Environmental Engineering Reference
In-Depth Information
of grassy surfaces around buildings, roads, and railways, and not only individual
trees or small groves in parks and along transportation corridors but often sub-
stantial tree cover in low-density residential suburban and exurban developments.
Perhaps the most accurate ISA total for the United States is found in a USGS study
that ended up with just over 90,000 km
2
for the 48 states (USGS 2000).
Elvidge et al. (2004) used remote sensing imagery to estimate that in the United
States, impervious surfaces covered about 113,000
13,000 km
2
, and Churkina,
Brown, and Keoleian (2010) ended up with a somewhat higher total of 141,000
±
±
40,000 km
2
for the year 2000. They also estimated that grass (with a carbon density
of just 0.5 t/ha) covers about 40% of urban and 66% of all exurban areas, and that
forest (with a carbon density of about 85 t/ha) accounts for just over 25% in each
of these categories (dei ned, respectively, as one housing unit for every 4,000 m
2
and
every 4,000-162,000 m
2
). These assumptions mean that urban and exurban areas
of the United States stored at least 3.5 (the most likely range being 2.6-4.6) Gt C
in the year 2000. For comparison, they also calculated that carbon stored in the
woody phytomass used in buildings and furniture was on the order of 1 Gt, and
that landi lls harbored about 2 Gt C.
The i rst global inventory of constructed ISA used a 1 km
2
grid of the bright-
ness of satellite-observed nighttime lights and population counts calibrated using
30 m resolution ISA of the United States (Elvidge et al. 2007). This study found
that the worldwide total of constructed ISA in the year 2000 was about 580,000
km
2
, or 0.43% of land surface (equivalent in area to Kenya or Madagascar), and
its total for the United States (about 84,000 km
2
) was very close to the USGS
study. The U.S. ISA was only a few percentage points smaller than the world's
largest national total, in China (87,000 km
2
); India, Brazil, and Russia completed
the top i ve.
The ranking is very different in per capita terms. After the small desert countries
of the Persian Gulf and even smaller-city states (whose ISA is exceptionally high a
share of the national territory) are set aside, the highest rates are in the Northern
Hemisphere's afl uent countries, and particularly in the higher latitudes: Canada led,
with about 350 m
2
per capita, followed by Finland (320 m
2
), the United States
(about 300 m
2
), Norway (235 m
2
), and Sweden (220 m
2
), while the ISA rates for
most other countries (both high- and low-income) were between 50 and 150 m
2
per
capita. Perhaps the most remarkable (and counterintuitive, for such a highly urban-
ized society) is Japan's low rate of just 114 m
2
per capita—but it is perfectly expli-
cable, given the exceptionally high density of housing and the constraints on
construction arising from the country's lack of nonagricultural l at land (85% of
Japan's surface is mountainous with often fairly steep slopes).
