When we burn wood or cut down trees and other plants, we put carbon back into the air. But when a plant is growing, it takes carbon out of the air. Every year the level of CO in the atmosphere goes up in the autumn, when plants are dying, and goes down in the spring, when plants are growing. Because of this, the large forests of the world are very important: they take CO, out of the air. Plants have a lot of carbon inside them, and the world's large tropical forests have nearly half of the carbon that is in the world's plants. If we cut down or burn the trees, their CO, will go back into the air and there will be fewer trees to take CO2 out of the air each spring We put carbon into the air, and about 25 per cent of it.