An introduction to climate change - Climate Change: Biological and Human Aspects - page 14

This net imbalance was both large and of the same order of magnitude as existing
global flows. Subsequent IPCC assessments did not estimate this imbalance in their
analogous tables, although there is still an imbalance. This imbalance is why some
think that a major route of carbon from the atmosphere has not been identified, or
alternatively that one or more of the existing carbon-flow estimates (Figure 1.3) is
considerably off the mark, or perhaps a bit of both. As regards the flow of fossil fuel
carbon to the atmosphere, this is quite well documented due to the economic attention
paid to the fossil fuel industry: we know how many barrels of oil are sold, tonnes of
coal are mined, and so forth, so we can be fairly certain that this estimate is broadly
accurate (note the smaller plus/minus error).
Looking at the other side of the equation, the accumulation of carbon in the
atmosphere is also as accurately charted as it can be, and has been directly monitored
over many years from many locations. It is because we know exactly how much extra
carbon we release, and have released, from fossil fuels into the atmosphere (again
note the smaller plus/minus error), and how much actually stays in the atmosphere,
that we can be certain that there is a shortfall and that some part of the carbon cycle
has either not been properly quantified or even perhaps not properly identified.
It is uncertainties such as this, and that the global climate-warming signal had to
be sufficient to be discernable from the background natural variation (noise), that has
helped some argue that global warming is not taking place. As we shall see over the
next few chapters, the climate has changed in the past (affecting biology and vice
versa) and the atmospheric concentrations of greenhouse gases have played a major
part in these changes.
Interestingly, the current year-on-year accumulation in atmospheric carbon dioxide,
as measured in either the northern or southern hemisphere, is not smooth. Rather,
there is an annual oscillation superimposed on the rising trend. The oscillation occurs
because of seasonality outside of the tropics. During winters in the temperate zone
there are no leaves on the trees and in the boreal zone too there is little photosynthesis
on land or in the sea (in the main by algae). But in the summer there is considerable
photosynthesis and so more carbon dioxide is drawn into plants and algae. In winter
respiration continues, even though photosynthesis is reduced, and so, on balance,
more carbon dioxide is released into the atmosphere. Thus there is an annual cycle
of waxing and waning of atmospheric carbon dioxide in the northern and southern
hemispheres (see Figure 1.4). Indeed, because while in one hemisphere there is
summer and the other winter, the carbon dioxide oscillations in the two hemispheres
are opposite and complement each other. However, the seasonal variation of carbon
dioxide in the southern hemisphere is not nearly so marked, as that hemisphere is
dominated by ocean, which has a strong ability to buffer carbon dioxide. Oxygen, as
the other gas concerned with photosynthetic and respiration reactions, also shows a
seasonal variation in each of the hemispheres but one that is more marked than that
for carbon dioxide (as oxygen is not buffered by the seas). Like carbon dioxide, the
seasonal variation of the atmospheric concentration of oxygen is equal and opposite
in the northern and southern hemispheres.
The changes in the atmospheric reservoir of carbon are quite well understood
(even though our knowledge of other reservoirs is not so complete) because we
can measure atmospheric carbon dioxide directly, and because atmospheric mixing