Day two of the Symposium convened by Pope Francis to call attention to climate change and ecocomic disparities began with a slide presentation by noted economist Jeffrey Sachs of Columbia University. It showed Boston and dozens of other cities swathed in ominous shades of red to signify when they will end up underwater in the 21st century if global warming is allowed to accelerate.

Boston Mayor Marty Walsh, sitting amongst other mayors civic officials from around the world called here by the pope, said that it was urgent that they act as a group to push policies aimed at slowing climate change.

"If we don’t deal with it, all of the coastal cities in the world are in trouble, not just Boston," Walsh said. "But I think as a city, we can set the bar and begin setting an example for other cities, but we need to be in this together and that's another message the pope was talking about, that we can't be in this individually,we got to be in this together."

This Vatican conference is noteworthy in how issues have been framed — not as singular concerns listed one-by-one or historically, but as problems symbiotically linked — and Sachs says they have to be faced all at once. Sachs, who also leads the United Nations Herculean (some would argue Utopian) effort to wipe out poverty worldwide, was asked by Pope Francis to organize the second day of the symposium, which addressed the intertwining issues of climate change and income inequality.

"What Pope Francis is saying is that the economy must operate within a moral framework," Sachs said. "That if you just allow markets and profits to guide everything, which is how many places operate right now, we’re going to go right off the rails, right over the cliff."

Figuratively for the economy, says Sachs, and literally, relative to the environment. Also speaking at the Vatican event was California Gov. Jerry Brown, whose state is in the midst of a drought. He says that for many Americans climate change is still an abstraction — but with each passing year, less so.

"It is very hard to grasp, but drought, the extreme weather event of Sandy, on the East Coast, where water rushes into the subways of New York, that can be understood," Brown said. "Farmers who have to fallow their land, that can be understood. People living in Tulare County, who turn on their tap, and no water comes out, so every day they have to bring bottles in, that can be understood."

Sachs says the American public gets it.

"It’s very interesting and surprising in the U.S. when you look actually at public opinion, in surveys, for example" he said. "The public has been saying now for more than a decade, 'tax the rich more.' The public has been saying, 'Get climate change under control.' The public has been saying, 'Make a move to renewable energy.' What doesn't happen, though, is legislative change in Washington."

That’s why Pope Francis sent out invitations to mayors around the world, including Walsh.

"I absolutely think it's strategic, there's no question about it. Here we are in the holy city, in the Vatican, with the pope, and religion wasn't spoken about. I think the pope understands the importance of getting these issues out there in the forefront. I think he understands this is where it needs to happen — on the local level, in city halls around the world."

Walsh says he hopes to lead by example. During a segment in the Vatican-sponsored program on economic development and poverty worldwide, Walsh laid out Boston’s vision for a greener, more equitable society to the audience of his fellow mayors in the audience.

"We're working on a housing plan to build 53,000 units of new housing by the year 2030," Walsh told the symposium. "We're working on a transportation plan. And we're looking at transit-oriented growth zones around train lines where we can build new housing."

But Walsh says he is also learning from other mayors who are dealing with the issues of climate change, migration, human trafficking and income inequality.

"I think Vancouver is a city we have some parallels with," Walsh said. "I was talking to the mayor of Vancouver, and really, his is a growing city like ours, he has high-tech industry coming there, a struggle with housing, moderate, low-income housing, so we talked about those issues. I think Rome could learn from us. Rome was talking today about LED in their lights and creating new pathways for bicycles, and new roadways and walkways. When people talk about where we are, we've done a lot of that already. We've already made the transformations. So it shows you that a young city like Boston can teach an old city like Rome."

With temperatures in Rome hovering near 100 degrees outside, and some elected officials here using notebooks as makeshift fans, attendees focused on next steps that mayors of cities big and small might take to put their collective imprint on policies that impact the environment. Some now plan to attend the Paris Conference on Climate Change in December, which already is riven with concerns that key proposals are being watered down by politics.