Children are not often invited to speak to the United Nations General Assembly. But there stood Felix Finkbeiner ... 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." At the time ... Finkbeiner was four years into leading a remarkable environmental cause that has since expanded into a global network of children activists working [to reforest] 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. The organization also prompted the first scientific, full-scale global tree count, which is now aiding NASA in an ongoing study of forests' abilities to store carbon dioxide. In many ways, Finkbeiner has done more than any other activist to recruit youth to the climate change movement. Plant-for-the-Planet now has an army of 55,000 "climate justice ambassadors," who have ... become climate activists in their home communities. Most of them are between the ages nine and 12. Meanwhile, he'll keep giving speeches. "It is in our own self-interest to get children to act," he says. "At the same time, I don't think we can give up on this generation of adults. All we can do is push them in the right direction."