Carlos Alvarado Quesada has heard all the naysayers before. In February, the 39-year-old president of Costa Rica committed to ridding the country of fossil fuels by 2050. If successful, Alvarado's plan could make Costa Rica the first zero-emissions country. But with a population of a mere 5 million, this leafy Central American nation is not a major contributor to the world's climate crisis.

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