Rather, it is a social encyclical, he told 70 mayors from big cities at a two day workshop at the Vatican this week addressing modern slavery and climate change.

It is a social encyclical, the Pope said, because the state of the environment is directly and intimately linked to the life and wellbeing of humankind.

He said huge migratory waves of peoples across the globe are triggered by environmental issues such as desertification and deforestation, which leave people and entire communities without the possibility of seeking a livelihood.

Thus - he said - the exodus that takes them into urban centres gives rise to human trafficking which brings with it diverse forms of exploitation (economic or sexual) of women, children and vulnerable people.

Pope Francis told the mayors that he has "a lot of hope" that negotiators at Paris climate talks later this year will reach an ambitious agreement to reduce global warming.

The Pope said he places great trust in the United Nations to bring about a good agreement.

But - he stressed - he also wants nations which belong to the UN to prioritise fighting both human trafficking and the exploitation of the world's most vulnerable people.

At the end of the meeting, the Pope and the mayors signed a common declaration of intent to combat environmental damage and human trafficking.

The declaration acknowledged that "human-induced climate change is a scientific reality, and its effective control is a moral imperative for humanity".

The declaration referred to the need for globally agreed Sustainable Development Goals.

Sources

• Vatican Radio
• Vatican Radio
• Image: Crisis Magazine