Carbon credits are permits that allow the holder to emit one ton of carbon dioxide. They can be awarded to environmental groups that reduce emissions through activities like planting trees. Companies with emissions quotas can purchase carbon credits from groups if they expect to exceed their quotas. Burning fossil fuels is a major source of industrial greenhouse gas emissions for many industries. Carbon dioxide is an important greenhouse gas that has increased in the atmosphere due to human activity and affects the climate. The concept of the carbon credit system was created to help control emissions levels.