Francis met with a delegation of Pacific leaders and told them he shares their concerns about rising sea levels and increasingly intense weather systems that are threatening their small islands.
He decried in particular the state of oceans, where overfishing and pollution by plastics and micro-plastics are killing fish stocks and sea life that are critical to Pacific island livelihoods.
While several causes are to blame, "sadly, many of them are due to shortsighted human activity connected with certain ways of exploiting natural and human resources, the impact of which ultimately reaches the ocean bed itself," the pope warned.
History's first Latin American pope has frequently spoken out against global warming and the impact it has in particular on poor and indigenous peoples. His landmark 2015 encyclical "Praise Be" denounced how wealthy countries exploit the poor, risking turning God's creation into an "immense pile of filth."
The Pacific leaders praised the encyclical for drawing attention to those most vulnerable to climate change, including residents of small Pacific islands for whom rising sea levels pose an existential threat.
The president of Nauru, Baron Waqa, told Francis that Pacific island leaders would urge negotiators at Bonn to uphold the Paris climate accord, where governments made commitments to keep global temperature rise this century below 2 degrees Celsius above pre-industrial levels, and pursue efforts to limit it to 1.5 degrees Celsius.
Waqa warned that the 1.5-degree rise was a crucial threshold: "There only remains a few years before we exceed carbon dioxide levels that will make temperature rise to levels that will see many parts of the Pacific disappear," he said.
Francis told the Pacific leaders that he hoped the Bonn talks would take their plight into consideration, and look for a shared strategy to confront the "grave problems" facing the environment and oceans.
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