ROME — Responding to a call from Pope Francis, mayors from around the world gathered at a Vatican summit on Tuesday declared that stronger action on climate change is “a moral imperative,” and called human trafficking a “crime against humanity.”

The mayors, from cities such as New York, Boston, Paris, and Stockholm, issued a joint declaration at the end of the first day of a two-day conference at which Pope Francis urged the gathered politicians to take action on both global warming and slavery, arguing that there’s a clear link between the exploitation of nature and of human beings.

“Ecology is human,” Francis told the mayors. “This is what I wanted to explain in my encyclical Laudato Si’: You can’t separate man from nature.”

In Laudato Si’, the first such teaching document ever devoted by a pope entirely to the environment, Francis wrote that global warming is both man-made and also inextricably linked to the plight of the world’s poor. That’s why, according to Vatican organizers, he wanted to combine the summits on these two topics.

The pontiff gave the closing remarks at the end of Tuesday’s session.

Both Tuesday’s workshop, titled “Modern Slavery and Climate Change: the Commitment of the Cities,” and Wednesday’s symposium, titled “Prosperity, People, and Planet: Achieving Sustainable Development in Our Cities,” are hosted by the Vatican’s Pontifical Academies of Sciences and Social Sciences.

Margaret Archer, a British sociologist who serves as president of the Academy for Social Sciences, said one goal of these events is to persuade the United Nations to include the fight against modern slavery into the body’s new development goals to be adopted this September in New York.

The goals, a set of targets that are supposed to drive national policies for the next 15 years, will be signed by more than 193 heads of state in September during a session of the UN General Assembly.

Predicted to be the largest meeting of its kind for the number of presidents and prime ministers in attendance, the assembly will open with an address by Pope Francis.

“I would say we need to get the United Nations interested,” Francis said. “The United Nations has to be interested in the phenomenon [of climate change], but above all on human trafficking produced by this environmental situation.”

“Human exploitation has to end,” Francis said.

Before Francis, more than a dozen people spoke on Tuesday, including California Gov. Jerry Brown, Paris Mayor Anne Hidalgo, and the United Kingdom’s anti-slavery commissioner, Kevin Hyland.

Brown denounced global warming deniers who, he said, are “bamboozling” the public with propaganda and putting “troglodytes” into government offices instead of environmentally responsible leaders.

Perhaps the most powerful moments of the day, however, came from two Mexican women: Karla Jacinto, forced into prostitution, and Ana Laura Perez Jaimes, who was trafficked into slavery and forced labor.

Jacinto said she was compelled to begin acting as a prostitute at the age of 12, and continued until she escaped at 16. In four years, she said, she was forced to have sex with thousands of men.

“Not one of them saw the face of a 12-year-old?” she asked the group. “Not one saw in me the face of a daughter, a wife, a mother, a woman?”

While in captivity, Jacinto said, she became pregnant and gave birth to a girl, who was taken away from her for more than a year. During the first eight months of her pregnancy, she said, she was compelled to continue acting as a prostitute.

“No one saw my baby bump?” she asked.

It was for her daughter, Jacinto said, that she shared her testimony with the mayors.

For her part, Perez said she was 16 when she was captured and chained to an ironing board. From that moment until two months ago, when she escaped at the age of 23, she said she’d been forced to iron 20 hours a day, to sleep in a standing position, to chew plastic bags to keep hunger at bay, and to drink from the water she used to remove wrinkles from other people’s clothes.

Perez urged the mayors to fight human trafficking and modern-day slavery, an illegal industry that experts estimate affect 36 million people. It is unthinkable, she said, that in the 21st century there still are enslaved humans.

“Not because you have money, because you have it all, are you more than me,” she said. “God made us [all] in his likeness.”

The two-day meeting is the latest in a series of events organized by the Vatican recently to try to influence not only the new UN development goals, but also a summit on climate change that will take place in Paris in December.

“I have high hopes for the Paris summit,” Francis said.

The pope also said the mayors had been invited because building awareness about the environment and human ecology will move those issues from the “peripheries to the center,” and the mayors can influence national and international leaders.

The host of the French climate change summit, Paris Mayor Anne Hidalgo, told the gathering that Pope Francis’ environmental text has “extraordinary power [that] already resounds in the world, and will weigh strongly in the global debate on this issue.”

New York Mayor Bill de Blasio urged resolve at the Paris meeting.

“The summit is just months away,” De Blasio said. “We need to see it as the finish line of a sprint, and take every local action we can in the coming months to maximize the chance that our national governments will act boldly.”

De Blasio called the pope’s encyclical not “a call to arms; it’s a call to sanity.” He also said that the gathering at the Vatican wasn’t designed for mayors to congratulate each other for what they’ve achieved, but to bring Laudato Si’ to life.

“How can we protect our people if we accept the status quo that is slowly killing our earth?” he asked.

De Blasio, who will host Francis in New York in September, announced that the city has committed to reducing 40 percent of its greenhouse gas emissions by 2030 and 80 percent by 2050.

Similarly, San Francisco Mayor Edwin M. Lee said the city and county of San Francisco will phase out the use of petroleum diesel in municipal fleets and replace it with renewable diesel by the end of the year.

There also were commitments on human slavery. Experts believe that 14,500 to 20,000 people, primarily women and children, are trafficked to the US annually.

After Pope Francis spoke, the mayors signed a declaration vowing to work to end abuse, exploitation, trafficking and all forms of modern slavery, calling them “crimes against humanity,” and to develop national resettlement and re-integration programs so that those who have been trafficked won’t be returned to dangerous situations in their home countries.

On the environment, the mayors’ declaration said “human-induced climate change is a scientific reality,” and called its effective control “a moral imperative for humanity.”