An introduction to climate change - Climate Change: Biological and Human Aspects - page 13

Table1.3 Estimatedannualcarbonemissionsin1980-9,1990-9(IPCC,2001a)and2000-5(IPCC,2007)to
theatmosphereandannualtransferfromtheatmospheretosinks.Thiswayyoucanseenotonlyhowwethink
thecarboncyclechangesascarbonmovesbetweenbiospherereservoirs,butalsohow
understanding/perceptionsofthecarboncyclehavechangedwithtime.(Thelatestunderstandingisgivenin
the2007WorkingGroupIassessmentinIPCC[2007],table1.7,p.516.)

Carbon transfer (Gt of carbon year − 1 )

1980-9 1990-9 2000-5

From fossil fuels to atmosphere 5.4 ± 0.3 6.3 ± 0.4 7.2 ± 0.3

From deforestation and net land-use change to atmosphere − 1.6 ± 1.0 − 1.4 ± 0.7 − 0.9 ± 0.6

Accumulation in atmosphere 3.4 ± 0.2 3.2 ± 0.1 4.1 ± 0.1

Ocean-to-atmosphere flux − 2.0 ± 0.8 − 1.7 ± 0.5 − 2.2 ± 0.5

Net imbalance in 1980s estimates 1.6 ± 1.4

third of the carbon dioxide in plants and in soil water. As we shall see, the way
these enzymes handle carbon dioxide molecules with different isotopes of carbon, or
oxygen molecules with different isotopes of oxygen, helps us in our understanding
of details of the carbon cycle.
It is important to note that there are some uncertainties in the estimates of the rate
of flows between the various sources and sinks (Figure 1.3). This is the subject of
ongoing research. Part of the problem (other than the cycle's complexity) is that the
carbon cycle is not static: there are varying transfers of carbon, so altering the amounts
within reservoirs. For example, during cold glacial times (such as 50 000 years ago)
the amount of atmospheric carbon (as both carbon dioxide and methane) was less
than today. Conversely, currently the atmospheric carbon reservoir is increasing. This
dynamism is not just because of human action; it also happens naturally, although
human action is the current additional factor critical to what is called global warming.
In practical terms today, carbon cycle uncertainties manifest themselves in a num-
ber of ways. Of particular concern is the mystery of where roughly half the carbon
dioxide released into the atmosphere by human action (from fuel burning and land-use
change) ends up: from measuring the atmospheric concentration of carbon dioxide
we know that half of what we burn does not remain in the atmosphere for very long.
This imbalance is so significant that there is debate as to whether a major carbon
cycle process has been overlooked. Alternatively it could be that the current estimates
as to the various flows have a sufficient degree of error that cumulatively manifests
itself as this imbalance.
The broad estimates (IPCC, 2001a, 2007; together with estimates of uncertainty)
of the contributions of burning fossil fuels and deforestation to atmospheric carbon
dioxide and the entry to carbon sinks from the atmosphere for the decades 1980-9 and
1990-9 are listed in Table 1.3. The estimates are derived from computer simulations.
As we can see from Table 1.3, in the 1990 IPCC report (average 1980-9 annual flux
column) there was this net imbalance between the estimates of carbon dioxide entering
and of those accumulating and leaving the atmosphere. More carbon dioxide was
estimated as being released into the atmosphere than was retained by the atmosphere
or thought to be absorbed by the oceans. Where was this carbon dioxide going?