Environmental Engineering Reference
In-Depth Information
Atmosphere
597
+ 165
120
Land
Use
Change
Land
sink
119.6
Weathering Respiration
0.2
1.6
2.6
6.4
GPP
70.6
70
22.2 20
Vegetation,
Soil & Detritus
Fossil Fuels
3700
−
2
44
2300
+ 101
−
140
0.8
0.4
Rivers
Surface Ocean
Marine Biota
50
39
900
+ 18
3
Weathering
0.2
1.6
90.2
101
11
Intermediate
& Deep Ocean
37,100
+ 100
0.2
Reservoir sizes in GtC
Fluxes and Rates in GtC yr
−
1
Surface sediment
150
Fig. 2.1 The global carbon cycle for the 1990s, showing the main annual fluxes in PgC/year:
preindustrial “natural” fluxes in black and “anthropogenic” fluxes in red. Gross fluxes generally
have uncertainties of more than
20% but fractional amounts have been retained to achieve
overall balance when including estimates in fractions of PgC/year for riverine transport,
weathering, deep ocean burial, etc. “GPP” is annual gross (terrestrial) primary production.
Atmospheric carbon content and all cumulative fluxes since 1750 are as of end 1994
(Figure from [
4
])
are closer to 120 and 70 Tg/year (the net fluxes are closer to zero). This suggests that
changes in the land or ocean carbon balance will be communicated rapidly through
the atmosphere.
Carbon dioxide is removed from the atmosphere, not through chemical reactions,
but through huge fluxes into the land or oceans, with an atmospheric lifetime of
several years (
Fig. 2.1
from [
4
]). In reality, however, the land and ocean fluxes are
reasonably well balanced and it is difficult for all the extra carbon to be taken up,
suggesting that the lifetime of the additional carbon dioxide emitted by humans is on
the order of a few hundred years [
4
,
5
]. However, there will be some carbon (perhaps
20-35%) that stays in the atmosphere for 3-7 kyr, thus the lifetime for atmospheric
carbon dioxide is ambiguous [
5
].
Carbon dioxide shifts between glacial and interglacial climates are one of the
most robust signals from the ice core records [
6
], and carbon dioxide tends to be
about 80-100 ppm smaller during glacial periods than during interglacial periods
(like today). These changes in carbon dioxide have not yet been explained, but are
likely to be caused by a combination of colder temperatures, changes in ocean
circulation, and changes in ocean productivity [
7
]. Indeed these changes in carbon
dioxide are important forcing agents for maintaining the cold temperatures in the
glacial periods [
8
], and thus must be an integral part of explaining the glacial/
interglacial cycles over the past several hundred thousand years.
