Because humans have been:
- Burning fossil fuels (coal, oil and natural gas). This releases carbon that had been buried safely underground for 300 million years.
- Deforestation. Cutting down trees which used to take carbon dioxide out of the atmosphere.
Human Activity is the primary reason for increase in carbon dioxide levels in the Atmosphere. These activities include:
- Burning coal for energy production
- Automobiles
- Deforestation - Destruction of trees and forests
- Industry
- Hydrocarbons used in refrigerators and other machines
