VATICAN CITY — For a 2,000-year-old institution hardly known for its mutability, there was a sense of urgency at the Vatican on Tuesday when scientists, diplomats and religious and political leaders discussed climate change and its impact on the world’s poor.

“We are the first generation that can end poverty, and the last generation that can avoid the worst impacts of climate change,” Secretary General Ban Ki-moon of the United Nations said at an international symposium on climate change organized by the Pontifical Academy of Sciences. The event presaged a keenly anticipated papal letter on the environment that Pope Francis is expected to issue in June.

Mr. Ban met with the pope ahead of the one-day conference here and told reporters afterward that the pope’s message in his scheduled papal teaching, known as an encyclical, would come at “a critical time,” one that “demanded a collective action.”

“Climate change is approaching much faster than one may think,” he said.

In September, the pope is scheduled to address Congress, as well as a United Nations summit meeting on sustainable development, where he is expected to reiterate his environmental message. The pope has said that climate change is “mostly” a result of human activity.