Environmental Engineering Reference
In-Depth Information
This is a density unmatched by any other vertebrate,
and it is 3 OM larger than that of large herbivorous
ungulates in Africa's richest grassland ecosystems. Even
more remarkably, this density surpasses even that of the
combined total of all microbial (archeal, bacterial, fungal,
and protozoan) biomass that is normally the dominant
category of heterotrophs in natural ecosystems. Large en-
ergy subsidies for food production and transportation
make the top consumer the most abundant heterotroph
in the densest urban ecosystems (in terms of total
biomass, not individual organisms). Densities of about
10,000 people/km
2
, common in megacities, are in bio-
mass terms 1 OM higher than the densities of inverte-
brate biomass in many farm soils, but they do not have
any preordained effects. Crowding has no simple relation
with any mechanism or response that has been measured
(Cohen 1980; Freedman 1980). Unlike in animal experi-
ments, human exposure to high density urban living
rarely produces extreme social pathologies (Lepore
1994): aberrant behavior, aggression, and criminality are
not at unsurpassed levels in Hong Kong or Kolkata.
Human territoriality has been elaborated to such a
high degree that it is hardly recognized as such (Malm-
berg 1980). Obviously, the territoriality of modern ur-
ban populations has no existential energetic foundation
(as all food comes from outside), but energetic reasons
alone are also insufficient to explain the preindustrial
quest for a defensible territory (Lopreato 1984). Even
among foraging humans, food needs (in arid areas,
including secure water supplies) were only one of many
determinants of territoriality. In turn, needs for individ-
ual space, delimited private ground, and spatial rules for
social interaction are clearly shared with many verte-
brates. Privacy, identity, dignity, anonymity, diversity,
status, security, and anxiety—all these powerful factors
enter the territorial imperative. Thanks to human behav-
ioral plasticity, high residential densities do not necessar-
ily produce an inordinate amount of social decay, and
hence there is no way to argue that globally we are near-
ing the physical limits of urban packing. Their large-scale
limits will be most likely due to intolerable metabolic
consequences of extraordinarily concentrated energy
conversion densities.
A revealing way to illustrate the space demands of
modern energy production and use is to focus first on
the disparities between the power densities of conver-
sions that harness renewable energies and those that rely
on fossil fuels. Many atmospheric, hydrospheric, and
lithospheric energy conversions proceed at very high
power densities: up to 10
3
W/m
2
for such spatially
restricted phenomena as tornadoes, thunderstorms, and
earthquakes, and about 10
2
W/m
2
for extensive latent
heat releases in hurricanes and monsoons (see chapter
2). But most of these impressive natural energy flows re-
main completely unusable (lightning, volcanic eruptions,
tsunamis, avalanches, landslides), and even the best com-
mercial techniques can capture only small fractions of
usable fluxes (see section 9.2).
In no case do the average power production densities
of renewable conversions surpass 100 W/m
2
. Flat plate
solar heat collectors in sunny locations come close; the
rates are mostly 5-15 W/m
2
for geothermal electricity,
upper-course hydrogeneration (high heads, small reser-
voirs), and wind-powered generation; about 1 W/m
2
for most lower-course hydrogeneration (large reservoirs);
and less than 1 W/m
2
for usable phytomass (see chapters
3, 6, and 10 for details on plant productivity). In con-
trast, extraction of fossil fuels produces coals, crude oils,
and natural gases with power densities of 1-10 kW/m
2
(fig. 11.2). Modern civilization is thus energized largely
