Pontiff follows encyclical on fossil fuels with environmental summit of mayors and links climate change to migration, slavery and ‘uncurtailed growth of cities’

Pope Francis said he had “great hopes” that a fundamental agreement to tackle climate change would be reached in Paris later this year and he believed the United Nations needed to play a central role in the fight against global warming.

“The UN really needs to take a very strong position on this issue, particularly the trafficking of human beings … [a problem] that has been created by climate change,” the pope said.

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The remarks followed a day-long meeting of mayors from around the world that was hosted by the Vatican to discuss environmental challenges and how disruptions in climate were contributing to a humanitarian crisis in migration and modern slavery. Speakers included Bill de Blasio of New York, George Ferguson of Bristol and Gustavo Petro of Bogotá, among dozens of others.

The conference began by hearing harrowing testimony from two Mexican women who were victims of modern-day slavery.

“It’s not possible that it still exists, that we remain blind” to the issue of modern slavery, said Ana Laura Pérez Jaimes, who spent five years chained up and forced to work 20 hours a day in Mexico. She showed the mayors photographs of some of the 600 scars she suffered as an indentured servant, forced to iron for hours a day without food or water. She said she had to urinate in a plastic bag.

A fellow Mexican, Karla Jacinto, described how she was physically and sexually abused by her family and forced into prostitution between the ages of 12 and 17. She was forced to have sex with more than 42,000 clients before she was rescued.

“I didn’t think I was worth anything. I thought I was just an object that was used and thrown away,” she told the hushed conference hall. The 22-year-old mother of two now campaigns on behalf of trafficking victims.

The meeting came about one month after the popular Argentinian pontiff released an encyclical – or church teaching – on the environment that called for the phasing-out of fossil fuels and for action to combat climate change and immoral consumption, which he said was putting humanity at risk.

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Francis said in remarks, which seemed to be impromptu and were not from a prepared text, that his was not a “green” encyclical, but rather a “social” one, which reflected an “attitude of human ecology”.

“We cannot separate man from everything else. There is a relationship which has a huge impact, both on the person in the way they treat the environment and the rebound effect against man when the environment is mistreated.”

He spoke of the “uncurtailed growth of cities”, a global phenomenon that was giving rise to “shanty towns and slums” on the periphery of big cities because there was not enough economic opportunity to sustain poor people in rural areas. “This needs to be denounced,” Francis said.

He criticised the rise in youth unemployment – which in countries such as Italy has reached rates of more than 40% – and lamented that the plight of the young and poor was leading to “meaningless lives”.

“If we project this to the future ... what kind of horizon can they look towards,” he asked. “Some turn to guerrilla activities, to find some meaning in life and also their health is jeopardised.”

He criticised the growth of black market labour and no-contract work, which he said ensured that people could not earn living wages and led to forms of “addiction”.

He emphasised the importance that the local mayors, who were gathered before him, had in shaping environmental debates at home, saying that true reform had to emerge from the periphery to be effective, and could not be imposed from above.