KANDAHAR, Afghanistan — Two years after the Taliban banned opium, Afghan farmers turning to alternative crops are discovering that many no longer grow easily here because of the impact of climate change, imperiling poppy eradication efforts.

       For decades, farmers in southern Afghanistan relied on opium poppies to make a living in their parched desert landscape. Even as prolonged drought dried out rivers and turned fields so salty that they glowed white in the sun, the hardy poppies flourished.