Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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By some calculations (Stocker & Schnittner, 1997), doubling of the present-day atmospheric
pCO 2
would halt oceanic circulation thereby decreasing CO 2
uptake by the
ocean by 30%. However, the interaction of oceanic circulation, SST, ice volume and
atmospheric pCO 2
are complicated by a number of side effects. A positive loop of elevated
atmospheric CO 2
→ rising SST → melting of polar ice → damping of oceanic heat
conveyor by meltwater → reduced CO 2
uptake by the ocean (Saltzman & Verbitsky,
1994) might switch to the negative mode by an enhanced vapour transfer to continental
glaciers (Oppenheimer, 1998).
Land biota stores about 830×10 9 Gt carbon, 90% of it in the standing crop of which
60% in the fast growing tropical rainforests, a major biospheric sink of atmospheric CO 2
.
Yet the role of the other biomes might prove to be grossly underrated. Seasonal atmospheric
CO 2
fluctuations, with the mid-latitude highs/lows in April and August respectively,
attest to significance of deciduous vegetation (Woodwell et al., 1978; Watson et
al., 1991).
The boreal forests store less in the arboreal phytomass, but more in the moss layer,
the annual production of which can be equal to that of the trees. Moreover, while the
tropical evergreen biomes store carbon in their standing crop biomass, the temperate
biomes shed much more to peat and soil, which in the long run are more durable carbon
sinks (D’Arrigo et al., 1987). Humus contains four times more carbon than vegetation,
most of it in the temperate forests and grasslands (Woodwell et al., 1978; Bolin, 1987).
The mid-latitude forest uptake of CO 2
is estimated as 3.7± 0.7 tons per ha per year, that
of grasslands about 11.1 tons per ha per year which is much more than previously expected
(Wofsy et al., 1993). The detrital pool contains three times more carbon than
living plants. Its role as a sink of atmospheric CO 2
depends on the balance of accumulation
and decomposition rates.
The latter are controlled both by climate and litter quality (Meentemeyer, 1978; Aerts,
1997). While climate (the variables affecting evapotranspiration) is a leading factor for
leaf litter, the Ca level and the C:N ratio (relatively high in conifer needles showing the
slowest decomposition rates) are more important for the roots (Silver & Mija, 2001).
Hence the long-term variation of litter decomposition rates are related to both climate
and evolution of higher plant biochemistry.
In marine ecosystems, zooplankton sinks carbon with fecal pellets to bottom deposits
where it is stored in anoxic sedimentary environments. This mechanism gains in importance
with a spread of near-bottom anoxy, partly correlated with an influx of fresh waters.
At the same time, planktic production increases with riverine discharge bringing
nitrates and iron. During glacial phases, iron comes also with dust from desert areas
(Kumar et al., 1995). An active circulation of oceanic waters enhances planktic production
by preventing bacterial denitrification of anoxic subsurface waters (Ganeshram et
al., 1995). Since plankton is also a source of dimetilsulphate aerosols that increase cloudiness,
the net albedo effect of an increased oceanic productivity is much greater than its
CO 2
effect.