Environmental Engineering Reference
In-Depth Information
In addition to carbon stored in the form of methane clathrates there is
also a substantial reservoir of organic carbon in near-surface land deposits.
The standard estimate of the amount of carbon in the top meter of the land
surface, plus living biomass, is about 2,100 gigatonnes of carbon, equivalent
to nearly 1,000 ppm of atmospheric CO
2
concentration before allowing for
ocean uptake (Trumper et al., 2009). This figure is probably an underestimate
of terrestrial organic carbon storage, as it doesn't fully account for carbon
storage in peatlands, and there is considerable potential that permafrost sys-
tems may store an additional 1,000 GtC or more of organic carbon (Schuur
et al., 2008). At all times, microbial respiration is oxidizing some of this
carbon and turning it to CO
2
, while photosynthesis is storing new carbon in
the terrestrial pool. Currently, it is the photosynthetic storage that wins, so
that the terrestrial ecosystem provides a moderate net sink of anthropogenic
carbon. Are there circumstances when the balance can change and the ter-
restrial ecosystem instead becomes a net source of atmospheric CO
2
?
Terrestrial carbon-cycle modeling was discussed in detail in Section
2.4. Many models predict that terrestrial ecosystems will continue to be a
modest sink of anthropogenic carbon, but this conclusion is dependent on
highly contested aspects of the CO
2
fertilization effect, and in particular the
possible role of nutrient limitation in inhibiting fertilization. At least one
carbon-cycle model predicts that terrestrial ecosystems can become a net
CO
2
source of carbon by 2050, with eventual releases of up to 5 GtC per
year (Cox et al., 2000). The best evidence that the Earth system can indeed
release several thousand GtC of organic carbon in the context of a warming
environment is provided by the Paleocene-Eocene Thermal Maximum.
The PETM event demonstrates that the Earth system can succumb to
destabilizing carbon-cycle feedbacks, in which an organic carbon pool (pre-
sumably on land) begins to oxidize rapidly and becomes a source rather than
a sink of atmospheric carbon dioxide. In the case of the PETM, the release
amounted to 3,000 GtC (Zeebe et al., 2009), which is eight times as much
carbon as has been released by all fossil burning to date, and comparable
to the higher range of estimates of what might be released by future fossil-
fuel burning. The processes that led to the PETM carbon release are not at
all understood, and so the risk that anthropogenic global warming could
trigger a similar catastrophic release cannot at present be quantified, save to
say that the risk is a real one. Estimates of carbon-cycle feedback based on
Pleistocene or Holocene carbon dioxide fluctuations suggest much smaller
carbon-cycle feedbacks than the PETM (Frank et al., 2010), but the PETM
provides the closest analogue to what might happen to the carbon cycle in
a climate substantially warmer than the present.
