Yvonne Aki-Sawyerr, mayor of Freetown, Sierra Leone, is on a mission to plant a million trees over the next two years, increasing vegetation in her city by fifty percent while shoring up eroding riverbanks and increasing biodiversity. "This isn't just about planting trees; it's about growing trees, and it's about ensuring that each one of us is part of the process," she says. "A million trees is our city's small contribution to increasing the much-needed global carbon sink."