Environmental Engineering Reference
In-Depth Information
diffusion of distance-shrinking prime movers, and con-
tinuing intensification of agriculture are the principal rea-
sons of this changed emphasis. But only colonization of
other celestial bodies can remove the constraints that the
Earth's finite area imposes on any civilization.
The intensifying use of land is highlighted by compar-
ing typical population densities during different stages of
human evolution. Even the most accomplished foragers
in the most favorable environments never surpassed 1
person/km
2
of exploited area, and most hunting and
gathering societies had just a few people for every 10
km
2
. Pastoralism could sustain 1-2 people/km
2
of
grassland or scrubland. Shifting farming pushed the rate
1 OM higher, to tens of people per square kilometer.
And traditional farming brought yet another tenfold in-
crease: several hundred people could be supported from
a square kilometer of arable fields and land under perma-
nent crops (see fig. 6.1). By the year 2005, despite the
nearly universal intensification of farming, agricultural
lands occupied roughly 12% of the Earth's ice-free sur-
face (1.54 Gha under annual species and permanent
crops) (FAO 2006) and supplied nearly 90% of food en-
ergy, the rest coming from grazing, forest foods, fish-
eries, and aquaculture.
Cultivated land thus fed on average just over 4
people/ha; China's rate was 8.5 people/ha. This means
that the global average of anthropomass (live weight of
humans, assuming an average of 45 kg/person for the
entire population) was almost 200 kg/ha of cultivated
land, and China's mean was nearly 400 kg/ha. In
China's most intensively cultivated provinces the mass of
humanity approached 500 kg/ha of arable land. In con-
trast, the average densities of the two large African
primates, chimpanzees and gorillas, are mostly less than
1 kg/ha of their now so limited and disappearing habi-
tats (Bernstein and Smith 1979; Prins and Reitsma
1989; Harcourt 1996). These comparisons demonstrate
the relentless ascent of the most adaptable as well as the
most destructive of all heterotrophic species, but they
also make it clear that this trend, predicated on rising
energy subsidies, cannot continue even if the requisite
amounts of energy were available.
In order to replicate the growth of yields during the
twentieth century, the global annual crop harvest would
have to be boosted nearly sevenfold, an achievement
that would imply average yields near or above the photo-
synthetic maxima even with the greatest possible expan-
sion of the cultivated land with intensive multicropping.
Only as yet unavailable genetic manipulation of photo-
synthesis could remove these energetic limits. The inten-
sification of energy subsidies in farming has been behind
the global rise of urbanization and its attendant high res-
idential densities. The high energy densities of fossil fuels
enabled centralized mass manufacturing, but the shift of
rural labor to cities could get under way only as field ma-
chinery and fertilizers began displacing animate power.
By 1800 only 3% of the global population of 1.2 billion
was urban; by 1900 the share was nearly 15% of 1.7
billion people; by 1950 it was roughly 30%; and by
2005 it was just above 50% of 6.5 billion. This profound
transformation—from an overwhelmingly rural, decen-
tralized, parochial, low-energy society to a predominantly
urban, centralized, globalized, high-energy culture—has
run its course in rich nations but is still accelerating in
modernizing countries.
Urban population densities can, of course, be orders
of magnitude higher than the anthropomass per unit of
cultivated land because cities increase their energy and
material footprints to large multiples of their areas. Simi-
larities exist only at the extremes. Residential densities
