BELEM, Brazil — World leaders descending on the United Nations annual climate summit in Brazil on Thursday will not need to see much more than the view from their airplane window to sense the unfathomable stakes.

Surrounding the coastal city of Belem is an emerald-green carpet festooned with winding rivers. But the view also reveals barren plains: Some 17 percent of the Amazon’s forest cover has vanished in the past 50 years, swallowed up for farmland, logging and mining.

Known as the “lungs of the world” for its capacity to absorb vast quantities of carbon dioxide, a greenhouse gas that warms the planet, the biodiverse Amazon rainforest has been choked by wildfires and cleared by cattle ranching.

It is here on the edge of the world’s largest remaining rainforest that Brazil President L

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