Environmental Engineering Reference
In-Depth Information
explain only a tiny fraction (0.048
C) of overall warming
per century (Peterson and Owen 2005). Similarly, Parker
(2004) concluded that UHI warming has not biased the
best estimates of global warming. UHI thus has negligi-
ble large-scale climatic effects, but it has undeniable local
impacts.
Well-documented effects include increased cloudiness
(up to 8% more), precipitation (up to 14% more), and
thunderstorms (up to 15% more) near and particularly
downwind of UHI-affected areas as well as decreased rel-
ative humidity (2%-8% lower), wind speed (20%-30%
lower annual mean), and insolation on horizontal sur-
faces (15%-20% lower due to structural shading). UHI
boosts emissions of both biogenic hydrocarbons (tree
isoprene) and fossil hydrocarbons stored in tanks, accel-
erates temperature-dependent photochemical smog reac-
tions, and aggravates summer heat waves. In turn,
excessive heat raises the frequency of premature deaths
(due to heat strokes and dehydration) and reduces
night-time cooling, thus making nights less tolerable for
everybody without air conditioning.
The most readily quantifiable effect of urban heat
islands is the increased used of electricity for additional
air-conditioning (and decreased winter heating). Regres-
sion of temperature and power load data for Los Angeles
showed that on warm afternoons electricity demand rises
nearly 3% for every 1
C rise in the daily maximum, and
the probability of smog increases much more steeply, by
5% for every 0.25
C rise (Heat Island Group 2000a).
UHI increases the number of cooling degree-days by
15%-35% in large U.S. cities and by as much as 90% in
Los Angeles (Taha 2004). A welcome effect has been
the extension of the growing season as far as 10 km
beyond the city's edges. Satellite-measured surface reflec-
tance of seasonal changes in plant growth around eastern
North American cities with areas larger than 10 km
2
found that growing seasons were 15 days longer and
that every 1
C of additional urban warmth during the
early spring brought the first blooms three days earlier
(Zhang et al. 2004).
Tree planting is the most efficient and esthetically
pleasing way of reducing UHI intensity. Urban parks are
0.5
C-2.5
C cooler than their surroundings. Lighter
pavements and cool roofs are also effective. Traditional
roofing surfaces have albedos ranging from 5% (asphalt
roofs of commercial buildings, dark house shingles) to
20% (green shingles), and more reflective light shingles
with albedos of 35%-40% can reduce the temperature
differences between roof and surrounding air by about
10
C (Heat Island Group 2000b). Konopacki et al.
(1997) calculated that the universal adoption of light-
colored roofing materials would reduce U.S.
air-
conditioning demand by about 10 TWh.
11.3 Energy and Water
The presence of large volumes of liquid water is one of
the key material preconditions of the Earth's biosphere,
and its peculiar properties have inestimable consequences
for the planet's energetics (M. W. Denny 1993; see sec-
tion 2.3). If water behaved like other fluids of a similar
molecular weight, it would boil at
91
C and freeze at
100
C, and its liquid range would be far too much be-
low the optima (30
C-40
C) for enzymatic energy con-
versions in plants and animals. Alcohols nearly match
water's extraordinary departure from the general rule of
decreasing boiling and freezing points with decreasing
molecular weight, but they have a much lower heat ca-
pacity. Water's large specific heat (at 4.186 J/g
C 67%
higher than that of ethanol) has played two critical evolu-
tionary roles: first, helping to maintain a relatively con-
