How do you plant a trillion trees? Start small, says 19-year-old Felix Finkbeiner. Ten years ago, Finkbeiner planted a single tree at his elementary school in Bavaria, Germany, after doing a school project on climate change. Soon after, he enlisted his classmates to help plant 1 million more trees. Before long, his project went viral online and within four years the millionth tree was planted. But that wasn’t the end of the story. Today, Finkbeiner’s idea has blossomed into a global movement called Plant for the Planet, and volunteers have planted 14.2 billion trees worldwide. Finkbeiner’s new goal: Plant 1 trillion trees. How will that combat climate change? Burning fossil fuels releases carbon dioxide, which builds up in Earth’s atmosphere, trapping heat and leading to warmer temperatures, scientists say. Trees absorb carbon dioxide, acting as natural filters. Finkbeiner’s group estimates that a trillion new trees might soak up a quarter of the carbon dioxide emitted each year. “Trees alone can’t solve the climate crisis,” says Finkbeiner, but “the more we make use of their abilities, the better off we’ll be.”