Burning Fossil Fuels
How Does the Earth System Respond to the Burning of Fossil Fuels?
Humanity is currently conducting an enormous experiment with the Earth. By the middle of this century we are set to have doubled the natural carbon dioxide concentration. The results should be interesting. It is debatable however, to say the least, whether it is wise to conduct such experiments on the only habitable planet available to us.
There are several serious consequences of pumping vast amounts of carbon dioxide into our atmosphere. Some of these consequences can be explored with the JModels: short-term global warming, ocean acidification and a long-term legacy of elevated CO2. On this page, the focus is on examining how the Earth System will recover from the perturbation. Carbonate compensation is a stabilising feedback, but how exactly will it interact with the CO2 perturbation? Will it compensate against anthropogenic CO2 to bring the system back to equilibrium?
Take-Up by Ocean
Start the carbon model and then click on the button to 'Add Fossil Fuels'. Select the option to add the fossil fuels from a sinewave and then choose to add a total of 4000 Gt C (one estimate of the total amount of recoverable fossil fuels) over a period of 400 years. Now 'Apply and Close' your fossil fuel selections and run the model. On the first page of plots you can see that atmospheric CO2 declines rapidly over the first few centuries as it gets incorporated into the deep ocean (look at changes in ocean DIC). Now go to the 'Physical' tab on the 'Model Parameters' window and set both of the ocean mixing coefficients to zero. This prevents atmospheric CO2 getting into the deep ocean. Re-run the model and you can see how atmospheric CO2 stays much higher.
Effect of Carbonate Compensation
Reset the mixing parameters to their default values, add fossil fuels as before, then click on 'Set Run Duration' and increase it to 20,000 years. Now run the model. It should take several minutes, but the model will accelerate for the later parts of the run due to an adaptive timestep. If you examine the three pages of output plots you will notice that atmospheric CO2 declines rapidly over the next few centuries as it gets incorporated into the deep ocean. After this time, it starts a slower decline, which is now driven by the process of carbonate compensation. Look at the second of the two pages of plots and examine the behaviour of the lysocline through time. Notice how it shallows dramatically due to ocean acidification. The bottom left panel shows how the total output of carbon (burial of organic carbon and CaCO3) becomes greatly depressed following the fossil fuel inputs. It falls to much lower values than the total input of carbon (in rivers). Carbonate compensation thereby causes a input-output imbalance and thus causes the ocean to increase its stocks of dissolved inorganic carbon (page 1 of plots), by preventing burial of CaCO3 on the seafloor.
To investigate the specific impact of carbonate compensation on the recovery, click on 'Save Simulation' and save the output results to a data file. Now repeat the run, but this time before running click on 'Model Parameters', go to the 'CaCO3' tab and select 'Fixed Lysocline'. This prevents carbonate compensation from operating. Re-run the model, save the results as before, then load both data files into Excel (or some other package) and plot out the atmospheric CO2 trajectories next to each other. The difference is due to carbonate compensation. Also compare the trajectories for (1) deep-box carbonate ion, (2) deep-box calcite saturation state, (3) deep-box DIC concentration.
Stabilisation of Saturation State not pCO2
Carbonate compensation is a negative feedback process that stabilises the ocean's saturation state with respect to calcium carbonate. Note how, in the run including carbonate compensation, the ocean's carbonate ion concentration and saturation states are returned to their original, pre-industrial values. Carbonate compensation is not, however, a direct regulator of atmospheric CO2 or of ocean DIC. Note how the increase in ocean DIC caused by its absorption of fossil fuel CO2 is in fact exacerbated by carbonate compensation as it prevents the ocean losing DIC through CaCO3 burial. Note also how atmospheric CO2 does not return to its pre-industrial levels but rather is kept at elevated levels indefinitely (or until other feedbacks, not included in the carbon model, bring it down).
20,000 years is not be long enough for carbonate compensation to return to equilibrium. If you have time then run the model for longer. You will see that even after hundreds of thousands of years atmospheric CO2 is still significantly elevated over the levels before fossil fuel emissions.
In this case the negative feedback appears to work very well, but only for the parameter it is directly controlling, i.e. ocean saturation state with respect to CaCO3. This is not equivalent to a stabilising control on atmospheric CO2. As it happens, carbonate compensation does counteract (to some extent) the perturbation to higher CO2 levels, but does not minimise it to zero.
Further reading
D. Archer (2005) Fate of fossil fuel CO2 in geologic time. Journal of Geophysical Research-Oceans, 110(C9): Art. no. C09S05.
T Tyrrell (2007) The long-term legacy of fossil fuels. Tellus B, 59: 664-672.