The term used to describe the flow of carbon (in various forms, e.g. as carbon dioxide) through the atmosphere, ocean, terrestrial biosphere and lithosphere. (IPCC)
A natural process remove about half of each year's anthropogenic CO2 emissions from the atmosphere, but it isn't totally understood where this carbon dioxide goes, the oceans are a major repository and land biological organisms absorb a great part. A decline as the world warms could lead to faster accumulation of CO2 in the atmosphere.