While Brazil 's presidential candidates have not made policy of the Amazon a major campaign topic, climate scientists and environmentalists say the winner of Sunday's race could determine whether, and how soon, the world's largest rainforest could reach a “tipping point.”

The key threshold, heavily debated among scientists, in simple terms would be when the dense, humid, moisture-generating forest shifts to a drier ecosystem. That would come with irreversible consequences for biodiversity and the climate, as the forest absorbs massive amounts of carbon dioxide, a greenhouse gas that drives global temperature to rise.…