The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 106

CO 2 levels show signifi cant fl uctuations that are directly related to the
interactions between the different reservoirs. These peaks represent the
time constants associated with geological and chemical mechanisms
affecting CO 2 in the atmosphere. The red peak is the experiment
humans are currently carrying out by burning fuel. Thus, the systems
analysis approach suggests a systematic way of approaching the ques-
tion of anthropogenic climate change: what will happen to the CO 2 lev-
els if we inject 1000 Gt of extra carbon into the atmosphere over a period
of 200 years by burning most of the fossil fuels?
Section 2
The biological carbon cycle
Our elementary biology course informs us that green plants use photo-
synthesis to convert CO 2 into biomass as part of the biological carbon
cycle. The natural rhythm of plant activity results in two cycles in atmos-
pheric CO 2 concentrations, one with a period of a single day, and another
with a period of a year.
Biological cycle: Diurnal cycle
Because photosynthesis requires sunlight, we expect to see changes in
the CO 2 concentration of the air around plants that follow the day and
night rhythm. During the day, photosynthesis removes the CO 2 from the
atmosphere, and during the night the decomposition of biomass (e.g., by
bacteria) continues, which causes CO 2 levels to increase. Indeed, the
peak on the shortest time scale in Figure 3.1.3 corresponds exactly to
this process [3.4]. Compared to the other fl uctuations, the amplitude of
this cycle is relatively small. Experiments that support this observation
are carried out in forests using large towers on which atmospheric CO 2
concentrations are measured at different heights (see Figure 3.2.1 ). An
example of the results of these experiments is depicted in Figure 3.2.2.