YOU MAY REMEMBER her as the 8-year-old who made headlines selling lemonade to help rescue children from slavery. Her mantra, “You don’t have to be big to change the world,” took fire on social media and earned her a spot ringing the bell of the New York Stock Exchange the day Twitter went public.

These days, Vivienne Harr is hoping her message to stand up for what you believe in will inspire other youths worried about climate change to join her in an audacious mission: to plant and protect 7 million trees across the width of Africa to create what is being called the Great Green Wall.

“If that continent is healthy, it’s going to make the whole world healthy,” says Harr, who at 17 is the chief executive officer of the youth-led movement Laudato Tree.

The Great Green Wall is an African- led initiative to plant and preserve a 10-mile-wide wall of trees that stretches nearly 5,000 miles across the semi-arid Sahel region. The greening of such a wide swath of land is meant to hold back the sprawling Sahara to the north and slow desertification.

“I hate the feeling of being hot and having no shade,” says Harr, who lives in the San Francisco Bay area. “I was reading about the people in the Sahel, and it’s like that for them all the time. They have no trees and no water.”

The effort resonated with Harr, who, for as long as she can remember, has sought peace and play beneath the enormous flowering catalpa tree in her front yard in Marin County, Calif.

The Laudato Tree movement was founded by Irish producer and author Don Mullan, who was inspired by Pope Francis’s Laudato Sì Challenge to the world to “care for our common home.” After seeing Harr speak at the Vatican, Mullan asked her to become the movement’s spokesperson last year.

Laudato Tree recently partnered with American Forests to engage Catholics across America in planting Laudato trees for the Great Green Wall in Africa, delivering Tree Equity in socio-economically disadvantaged neighborhoods across United States cities, and planting in their own communities and parishes in ways that deepen their relationship to nature. These missions are increasingly important, given the changing climate.

“I am an optimist,” Harr says. “I believe that while we are in a difficult position right now, we can still pull it together. We don’t have a long time but we still can.”