​Human activities have introduced historic levels of carbon dioxide into the atmosphere, speeding destructive changes such as global warming. At the same time, tropical forests — the world's largest carbon storehouses — are being decimated. Tropical deforestation is responsible for roughly 15 percent of global greenhouse gas emissions — more than all the world's cars, trucks, planes and ships combined. Conserving forests is one of the least expensive and most immediate ways to reduce emissions. It can provide close to one half of the world’s needed emissions reductions through 2020.