Its the total amount of biomass on the planet that contributes to the Carbon Cycle. Because of agriculture, this biomass is actually increasing with time (which contributes to the methane problem). Trees are temporary storage only; they fall down and rot and release Carbon eventually

Combustion + deforestation produce an imbalance and a net flux. Without them we would be experiencing a mild reduction of CO2 as uptake on the land + ocean = 194 units while land and ocean sources are 190 units. This small imbalance is typical of the glacial/interglacial cycle. The natural system is never in exact equilibrium with respect to maintaining a constant level of CO2. However, there is no sink presently available that absorbs the human induced source increase.

Residence time in the atmosphere varies between 5 and 200 years depending on many, many factors.

Not all of the carbon dioxide that has been emitted by human activities remains in the atmosphere. The oceans have absorbed some of it because as the carbon dioxide in the atmosphere increases it drives diffusion of carbon dioxide into the oceans. However, when we try to account for sources and sinks for carbon dioxide in the atmosphere we uncover some mysteries. For example, notice in Figure 1 (schematic of the carbon cycle) that fossil fuel burning releases roughly 5.5 gigatons of carbon (GtC [giga=1 billion]) per year into the atmosphere and that land-use changes such as deforestation contribute roughly 1.6 GtC per year. Measurements of atmospheric carbon dioxide levels (going on since 1957) suggest that of the approximate total amount of 7.1 GtC released per year by human activities, approximately 3.2 GtC remain in the atmosphere, resulting in an increase in atmospheric carbon dioxide. In addition, approximately 2 GtC diffuses into the world’s oceans, thus leaving 1.9 GtC unaccounted for.

Course Page