<P> Because carbon uptake in the terrestrial biosphere is dependent on biotic factors, it follows a diurnal and seasonal cycle . In CO measurements, this feature is apparent in the Keeling curve . It is strongest in the northern hemisphere, because this hemisphere has more land mass than the southern hemisphere and thus more room for ecosystems to absorb and emit carbon . </P> <P> Carbon leaves the terrestrial biosphere in several ways and on different time scales . The combustion or respiration of organic carbon releases it rapidly into the atmosphere . It can also be exported into the ocean through rivers or remain sequestered in soils in the form of inert carbon . Carbon stored in soil can remain there for up to thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration . Between 1989 and 2008 soil respiration increased by about 0.1% per year . In 2008, the global total of CO released from the soil reached roughly 98 billion tonnes, about 10 times more carbon than humans are now putting into the atmosphere each year by burning fossil fuel . There are a few plausible explanations for this trend, but the most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO . The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of climate change . From pre-industrial era to 2010, the terrestrial biosphere represented a net source of atmospheric CO prior to 1940, switching subsequently to a net sink . </P> <P> The ocean contains the greatest quantity of actively cycled carbon in this world and are second only to the lithosphere in the amount of carbon they store . Its surface layer holds large amounts of dissolved inorganic carbon that is exchanged rapidly with the atmosphere . The deep layer's concentration of dissolved inorganic carbon (DIC) is about 15% higher than that of the surface layer . DIC is stored in the deep layer for much longer periods of time . Thermohaline circulation exchanges carbon between these two layers . </P> <P> Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, which is converted into carbonate . It can also enter the ocean through rivers as dissolved organic carbon . It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the ocean's deeper, more carbon rich layers as dead soft tissue or in shells as calcium carbonate . It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation . </P>

The biggest reservoir of carbon on earth is in the