Environmental Engineering Reference
In-Depth Information
Janssen (2010), the total built-up area of the cities rose from about 47,000 km
2
(an area a bit larger than Denmark) to 538,000 km
2
(an area a bit smaller than
France). The last total is just one of eight recent studies that calculated the total
global urban area for the year 2000, with results ranging over an order of magni-
tude, from 270,000 km
2
to as much as 3.52 Mkm
2
. Potere et al. (2009) assessed the
accuracy of these studies by using a random sample of 10,000 high-resolution vali-
dation sites and 140 medium-resolution city maps and concluded that the assess-
ment based on the new MODIS 500 m resolution global urban map is the most
accurate: it put the total urban land at 657,000 km
2
in 2001 (Schneider, Friedl, and
Potere 2009). This study classii ed as urban all areas where built structures occupied
more than 50% of the total area, and it included all nonvegetated, human-constructed
elements with a minimum area of more than 1 km
2
.
This i nding has several interesting implications. First, the total is only about 20%
higher than the one cited in the last paragraph that was used to derive the historical
trend of urban land claims, and hence it basically coni rms the validity of the pro-
gression of land claimed by cities from a total on the order of 10,000 km
2
in 1500
to 50-60 times as much half a millennium later. Second, the claim of some 660,000
km
2
equals only about 0.44% of the Earth's continental area. and even when
allowing a 50% error, the total would be no more than 1 Mkm
2
or less than 0.7%
of ice-free land. This surprisingly small share of land needed to house half of human-
ity (the 50% mark was surpassed in 2007) is due to very high residential densities
(Demographia 2010).
In the year 2000, citywide means of population densities were about 13,000/km
2
in Tokyo, 16,000/km
2
in Seoul, 20,000km
2
in Paris, and 40,000/km
2
in Manila.
Hong Kong's most densely populated district (Kwun Tong) had 50,000/km
2
, and
Tokyo's four central wards had a daytime density of about 55,000/km
2
, as high as
the daytime population of Manhattan, where the Wall Street area packs in close to
250,000 people/km
2
during working hours. Residential densities on the order of
50,000 people/km
2
imply more than 2 kg/m
2
of anthropomass, the rate unmatched
by any other vertebrate and three orders of magnitude higher than the peak zoomass
recorded for large herbivorous ungulates in Africa's richest grassland ecosystems.
What is even more stunning is that such human densities surpass even those of all
microbial biomass that normally dominates the weight of heterotrophs in natural
ecosystems.
Urban and industrial areas and transportation corridors are, of course, far from
homogeneous: they include such impervious surface areas (ISA) as buildings, roofed
and paved surfaces (roads, sidewalks, and parking lots), and aboveground storages
of materials (fossil fuels, ores) that are devoid of any vegetation, as well as plenty
