A "carbon source" refers to processes that<u><em> hastens carbon release into the atmosphere more than what they absorb.</em></u> Example of this is the burning of nonrenewable resources like fossil fuels.
The combustion of fossil fuels<em> increases the release of carbon dioxide.</em> Carbon is then being trapped into the atmosphere. This contributes to <em>"global warming."</em>
This is<u> opposite to</u><em><u> "carbon sink,"</u></em><em> </em>which absorbs most of the carbon than they emit.