An introduction to climate change - Climate Change: Biological and Human Aspects - page 15

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Fig. 1.4 Outlineofnorthernhemisphericseasonalityefectsofatmosphericcarbondioxideconcentration,1958-2011

throughtotheearly21stcentury.Notethatinadditiontotheannualwaxingandwaning,thereisalsoan
overalltrendofgrowthinatmosphericconcentration.ppmv,partspermillionbyvolume.

within hemispheres is reasonably thorough. We can therefore be quite certain that
our knowledge of this atmospheric part of the carbon cycle is fairly accurate. The
problem of the missing carbon seems to be associated with one or more of the
other carbon sinks: possibly the accumulation of carbon by terrestrial plants, or
alternatively by absorption into the oceans. Nonetheless, the annual waxing and
waning of atmospheric carbon dioxide does illustrate the power of the photosynthetic
carbon pump (the power of plant and algal photosynthesis to drive the carbon cycle).
Turning to human impacts on terrestrial reservoirs of carbon, estimates of deforest-
ation/land use do have greater error associated with them than changes in atmospheric
gas concentrations, which can be measured directly. So it may well be that part of
the missing carbon flow in the carbon cycle is associated with this component of
the cycle. However, instances of more detailed scrutiny of, for example, deforesta-
tion data have revealed that here official estimates are invariably underestimates, and
that deforestation probably accounts for a greater contribution to atmospheric carbon
dioxide, not less, and so the amount of missing carbon is greater. This means that, if
anything, our understanding of the cycle's carbon imbalance is not even as good as
we think. Of course, deforestation is but one dimension, of a number, to the changes
of the terrestrial reservoirs of carbon.
The importance and power of the photosynthetic pump driving part of the carbon
cycle is corroborated by the magnitude of the seasonal oscillation in carbon dioxide.
Nonetheless land-use change, along with terrestrial-biome change and ocean accu-
mulation, are key areas of uncertainty (either singly or together) that might account
for the missing carbon. It could be that oceans are accumulating more carbon than we
think and/or that the increased atmospheric carbon dioxide along with global warm-
ing is encouraging terrestrial photosynthesis, drawing down carbon into plants over
much of the globe. As stated above, because of the magnitude of seasonal variation
in atmospheric carbon dioxide outside of the tropics we can see how powerful the