The Carbon Cycle - Introduction to Carbon Capture and Sequestration - page 108

The data give the CO 2 levels and the solar radiation as a function of the
time of the day. We see that, at levels up to 20 m (typically the height of
tree and forest growth) CO 2 levels decrease during the day and recover
during the night. We also see that these diurnal variations become
smaller as we get to the higher levels, with CO 2 concentrations nearly
constant at the highest levels (21.5 m and above). This illustrates the fact
that the effect of the diurnal cycle on the overall CO 2 levels in the atmos-
phere is small (see the diurnal peak in Figure 3.1.3 ).
Biological cycle: Annual cycle
The next fl uctuation has a period of a year. The experimental data upon
which this “peak” is based is demonstrated by measurements of CO 2
levels on Mauna Loa in Hawaii. We are fortunate to have highly accurate
CO 2 concentrations recorded since 1970. These historical data are
shown in Figure 3.2.3 [3.5]. From this data, a steady increase of overall
CO 2 levels in the atmosphere is immediately apparent. In addition to this
general increase we observe a modulation with a period of a year. The
data for the year 2012, shown in Figure 3.2.3, clearly demonstrate that
during the summer CO 2 levels are lower than during winter. This is con-
sistent with the consumption of CO 2 by growing biomass in the summer
[3.5]. The size of the fl uctuations is of the order of 10 ppm, or 3% of the
total amount of carbon in the atmosphere.
Question 3.2.1 Hawaii is in the middle?
Those who have visited Hawaii know it is located very close to the equator.
As winter and summer are inverted in the Northern and Southern hemi-
spheres, one would expect to measure CO 2 levels that are the average of
summer and winter in Hawaii. Why do the graphs in Figure 3.2.3 show
otherwise?