Bill de Blasio, the mayor of New York City, has pledged to reduce his city’s carbon footprint by 40% by 2030 in a speech in which he praised Pope Francis – who he called the “highest moral authority” – for calling on the world to take bold action on climate change.

The remarks were presented at a rare Vatican meeting of mayors from around the world, who gathered to discuss the environmental challenges facing their cities and the correlation between global warming and modern slavery.

De Blasio, who arrived at Vatican City a few hours late after his flight from New York was diverted by fog, said it had become “painfully obvious” that political leaders had to set “difficult goals for ourselves” in the fight against global warming.

He emphasised that, even as national leaders waffled on the issue, local mayors had to take matters into their own hands and “jolt our national paradigms”.

He praised Pope Francis’s encyclical on the environment, a sweeping, nearly 200-page document that calls for leaders to phase out dependence on fossil fuels and criticises consumerism and consumption that propagates global warming. The encyclical takes aim at the developed countries that have exploited the environment at the expense of the poor.

“The encyclical is not a call to arms. It is a call to sanity,” De Blasio said.

He began his speech with a bold “Buongiorno” to the gathered guests. De Blasio, who has Italian roots, is something of a local hero in Italy, which he visited last year on a 10-day family vacation. He also gave a warm greeting to Rome’s embattled mayor, Ignazio Marino, who is under pressure to resign because he is seen by some – including members of his own party – as incapable of coming to grips with a huge corruption scandal which has engulfed Rome, dating back to his predecessor, who is now under investigation.

De Blasio sent a tweet in support of Marino last month.

.@IgnazioMarino’s record of cleaning up a corrupt system is commendable—we must continue to stand up for leaders who stand up to corruption. — Bill de Blasio (@BilldeBlasio) June 23, 2015

New York’s mayor is a founding member of a group of local leaders who have said they would reduce carbon emissions by 80% by 2050, but De Blasio used the Vatican conference to issue an interim goal: a reduction of 40% by 2030.

He also highlighted the role December’s climate summit in Paris would play.

“We look ahead to the Paris summit, just months away. We need to see it as a finish line to a sprint,” he said. “Paris may constitute our last best hope.”

When asked if Wall Street was a good partner in New York’s fight against global warming, the mayor said the city was just beginning to engage the business community to reach its goals.

“I’ve said to the business community that we would love to meet these goals together, but if there is any hesitation on the part of the real estate community, business community, we will mandate action ... It is about survival,” he said.

“My message is clear. We come in peace; we would like to do this cooperatively. If we see a lack of commitment, we will mandate it,” he said.

De Blasio said he hoped to spend time with Pope Francis during the pontiff’s September trip to New York City.

“This is a leader that we haven’t seen before. He is saying things so clearly and so powerfully. He is moving people on an extraordinary level,” De Blasio said.

“He is literally saying we have to reset our assumptions and think more deeply and more morally about our actions because our current sense of practicality is digging our graves,” he said, adding that he hoped the pope’s visit to Washington DC would have a “cleansing effect” on the US Congress.

De Blasio was not the only mayor representing a North American city at the Vatican. Others included San Francisco’s Edwin Lee, Seattle’s Ed Murray, Boston’s Martin Walsh and Mitch Landrieu of New Orleans. Vancouver’s mayor, Gregor Robertson, was the only Canadian at the meeting.

While each represented a different geographical area and faces unique environmental challenges, they have one thing in common: all are Democrats. While some Republican mayors were invited to attend the function, a person familiar with the organisation of the conference said that none accepted.

