Geoscience Reference
In-Depth Information
cycle, the phosphorus cycle and the hydrological cycle.
The Swedish scientist Svante Arrhenius speculated in
the early 1900s that human activities would increase the
concentration of carbon dioxide in the atmosphere and
that this would lead to global warming. Not until the
1950s did David Keeling initiate accurate measurements
of CO
2
at stations like Mauna Loa, Hawaii, which showed
that the concentration was continually increasing, with a
strong seasonal cycle (the 'Keeling curve'). The seasonal
cycle of CO
2
represents a temporal imbalance between
photosynthesis and respiration during the year. Unlike
other greenhouse gases involved in the Earth's radiation
balance such as methane (CH
4
) and nitrous oxide (N
2
O),
there is no finite lifetime for CO
2
. The atmosphere
continually cleanses itself of CH
4
in about nine years by
photochemical oxidation. In contrast, CO
2
may dissolve
in oceans, or get turned into organic tissue during growth,
but in general it turns back into atmospheric CO
2
on a
large scale very easily.
The global carbon cycle and the exchange of carbon
between the atmosphere and various natural and anthro-
pogenic compartments in the cycle are shown in
Figure
and the atmosphere through photosynthesis, respiration,
decomposition, and combustion. Much carbon is stored
in the huge geological reservoirs of coal and carbonate
rocks, but the exchange between these reservoirs and the
three active reservoirs of atmosphere, biosphere and
oceans is very slow. The Earth contains about 10
23
g of
carbon. The largest fluxes in the global carbon cycle are
those that link atmospheric CO
2
to terrestrial vegetation
and the oceans. Emissions produced by burning fossil
fuels and by land use changes are 'stuck' in the three active
systems. A steady state develops between the atmosphere
and each of the other two reservoirs, with very large bi-
directional flows between them.
It has long been known that the rate of increase in
atmospheric CO
2
is less than the rates of anthropogenic
emissions and land use change. This 'missing sink' is now
known to be mostly due to two factors. First, there is
uptake in the terrestrial biosphere, stimulated by forest
regeneration, fertilization, nitrogen deposition and
climatic effects. It amounts to about 0.1 Pmol C yr
-1
. This
is the process of
sequestration,
or the 'locking up' of an
element in a store, or sink. Ecologists argue from theory
that most carbon is sequestered by the net primary
production of tropical and temperate forests. These
forests act as a carbon sink, holding the CO
2
they take in
via photosynthesis before returning it to the atmosphere
as their dead wood decays or as they are burnt in fires
(
Plate 21.2
).
Forests
Tropical rain forest
42
Tropical seasonal
14
Tropical evergreen
10
Temperate deciduous
11
Boreal
13
Grassland and desert
Savanna
4
Temperate grassland
1
Tundra and alpine
0·3
Desert and semi-desert
1
Aquatic
Open ocean
0·5
Reefs
0·6
Estuaries
0·7
Ecosystem type
LAI
% NPP
Ratio
(m
2
m
-2
)
consumed
NPP/B
by herbi-
vores
Tropical rain forest
6-16
7
0·04
Tropical evergreen
5-14
4
0·04
Temperate deciduous
3-12
5
0·04
Boreal forests
7-15
4
0·04
Savanna
1-5
15
0·23
Temperate grassland
5-16
10
0·33
Lakes
20
25
Open ocean
40
42
Reefs
15
2
Estuaries
15
2
between forest biomes, grassland biomes and aquatic
ecosystems, with increasing turnover rates in these groups.
Similarly the ratio NPP/B contrasts the rapid turnover
rates of lakes and oceans with the very low figures for
terrestrial ecosystems, especially forests.
GLOBAL CARBON CYCLE
Since the 1990s there have been significant advances in
knowledge about the pools and fluxes in the global carbon
cycle. There is also more understanding of the interactions
between this cycle and other global cycles like the nitrogen
