From long-term measurements of carbon dioxide concentrations in the atmosphere, we know levels were around 280 parts per million by volume at the beginning of the Industrial Revolution. Sources produce and release carbon dioxide and increase its concentration in the atmosphere. Sinks sequester carbon, taking it from the atmosphere and making it unavailable for release, leading to a net reduction of carbon dioxide in the atmosphere. Fossil fuel combustion is the human activity which results in the greatest emissions of carbon dioxide to the atmosphere. Changes in land-use and forestry activities affect the only significant sink for carbon dioxide within human control, our forest ecosystem biomass.
Forest ecosystems act as sources and sinks of carbon dioxide at the same time.
Carbon dioxide is removed from the atmosphere by growing vegetation during photosynthesis while using solar energy to produce biomass. But forest ecosystems also release carbon dioxide to the atmosphere through respiration when photosynthesis is shut down at night and through the process of biomass decay. Biomass accumulates in tree trunks, the understory of the forest, forest floor litter, and in forest soils. When decay or oxidation occurs in these systems, carbon sequestered in the biomass is returned to the atmosphere in the form of carbon dioxide. The difference between how much carbon dioxide is released to the atmosphere and how much is removed from the atmosphere is called the net annual flux.