Doing this because I know I'm kickass at Chemistry.
One source would be human emissions. As humans, we use a lot of machines (like power plants) that produce large amounts of carbon dioxide into the atmosphere. The ocean also produces a lot of carbon dioxide via the evaporation cycle. Polluted waters eventually will go into the atmosphere. Volcanoes, even when not erupted, emit a lot of carbon dioxide as well. Dormant underground magma releases carbon dioxide through the Earth's pores, and even when a volcano erupts, it produces carbon dioxide as well.
