PLANCHÓN, Peru—More than a decade ago, Leonardo Racua was one of the first people in Peru to agree to help preserve the Amazon rainforest in exchange for cash from the sale of carbon credits. He built a thatched-roof home in the country’s southern Madre de Dios state and harvested Brazil nuts that fall from massive trees deep in the jungle. It wasn’t until last year that Mr. Racua received an $8,000 payment, his first cash from the sale of carbon credits.

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