Imagine a world without forests. No shade on a hot day. No wood for building. No clean air to breathe. Forests cover about 31 percent of Earth's land, and every March 21, the world celebrates the International Day of Forests. Trees play a critical role in Earth's atmosphere. Through a process called photosynthesis, trees absorb carbon dioxide and release oxygen. A single large tree can produce enough oxygen for two people for an entire year. Forests also act as giant filters. Their roots clean water as it passes through the soil. The Amazon rainforest alone produces roughly 20 percent of the world's oxygen, earning it the nickname "the lungs of the planet." Forests provide habitat for over 80 percent of all land animals and plants. From tiny insects to large bears, countless species depend on forests for food and shelter. When forests are cut down, these animals lose their homes. Deforestation is one of the biggest threats to forests today. Every year, roughly 10 million hectares of forest are destroyed, mostly for farming and building. That is an area roughly the size of South Korea lost every single year.