Children are not often invited to speak to the United Nations General Assembly. But there stood Felix Finkbeiner, German wunderkind in his Harry Potter spectacles, gray hoodie, and mop-top haircut—with a somber question about climate change.

“We children know adults know the challenges and they know the solutions,” he said. “We don’t know why there is so little action.”

The children came up with three possible reasons to explain the lapse, he said. One is differing perspectives on the meaning of the word “future.”

“For most adults, it’s an academic question. For many of us children, it’s a question of survival,” he said. “Twenty-one hundred is still in our lifetime.”

Another explanation is climate denial. The third possibility can be glimpsed in an