If journalists aboard the papal plane today were hoping to draw Francis into a discussion of those challenges, they came away disappointed.
Francis walked back to the press compartment shortly after takeoff and spoke for only five minutes, focusing on the risks of a "throwaway society" that neglects both its youth and its elderly.
"I don't give interviews," the pope said by way of explanation.
"Why, I don't know, but I can't ... It's a little difficult for me, but I'm grateful for your company," he said.
Yet part of Francis' charm is that this skittishness didn't come off as a snub, because the pope proceeded to spend the better part of the next hour standing in the front of the economy cabin of the papal plane to personally greet each of the roughly 70 journalists on board.
Monday afternoon, Francis was scheduled to meet Brazilian President Dilma Rousseff at the Presidential Palace, and then the 76-year-old pontiff will rest on Tuesday before heading north to the famed Brazilian Marian shrine of Aparecida on Wednesday.
CNN's Barbara Arvanitidis contributed to this report.
John L. Allen Jr. is CNN's senior Vatican analyst and a senior correspondent for the National Catholic Reporter.