Environmental Engineering Reference
In-Depth Information
sulfur is an indispensable building ingredient, the fortifier
responsible for the three-dimensional structure of pro-
teins. The three elements are locked in the lithosphere
and hydrosphere in carbonates, nitrates, and sulfates,
and, to list just the principal members of airborne seg-
ments of their respective cycles, in CO, CO
2
,CH
4
,
N
2
O, NO, NO
2
,NH
3
,NO
3
,SO
2
,H
2
S, and SO
4
. Dur-
ing the preindustrial era human interference in the three
cycles was limited to burning of biomass and conversion
of natural ecosystems to cultivated lands (both essentially
a locally accelerated release of plant C, N, and S), and
some concentrated dumping and recycling of organic
wastes.
Fossil-fueled civilization brought radically different
interventions. The combustion of fossil fuels reintro-
duced long-dormant stores of C and S into the atmo-
sphere and generated increasing amounts of nitrogen
oxides. In addition, agricultural intensification rested on
the expanding use of inorganic nitrogen fertilizers. As a
result, anthropogenic fluxes of the three elements now
form large shares of their total biospheric flows, especially
in industrialized or intensively farmed areas (Smil
2000a). By far the most worrisome interference in
the global cycle is rising atmospheric concentrations of
CO
2
from combustion of fossil fuels and land use
changes (fig. 11.7). These concerns arise from the critical
role the gas has played in determining the biosphere's
temperature.
Water vapor, the most important greenhouse gas,
could not have maintained relatively stable temperatures
because its changing atmospheric concentrations amplify
rather than counteract departures from surface tempera-
tures; evaporation declines with cooling and rises with
warming. In addition, changes in soil moisture do little
to chemical weathering. Only long-term feedback be-
tween CO
2
, surface temperature, and the weathering of
silicate minerals explain the limited variability of mean
tropospheric temperature. Lower temperatures and de-
creased rates of silicate weathering result in gradual accu-
mulation of CO
2
and in subsequent warming (Berner
1999). A reliable record of atmospheric CO
2
is available
for the past 420,000 years, thanks to the analyses of air
bubbles from ice cores retrieved in Antarctica and Green-
land. Preindustrial CO
2
levels were never below 180
ppm and never above 300 ppm (fig. 11.8) (Raynaud
et al. 1993; Petit et al. 1999). And between the begin-
nings of the first civilizations 5000-6000 years ago and
the onset of the fossil-fueled era, these levels fluctuated
narrowly between 250 ppm and 290 ppm.
The post-1850 rise of fossil fuel combustion (including
relatively small contributions by cement production and
natural gas flaring) brought global carbon emissions (1 t
C
¼
3.66 t CO
2
) from less than 0.5 Gt in 1900 to 1.5
Gt in 1950, and to over 6.5 Gt C by the year 2000,
with about 35% originating from coal and 60% from
hydrocarbons (Marland, Boden, and Andres 2005).
Many studies have also evaluated life cycle emissions of
CO
2
, or more precisely, CO
2
equivalents of other green-
house gases (Lenzen 1999; Meier 2002; Gagnon,
B
´
langer, and Uchiyama 2002; IHA 2003). The lowest
values (rounded to avoid unwarranted impressions of ac-
curacy and expressed in CO
2
-equivalent t/GW
e
h) are for
wind (@10), nuclear fission and large-scale hydrogenera-
tion (@15, with the latter up to 50). Biomass-generated
electricity rates just over 100, combined cycle gas tur-
bines at nearly 500, diesel generators at almost 800, and
conventional fossil-fueled plants around 1,000. Analyses
of solar-thermal and photovaltaic generation are rela-
