VATICAN CITY — Pope Francis leaves on Monday for a visit to four island nations across southeast Asia, an ambitious trip to urge global action on climate change that may test the strength of the 87-year-old head of the global Catholic Church. Over 12 days from Sept. 2-13, Francis will travel nearly 33,000 km (20,500 miles) to visit Indonesia, Papua New Guinea, East Timor and Singapore .

It is the longest trip yet by the pontiff, who now regularly uses a wheelchair due to knee and back pain. Francis pushed hard for the 2015 Paris climate agreement and aides say he wants to continue his appeals to confront the dangers of a rapidly warming world, and especially to support the most vulnerable . In the countries on his tour, these dangers include rising sea levels and increasingly severe and unpredictable heat waves and typhoons.

Jakarta, the Indonesian capital where the trip begins, has experienced disastrous flooding in recent years and is slowly sinking, prompting the government to build a new $32-billion capital on Borneo. Francis is scheduled to headline more than 40 events during the voyage and some observers say that, beyond his specific itinerary, he wants to show he is still capable of leading the 1.4-billion-member Church, despite his age and bouts of ill health.

“It is a show of strength for Pope Francis,” said Massimo Faggioli, an Italian academic who has followed the papacy closely. What does the Pope hope to achieve? Faggioli, a professor at Villanova Universit.