WASHINGTON (CNS) -- Jeffrey Sachs has a thick resume.

He holds the rank of university professor at Columbia, where he heads the Center for Sustainable Development and had been director of its Earth Institute.

He also co-founded and served as chief strategist for the Millennium Promise Alliance and was director of the U.N. Millennium Project's work on the Millennium Development Goals.

And Sachs has been sought out by the Vatican under three popes to provide guidance and impetus.

"Of course everybody wants to go to the Vatican," Sachs said Dec. 15 during a webinar sponsored by Georgetown University's Berkley Center for Religion, Peace and World Affairs, titled "Interreligious Perspectives on Peace and Development: A Conversation with Jeffrey Sachs."

In a conversation with Berkley Center senior fellow Katherine Marshall, Sachs said he first got involved with the Vatican in 1991.

"I was invited at that time to be an expert to advise Pope John Paul II on what would be the encyclical 'Centesimus Annus,'" he recalled. "'Centesimus Annus' was the encyclical (issued on) the 100-year anniversary of 'Rerum Novarum,' which Pope Leo XIII issued in 1891 about the church's teachings about industrialization and is considered the church's breakthrough model on social teaching."

Besides Sachs' expertise, what also might have helped him to get the invitation was that he was an adviser to Poland's first post-communist government, the Iron Curtain having fallen just two years before. Sachs said the Polish-born pope asked him, "What's going on in Poland?" "I was able to brief him in a room outside the Sistine Chapel," he disclosed.

Sachs' next visit to the Vatican was later in the 1990s, when he went with Bono, the lead singer of the rock band U2, to discuss the debt crisis with the pope. After their meeting, "John Paul declared a jubilee year, which in biblical terms calls for the cancellation of debts," Sachs said.

"Bono and I came back from the meeting at Castel Gandolfo," the pontiff's summer home, "and went to a Senate Banking Committee meeting," where he said their testimony "melted the hearts of even some of the most hardened members of Congress."

Sachs' next visit came in 2012 during the pontificate of Pope Benedict XVI. At that time, "the Pontifical Academy of Science was starting to prepare for the 2015 Paris negotiations, the climate issue and what would be the Sustainable Development Goals," he recalled.

He credited Argentine Bishop Marcelo Sánchez Sorondo, then chancellor of the Pontifical Academy of Sciences and the Pontifical Academy of Social Sciences, for having "played a leading role" in forging consensus.

"Science, theology, pastoral guidance, all of this wrapped together -- plus a strong call for global action," Sachs said. "I know it moved the national agenda to close the deal on Paris."

The two academies of top-level scholars and experts promote studies on issues of concern to the Vatican.

He added, "I worked together with Bishop Sánchez Sorondo bringing together Pope Francis and U.N. Secretary-General Ban Ki-moon" for when the pope addressed the U.N. General Assembly Sept. 25, 2015, as part of his U.S visit.

"This was a fantastic assembly of voices," Sachs said. "It was an extraordinarily powerful moment. I have very many pictures in my camera of that very special day."

As of late, Sachs, when not writing, teaching or lecturing, he just may be in Rome to work with the Community of Sant'Egidio, known for its social service and peace efforts.

But he's clearly much in demand, as the Dec. 15 webinar was cut short by an hour due to what he described as an emergency call with political leaders, giving no further details.

Sachs, who once served on the Pontifical Academy of Social Sciences, has a new book just out titled "Ethics in Action for Sustainable Development," which examines sustainable development over a three-year period.

"There's also an interreligious gathering that emphasizes the common call for peace by religious leaders," Sachs said, with "the religious leaders saying, 'These wars, they are not on us. We can't stop them. ... They are not religious wars.' I take them at their words."

Still, "I'd like to see Pope Francis play some role in mediating this Ukraine disaster, I hope they would bring Ukraine and Russia together for some kind of trialogue," Sachs said.

Another project that occupies much of Sachs' time: "I'm working a lot with various U.N. agencies that have a real on-the-ground mandate, but they don't have the presence on the ground that a parish does, or a priest or an imam. We can think of other ways to do it. It could be extremely fruitful, whether it's getting kids in school, or addressing other political challenges."