The document discusses the greenhouse effect and its natural occurrence in trapping heat in the atmosphere, keeping the average temperature of Earth at 15°C instead of -18°C without an atmosphere. It explains that greenhouse gases like water vapor, carbon dioxide, methane, and nitrous oxide absorb and reemit infrared radiation from the surface, preventing it from escaping into space. Additionally, it describes the human impact on the carbon cycle through activities like burning fossil fuels, which contain carbon that was previously locked away and is now being added to the active carbon cycle.