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					            Carbon Cycle Basics

                            Ranga Myneni
                           Boston University

Egon Schiele (1890-1918)
    Autumn Sun 1

                               Carbon Pools
Pool                                     Amount in Billions of Metric Tons
Atmosphere                               578 (as of 1700)- 766 (as of 1999)
T errestrial Plants                      540 to 610
Soil Organic matter                      1500 to 1600
Ocean                                    38,000 to 40,000
Fossil Fuel Deposits                     4000
Marine Sediments and Sedimentary Rocks   66,000,000 to 100,000,000

Carbon is stored on our planet in the following major pools:

• as organic molecules in living and dead organisms found in the
• as the gas carbon dioxide in the atmosphere;
• as organic matter in soils;
• in the lithosphere as fossil fuels and sedimentary rock deposits such as
  limestone, dolomite and chalk;
• in the oceans as dissolved atmospheric carbon dioxide and as calcium
  carbonate shells in marine organisms.
                   Global Carbon Cycle

Carbon is exchanged between the active pools due to various processes –
photosynthesis and respiration between the land and the atmosphere, and
diffusion between the ocean and the atmosphere.                         3/12
         Atmospheric CO2 Concentration-1

Accurate and direct measurements of the concentration of CO2 in the atmosphere
began in 1957 at the South Pole and in 1958 at Mauna Loa, Hawaii.         4/12
          Atmospheric CO2 Concentration-2

In 1958, the concentration of CO2 was about 315 ppmv, and the growth rate was
about 0.6 ppmv/yr. This growth rate has generally been increasing since then; it
averaged 0.83 ppmv/yr in the 1960s, 1.28 ppmv/yr during the 1970s, and 1.53
ppmv/yr during the 1980s. The concentration in 2006 was over 380 ppmv.

The annual cycle in the Mauna Loa record is due to the seasonality of vegetation.
In early spring, the concentration of CO2 is at its maximum, and as the plants
green-up, the concentration drops, reaching a minimum value towards the end of
the summer, and when leaves fall, it starts to build up again. This swing in the
amplitude is most pronounced in the records from the northern high latitudes,
where it can be as large as 15 ppmv.

          Atmospheric CO2 Concentration-3
There are at least three arguments to be made for the case that the observed
increase in atmospheric CO2 concentration is due to emissions related to human

(1) The rise in atmospheric CO2 concentration closely follows the increase in
    emissions related to fossil fuel burning.

(2) The inter-hemispheric gradient in atmospheric CO2 concentration is growing
    in parallel with CO2 emissions. That is, there is more land mass in the
   Northern hemisphere, and therefore more human activity, and thus, higher
   emissions, which is reflected in the CO2 growth in the Northern hemisphere
   (compared to the SH).

(3) Fossil fuels and biospheric carbon are low in Carbon 13 (an isotope). The ratio
   of carbon 13 to carbon 12 in the atmosphere has been decreasing.

  Historical Atmospheric CO2 Concentration

This figures shows that the concentration of CO2 has never been grater than 300
ppmv for the past 400,000 years.
                 Terrestrial Carbon Processes-1

Schematic representation of the terrestrial carbon cycle. Arrows indicate fluxes; boxes indicate pools.
   The size of the boxes represents differences in carbon distribution in terrestrial ecosystems. CWD,
   coarse woody debris; Rh, heterotrophic respiration by soil organisms; PS, photosynthesis. Credits:
   Schulze et al. (2000), Managing forests after Kyoto, Science, 289:2058-2059.
              Terrestrial Carbon Processes-2
Gross Primary Production (GPP): The amount that is fixed from the atmosphere, i.e.,
  converted from CO2 to carbohydrates during photosynthesis, is called GPP, which
  is carbon assimilation by photosynthesis ignoring photorespiration. Terrestrial GPP
  has been estimated to be 120 Gt C/yr.

Net Primary Production (NPP): Annual plant growth is the difference between
   photosynthesis and autotrophic respiration (Ra), and is referred to as net primary
   production (NPP). NPP is the fraction of GPP resulting in plant growth, and can be
   measured through sequential harvesting or by measuring plant biomass, provided
   turnover of all components (e.g., fine roots) is included. Global terrestrial NPP has
   been estimated to be 60 Gt C/yr, that is, about half of GPP is incorporated in new
   plant tissue. The other half is returned to the atmospheric as CO2 by autotrophic
   respiration, that is, respiration by plant tissues.

              Terrestrial Carbon Processes-3
Net Ecosystem Production (NEP): is the difference between NPP and heterotrophic
   respiration (Rh), which determines the amount of carbon lost or gained by the
   ecosystem without disturbances, such as harvests and fire. NEP can be estimated
   from measurements of CO2 fluxes over patches of land. Global NEP is estimated at
   about 10 Gt C/yr.

Net Biome Production (NBP): is the carbon accumulated by the terrestrial biosphere
   when carbon losses from non-respiratory processes are taken into account, including
   fires, harvests/removals, erosion and export of dissolved organic carbon by rivers to
   the oceans. NBP is a small fraction of the initial uptake of CO2 from the atmosphere
   and can be positive or negative; at equilibrium it would be zero. NBP is the critical
   parameter to consider for long-term (decadal) carbon storage. NBP is estimated to
   have averaged 0.2 +/- 0.7 Gt C/yr during the 1980s and 1.4 +/- 0.7 Gt C/yr during
   the 1990s.

                         Global Carbon Budget
                                                     1980s (Gt C/yr)       1990s (Gt C/yr)
Emissions (fossil-fuel buring, cement manufacture)       5.4 +/- 0.3          6.3 +/- 0.4
Atmospheric increase                                     3.3 +/- 0.1          3.2 +/- 0.1
Ocean-atmosphere flux                                   -1.9 +/- 0.5         -1.7 +/- 0.5
Land-atmosphere flux                                    -0.2 +/- 0.7         -1.4 +/- 0.7
Emissions due to land-use change                                o
                                                      1.7 (0.6 t 2.5)     Assum e 1.6 +/- 0.8
Residual terrestrial sink                            -1.9 (-3.8 to 0.3)        -2 to -4

During the 1980s, carbon emissions totaled 5.4 +/- 0.3 Gt C/yr (Giga tons or 109
   tons of carbon per year) from fossil-fuel burning and cement manufacture,
   and 1.7 (0.6 to 2.5) Gt C/yr from land-use changes.
The net carbon flux into the oceans is estimated to be 1.9 +/- 0.5 Gt C/yr, and 0.2
   +/- 0.7 Gt C/yr into the land.
Because the atmospheric carbon increase is observed to be 3.3 +/- 0.1 Gt C/yr,
   there is still a 1.7 Gt C missing sink per year.
For the 1990s, the estimates are somewhat similar, except for a larger land carbon
   sink. Many studies suggest 1 to 2 Gt of carbon sequestered in pools on land in
   temperate and boreal regions.

           Spatial Pattern of Carbon Uptake

This figure shows the zonal distribution of terrestrial and oceanic carbon fluxes.
   Results are shown for the 1980s (plain bars) and for 1990-1996 (hatched bars).
   Positive numbers are fluxes to the atmosphere.
This figure represents our current understanding, that is, about 1 to 2 billion tons
   of carbon are somehow sequestered in sinks on land north of 30N.
Elsewhere, the land is neutral, where sources nearly match sinks. The geographic
   distribution of the northerly land sink remains unknown.

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