From the very start of the day, Boston Mayor Marty Walsh said he knew it would be one of the more unusual 12 hours of his lifetime, and not just because he would be in the presence of the pope by the end of it.

It began with a shared ride to the Vatican with fellow mayor Gregor Robertson, of Vancouver. It started with small talk.

"We talked about hockey, actually," Robertson said. "The Bruins and the Canucks going at it [in the Stanley Cup Finals] a few years ago to Game 7."

But by the time they were whisked through the massive gates and passed the Swiss Guards, Walsh and Robertson were discussing what they can do to help halt human trafficking and reduce global warming in their respective cities.

"We have an obligation to take up these two issues, when it comes to human trafficking, sex exploitation, slave labor, as we heard one of the women testify today about today, global warming," Walsh said. "There really is an opportunity for us as mayors to make a difference."

"The sex trade, as it moves women around the globe against their will, has to be eradicated," Robertson said. "There is no excuse, there is no way you can tolerate that in any city. The rest of the subject of prostitution is obviously up for debate, and it varies from community to community, and it's a challenging one for cities to grapple with."

A two-day symposium on modern day slavery and climate change continues Wednesday at the Vatican with Walsh expecting to take the podium. Monday the assembly heard from Pope Francis, who encouraged the mayors to do all they can to compel their respective governments to cut carbon emissions and save the planet.

Sixty mayors from around the world then took their seats, placed headphones over their ears and listened as interpreters translated remarks from Spanish to Portuguese, Pashtun to Swahili, and from French to English. Two women began the day describing to this international audience how they had been held against their wills for years and enslaved in the so-called sex industry. There was silence and visible dismay, even among some of the more hardened politicians in the room.

Over the next few hours, municipal leader after municipal leader took to the podium to describe what they and others were doing in their respective cities to address the twin problems of human trafficking and climate change.

Social media criticism of the conference has centered on what some critics said was a nonexistent relationship between these two concerns. New Orleans Mayor Mitch Landrieu couldn’t disagree more. He used Hurricane Katrina to connect the dots.

"When it gets hot, the poor get a lot hotter, and when it gets cold, the poor get a lot colder," he said. "That's a quote from Gen. [Russel L.] Honoré. The point of this is, when climate change comes, when we feel its dramatic impact, and it forces people to move out of their homes, like they did after Katrina, people are put in really, really difficult situations where they have to make bad decisions, and in very impoverished countries one of the decisions they make is to sell their bodies, or to give themselves over to somebody else to abuse them, and quite technically enslave them into owing them money for the rest of their lives, which evidently turns into a very large economy for human trafficking across the world."

New York Mayor Bill De Blasio also addressed the Symposium. His comments centered on the devastation wrought by Hurricane Sandy from New York to Massachusetts’ South Shore.

"This tragedy made the people of New York City understand climate change in a very different way," he said. "It became personal. It became real. It was no longer an abstraction."

De Blasio spoke to reporters outside Vatican Hall about New York’s plans to reduce greenhouse effects, and was asked how he thought the message of this conference will be received by Congress.

"I hope it will have a cleansing effect on the U.S. Congress," he said. "The pope's message needs to be heard in Washington. It has been ignored. The very ideas of addressing inequality and creating a more fair economic reality — obviously, so much of that has been ignored in the Congress. So much of the challenge of global warming has gone unaddressed by the Congress. I think the timing couldn't be better."

When Pope Francis finally arrived at the Vatican Symposium shortly after five, 30 mayors from around the world, including Sao Paulo, Paris, San Francisco and Oslo had preceded him at the podium.

Pope Francis has twice previously convened meetings of worldwide religious leaders and pledged to eradicate human trafficking and to slow the effects of climate change.

None of the mayors was given the opportunity to meet the Pope face to face. That honor went to the two victims of human trafficking who shook hands with the pontiff, accompanied by a chorus of applause. At the conclusion of his presentation, Pope Francis asked the mayors to come forward.

Walsh, De Blasio, Robertson, Landrieu and the other municipal leaders worldwide then — one-by-one — signed a compact pledging to do what their respective governments have fallen short of accomplishing: reducing carbon emissions in their respective municipalities and working just that much harder to end forced prostitution and labor without pay or very little pay.

Today is Mayor Walsh’s turn at the podium. He was added to the speakers roster after his omission was pointed out to organizers of the conference. One official said it was not a slight, but that cities facing the most severe crises of climate change and human trafficking were given priority. Walsh will talk about economic development in Boston and the relationship between antipoverty policies and efforts to end human trafficking in the Hub.