Starting his project as a nine-year-old, Felix Finkbeiner aims to restore the world’s forests.

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 animal parable about monkeys that made an especially sharp point in the way that only a child delivering the message can.

“If you let a monkey choose if he wants one banana now or six bananas later, the monkey will always chose the one banana now,” he said. “From this, we children understood we cannot trust that adults alone will save our future. To do that, we have to take our future in our hands.”

At the time of his speech, Finkbeiner was four years into leading a remarkable environmental cause that has since expanded into a global network of children activists working to slow the Earth’s warming by reforesting the planet.

Today, Finkbeiner is 19—and Plant-for-the-Planet, the environmental group he founded, together with the UN’s Billion Tree campaign, has planted more than 14 billion trees in more than 130 nations. The group has also pushed the planting goal upward to one trillion trees—150 for every person on the Earth.