An introduction to climate change - Climate Change: Biological and Human Aspects - page 16

photosynthetic and respiratory carbon pumps truly are. If this missing carbon sink
is terrestrial (and given that much of the planet's land is in just one, the northern,
hemisphere) it appears that the carbon may be being sequestered not in a temporary
way by annual plants but in a longer-term way by perennials, and especially temperate
and boreal trees. Then there is the carbon stored in soils and detritus. The total carbon
store in soils, at 1500 gigatonnes of carbon (GtC), is over twice that stored as biomass
and is also more than the atmospheric carbon reservoir. Currently the reservoir of soil
carbon is a net sink, although carbon flows to and from it are far less than those to and
from either biomass carbon or atmospheric carbon reservoirs. It is important to note
that currently soil acts as a global net carbon sink. 'Currently' because a world that
is just a few degrees warmer could see the soil reservoir act as a net source of carbon
as it would be released as carbon dioxide into the atmosphere. In such an instance
soils would act to further global warming. (We will return to carbon in soils later in
this chapter and again in Chapters 5 and 7.)
Again, as mentioned above, alternatively (or in addition: we just do not know) the
oceans could be a greater sink of carbon than we realise, so they could account for
the carbon imbalance. Either way, it is almost certain that the driving force behind
this missing sequestration of additional atmospheric carbon is photosynthesis, even
if we are unable to say whether it is marine, terrestrial or both, let alone where on
Earth it is taking place. (Having said that, there is some fascinating progress in the
area of satellite monitoring that shows seasonal changes in the photosynthetic pump
in various parts of the world, although there is a little way to go yet before we can get
meaningful quantitative data.)
With regard to the scale of carbon flow between carbon cycle reservoirs due
to photosynthesis, the observed annual natural seasonal variation in atmospheric
drawdown and replenishment is considerable, and these are greater than the year-on-
year increasing trend in atmospheric carbon dioxide due to the human addition of
fossil fuels and land-use change. Just as human action (the action of just one species)
is responsible for the current growth in atmospheric carbon dioxide, so one of the
most fundamental biological processes - photosynthesis - (through many species)
can almost certainly be involved in its amelioration. This re-emphasises that biology
and climatology are closely entwined.
Here the ways that photosynthetic and respiratory enzymes handle carbon and
oxygen are beginning to illuminate the problem of missing carbon. As stated above,
Rubisco is the enzyme globally responsible for fixing atmospheric carbon dioxide as
part of the photosynthetic process. However, not all atmospheric carbon is in the form
of the 12 C isotope. Around 1% is 13 C. Rubisco has evolved the greatest specificity
for the almost universal 12 C and so discriminates against 13 C, leaving it behind in
the atmosphere. If photosynthetic activity increases (as it does every summer in
each hemisphere) then the increase in atmospheric 13 C left behind can be measured.
This also works if photosynthesis increases due to global warming, because in a
warmer world the thermal growing season (TGS) is longer. On the other hand,
isotopes of 12 C and 13 C dissolve more or less equally well in sea water: in fact, if
anything 13 C dissolves slightly more easily. Consequently, if we detect changes in
atmospheric 13 C above and beyond the expected seasonal changes in a hemisphere, we
can see whether the photosynthetic pump is working harder or not. Similarly, carbonic