LA VIRGEN, Costa Rica — Over just a few decades in the mid-20th century, this small country chopped down a majority of its ancient forests. But after a huge conservation push and a wave of forest regrowth, trees now blanket more than half of Costa Rica.

Far to the south, the Amazon forest was once being quickly cleared to make way for farming, but Brazil has slowed the loss so much that it has done more than any other country to limit the emissions leading to global warming.

And on the other side of the world, in Indonesia, bold new promises have been made in the past few months to halt the rampant cutting of that country’s forests, backed by business interests with the clout to make it happen.

In the battle to limit the risks of climate change, it has been clear for decades that focusing on the world’s immense tropical forests — saving the ones that are left, and perhaps letting new ones grow — is the single most promising near-term strategy.