Environmental Engineering Reference
In-Depth Information
TABLE 6.2
Balance of atmospheric CO
2
during the 1990s.
Flux of CO
2
Category
Rate (Pg C/y)
Comments
Fossil fuel plus forest clearing
emissions to the atmosphere
6.5
Estimated from national records and additionally confirmed
by changes in
13
C and
14
C of the atmosphere.
Measured increase in
atmosphere
3.1
Direct measurement. Known well from Mauna Loa and
other observatories.
Ocean CO
2
uptake
1.5
Semidirect measurement based on both DIC concentrations
and the penetration of bomb
14
C into the ocean. Largely a
physical-diffusion sink.
Net “terrestrial sink”
1.9
Estimated by difference between the other terms. Poorly
constrained. Must be biological if on land.
Positive values are inputs into the atmosphere; negative values are losses from the atmosphere. The net “terrestrial sink”
sometimes called the “missing sink” is inferred by the difference among the other sources and sinks. (Data from
Schimel et al. 2001
.)
sequestered into the oceans (1.5 Pg C/y) or on the continents (1.9 Pg C/y).
The estimates of both fossil fuel combustion and oceanic uptake are considered to be
relatively precise. The sequestration on the continents is basically estimated by
difference and not considered to be very precise. In fact, this net terrestrial
sequestration in literature published in the 1980s and 1990s is referred to as the
“missing C sink” largely because it has proven difficult to locate where on land or by
what processes this sequestration occurs (
Hobbie et al. 1985; Woodwell et al. 1998; Fan
et al. 1998
). While there is active research into the missing sink, the processes causing it
have not yet been resolved but the sink is likely on land (
Tans et al. 1990
).
2. The changes in C isotopes (
13
C, a stable isotope, and
14
C, a radioactive isotope) in the
atmosphere. The isotopic make-up of atmospheric CO
2
provides a second line of
evidence (
Tans et al. 1979
). Photosynthetic uptake strongly favors the lighter isotope of
C(
12
C). Thus, any plant biomass or subsequent product is depleted (depleted here
means has less of the particular isotope) in both
13
C and
14
C compared to CO
2
in the
atmosphere. The decline over time of
13
C in the atmosphere shows that the new source
of CO
2
ultimately had a photosynthetic origin (
Bacastow et al. 1996
). Radioactive decay
of
14
C further depletes this isotope from material that was produced by photosynthesis
in an earlier time (
Jain et al. 1995
). With a half-life of 5780 years, material that is many
millions of years old, such as oil and coal, has essentially no remaining
14
C. The decline
over time of
14
C in the atmosphere tells us that the source of the increased CO
2
was
highly depleted in
14
C, which points to fossil fuel as the likely source. The isotopic
changes in atmospheric CO
2
over time, first reported by
Suess (1980)
, are also in
quantitative agreement with the mass balances of total CO
2
.
The Role of the Oceans
The sequestration of CO
2
on land and in the ocean are dominated by fundamentally
different processes. On land the major sink is storage or export of organic matter ulti-
mately from photosynthesis. What remains after plant, animal, and microbial respiration