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 Global
Scale: Forestry and Agriculture in the Global Carbon Cycle
The Earth has a natural carbon cycle. Carbon is naturally exchanged between
terrestrial vegetation and the atmosphere through photosynthesis and respiration.
Carbon flows from one reservoir to another over time scales ranging from
days to decades to millennia. The major components or reservoirs of carbon
include the oceans, terrestrial vegetation and soils, and the atmosphere.
This carbon cycle helps regulate the amount of carbon dioxide (CO2)
present in our atmosphere, and is therefore a major component of the climate
system.
Over the millennium prior to the Industrial Revolution, atmospheric
concentrations of CO2 were relatively stable. This is because
the two major carbon fluxes—between terrestrial vegetation and
the atmosphere; and between the ocean and the atmosphere—were
generally in equilibrium (see diagram below).
The burning of fossil fuels and deforestation has introduced an additional
flux into the carbon cycle (see diagram below). These activities combined
now emit almost eight billion metric tons of carbon to the atmosphere
every year, about 20% of which is the result of land-use change such
as tropical deforestation. Roughly half of these human-induced carbon
emissions remain in the atmosphere (for up to a century or more), while
the remainder is taken up in nearly equal portions by the oceans and
land vegetation.
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