Environmental Engineering Reference
In-Depth Information
(@42 TW) and its average power density (80 mW/m
2
)
are minuscule. But these low-power flows have been
reshaping ocean floor, breaking apart and reassembling
the continents, building mountain ranges, and recy-
cling plates into the mantle. Dramatic displays of con-
centrated geothermal releases, earthquakes and volcanic
eruptions, have also had a great impact on the bio-
sphere's evolution.
There is a surfeit of free energy in the biosphere be-
cause only a fraction of available solar radiation is needed
to energize the water cycle and atmospheric motion, and
tectonic processes require only a very small part of plane-
tary heat. This is also true as far as photosynthesis is con-
cerned. Photosynthesis is not, with obvious exceptions of
high-latitude winters and plants in dark forest under-
growth, generally limited by the availability of solar
energy but rather by a relatively low concentration of at-
mospheric CO
2
, by temperature, and by water and nutri-
ent supply. Reaction losses and autotrophic respiration
limit photosynthetic efficiency to no more than about
5%, even under optimum conditions for the most effi-
cient plants. Species with suppressed photorespiration
(using the C
4
photosynthetic pathway) have an efficiency
advantage. C
4
plants also have substantially higher water
use efficiencies. Unfortunately, most of the major crops
are C
3
species. The CAM pathway is an excellent adapta-
tion to extreme temperature and water stresses, but its
productivity is necessarily very low.
Assessments of phytomass stores (dry matter energy
densities 17-20 kJ/g) can be only approximate. The
global aggregate is about 1 Tt, or 20 ZJ, with more
than 80% of the total in woody tissues. Continuing de-
struction of forests is the fastest way to reduce the Earth's
phytomass and, in the case of species-rich tropical ecosys-
tems, to lower dramatically the diversity of life. Small
inquiries and compartmentalization of knowledge. This
final, integrative, chapter does the very opposite; it aggre-
gates the most important findings of general energetics.
13.1 Energy in the Biosphere
The Sun floods the Earth with a surfeit of energy. Of
174 PW intercepted by the planet, about 122 PW (240
W/m
2
) is absorbed by the biosphere. The fate of this
absorbed radiation is strongly influenced by a highly un-
likely atmospheric composition that makes it possible
to have huge amounts of liquid water. In turn, water's
unique energetic attributes—high specific heat, high
heat capacity, and high heat of vaporization—are decisive
in storing and redistributing absorbed solar radiation.
Slightly more than one-third of the absorbed flux (45
PW) drives the planetary water cycle, and only a small
share of it (3.5 PW) is needed to keep the atmosphere
in motion (fig. 13.1). Power densities of solar flows
range from 10
3
W/m
2
for peak (noontime) direct radia-
tion to 10
0
W/m
2
in ocean waves, river runoff, and
winds. The kinetic energies of air and water can be highly
destructive because vertical power densities of cyclonic
flows and floods commonly surpass 10 kW/m
2
and
reach up to 1 MW/m
2
. These violent kinetic events
have shaped the evolution of ecosystems, yet in aggre-
gate terms all rain-carrying cyclones release much more
energy as latent heat. But the kinetic energies of precipi-
tation and the potential energies of water have been key
agents of geomorphic and ecosystemic change.
The Earth's solid crust is made up of huge rigid plates
subjected to slow cycles of creation and destruction,
which are energized by basal cooling of the planet and
by radioactive decay of several isotopes. Lunar and solar
gravity are responsible for tidal friction. In comparison
with solar flows, both the aggregate terrestrial heat flux
