Carbon is in the atmosphere attached to oxygen as the form of carbon dioxide. Plants use carbon dioxide as a food source, so then the carbon becomes a part of that plant. When the plant dies, the stored carbon then releases back into the atmosphere. That's why when we cut down forests, we release mass amounts of carbon into the atmosphere. It's not just burning fossil fuels, its taking away carbon storage. The carbon dioxide currently in the atmosphere is 400 parts per million, increasing 30% since before the industrial revolution. Carbon is the most abundant element on earth, and although without carbon the earth would be a frozen ice cap, maybe humans help of releasing it into the atmosphere is making it a little too warm.