Carbon Cycle Plot

This shows the emissions and sinks of CO2 to/from the atmosphere, and the concentration of CO2 in the atmosphere, from 1750 to 2300.
  • Java source code

    Note, only CO2 concentration is shown at the simplest complexity level.

    Curves

    (All from Carbon module)

    CO2 Emissions

  • Red: from burning fossil fuel
  • Yellow: from land use change
  • Brown: Total (fossil + landuse)

    Note: fossil emissions are the same as the total in the regional emissions plot, but the timescale here is longer

    CO2 Sinks

    (net flux out of the atmosphere)
  • Blue: Ocean
  • Green: Land (terrestrial biosphere)
  • Cyan: Total (ocean + land)

    Atmospheric CO2

    (Concentration in ppm, right scale)
  • Black: Calculated by model
  • Grey: Measured historical data (maybe underneath the black curve)

    The annual change in CO2 concentration is simply the sum of the emissions, minus the sinks. So, if the brown curve is above the cyan curve then the concentration will rise, or vice versa.

    Units

  • Sources and sinks are measured in GtC/yr, on the left hand scale (see how much is a GtC?)
  • Atmospheric CO2 is measured in parts-per-million (by volume, or by molecules), shown on the right hand scale.

    Controls and options

    Sinks model parameters

    See Carbon Module
  • Green: biosphere sink fertilisation
  • HD Option: historical landuse by mass balance
    Ocean mixing:
  • Blue: vertical diffusivity
  • Cyan: high-latitude mixing
  • Yellow: horizontal advection
  • Red: upwelling rate
  • Orange: gas exchange rate
    Carbonate chemistry:
  • CF Option: feedback from temperature?
  • Carbchem menu: various calculation methods

    Mitigation policy parameters

    (Available if "Stabilise CO2 Concentration" option is chosen from the mitigation menu)
    See also
  • Mitigation module
  • Stabilisation scenarios

  • Black 4-point arrow: controls the endpoint of a target CO2 stabilisation curve
  • Stabilisation menu: fixed endpoints corresponding to IPCC scenarios
  • WRE option: delayed start, target curve initially follows IS92A (WRE= Wigley Richels & Edmonds).

    Note that controls to stabilise CO2 emissions may be found on the regional emissions plot

    Discussion

    See also
  • Carbon Module, how it works
  • Carbon Storage Plot showing contents of each box

    Emissions

    Fossil and landuse change emissions are derived either from Mitigation or from SRES modules. For mitigation scenarios, land-use is simply a constant fraction of the total. Note that in SRES scenarios, land use emissions can become negative, implying net regrowth / sequestration.

    Dynamic response of sinks

    Both sinks increase in response to rising atmospheric CO2.

    It is easier to understand see this effect in a "forward" calculation, applying the no-policy SRES scenarios or the "stabilise emissions" option (mitigation menu).

    Then, if you increase one of the sinks by adjusting the model parameters, the atmospheric CO2 falls slightly, and so the other sink drops.

    However, when you run the model in inverse mode, adjusting the sink parameters will cause the emissions to change, in order to continue to reach the target concentration or temperature curve.

  • See stabilisation scenarios -inverse calculations

    Biosphere sink

    The green curve shows the amount of extra (anthropogenic) carbon taken up by the terrestrial biosphere (green plants, wood and soil) due to the "CO2 fertilisation" effect (photosynthetic carbon fixation is slightly more efficient at higher CO2 concentrations). The green arrow adjusts this fertilisation effect.

    Later, the biosphere sink begins to "saturate", as other factors such as water, sunlight and nutrients become more rate-limiting than CO2 for photosynthesis.

    Ocean mixing

    The ocean has a very large capacity to store CO2 (due to chemical buffering - see below).However the mixing between surface water and deep water is very slow, so the sink of anthropogenic CO2 is dependent on the mixing rate.

    Only one parameter, the eddy diffusivity factor (blue arrow), is shown at the "basic" complexity level. This controls the rate at which CO2 mixes vertically in the bulk of the ocean.

    If you select "expert" from the complexity menu, you can see more controls, and can compare the relative importance of processes. The mixing is dominated by the vertical diffusion and horizontal advection. The upwelling loop makes only a small difference. The effect of the gas-exchange rate is also small (unless you cut it altogether), since the mixed surface layer quickly catches up with the atmosphere.

    Note that the upwelling is more important in the climate model which has no horizontal advection. There is some physical sense in this difference structure, since mixing depends on density gradients which depend on temperature, so this effect supresses mixing of heat in a way that does not affect CO2.

    Carbonate Chemistry

    CO2 reacts with alkaline seawater to form bicarbonate ions

    CO2 + H2O <=> HCO3- + H+

    This conversion reduces the partial pressure of CO2 in seawater, giving the ocean has a vast capacity to store CO2. Currently the ocean holds about 50 times more inorganic carbon than the atmosphere, 99% of it in the form of HCO3-.

    However adding CO2 to seawater makes it more acidic, which reduces this "buffer capacity". Increasing the temperature also affects the chemical equilibria and reduces the solubility of CO2 in seawater. Both these feedbacks (acidification and temperature) act to decrease the future ocean sink.

    If you select "linear" from the "carbchem" menu (expert level) you will switch off the acidification effect.

    If you disable the "CF" option, you will switch off the feedback from temperature. You may observe that this removes the spikes in the historical ocean sink, which are caused by equivalent spikes in radiative forcing, especially volcanos.

  • See also Cause-effect relationships