affect the poor.

Francis has tasked his fellow Argentine, Monsignor Marcelo Sanchez Sorondo, to use his Pontifical Academy of the Sciences to engage a broad cross-section of society — Nobel laureates, faith leaders, the United Nations and law enforcement — to rally a global movement in support of his dual campaign.

Sanchez Sorondo told reporters Wednesday he realized he needed to include mayors after hearing from police chiefs battling human trafficking that they needed their bosses to get on board.

Some mayors have already made climate change a key issue: Rio de Janeiro Mayor Eduardo Paes chairs the C40 Cities initiative, which focuses on changing urban action to reduce greenhouse gas emissions and promote economic opportunities.

He will be joined, among others, by the mayor of the tiny Italian island of Lampedusa, which has been periodically overwhelmed by waves of migrants fleeing Africa and the Mideast.