Environmental Engineering Reference
In-Depth Information
with rates for individual events mostly between 10 EJ
and 50 EJ (120-580 TW); the kinetic energy dissipated
by an average Atlantic hurricane is about 3 TW and by a
Pacific supertyphoon ten times as much (Emanuel 1998).
The Earth's largest seasonal landward water-borne
transfer of latent heat is the Asian monsoon that origi-
nates in the heating of the Pacific Ocean waters (Gadgil
2003). Its course affects the lives of about 2 billion peo-
ple in an area totaling nearly 10 Gm
2
. Continental pre-
cipitation averages about 10 Gm
3
during six months, a
latent heat transfer amounting to almost 1.5 YJ at a rate
of 1.5 PW and land power density of some 150 W/m
2
.
All but about 10% of water evaporated from the ocean is
precipitated back onto sea surfaces. In the long run oce-
anic evaporation is also influenced by the mean sea level:
during the last glacial maximum, 18,000 years ago, that
level was 85-130 m lower than it is now (CLIMAP
1976).
Direct estimates from tide gauges indicate a global sea
level rise of 1.5-2 mm/a during the twentieth century,
although indirect estimates based on changes in mass and
volume suggest much lower (
<
0.5 mm) annual rates.
Miller and Douglas (2004) found that gauge measure-
ments are not biased by any above-average local warning.
For comparison, measurements by satellite altimeters for
the period 1993-2003 indicate a very similar annual rate
of about 2.5 mm. Shoreline retreat is the most obvious
consequence of sea level rise, with coastal plains typically
experiencing losses of 30-100 cm/a (Pilkey and Cooper
2004). Longer-term effects, related to more substantial
global warming-driven sea level rise, would be much
more worrisome.
There is no way to ascertain directly the share of plan-
etary heating that powers the atmospheric motion in
order to offset energy dissipated in turbulence (aloft)
and friction (at the surface). The best estimate derived
from the parameters of general circulation theory puts
the share at about 2% (Lorenz 1976), or roughly 7 W/
m
2
and 3.5 PW for the whole planet, but Peixoto and
Oort (1992) estimated that energy transferred to wind
and dissipated as friction is no more than 870 TW. With
the average density of 1.2 kg/m
3
and mean speed of
about 10 m/s, the kinetic energy of 1 m
3
of moving air
equals about 60 J. For the whole atmosphere (5.1 Em
3
)
it adds up to roughly 300 EJ (9.5 TW), a total smaller
than annual global commercial energy use in 2000. But
this figure is not an equivalent of a resource base to be
used for estimates of eventual extraction. Rather the en-
tire solar recharge (at least 870 TW) is the absolute theo-
retical limit on any utilization of wind energy. More
practically, the downward flux of kinetic energy, averag-
ing about 1.5 W/m
2
over the land surface (@220 TW
in total), puts the limit on extractable power (Best 1979).
2.4 Water and Air in Motion: Kinetic Fluxes
Kinetic energies of ocean water include the currents that
redistribute the absorbed heat, wind-generated waves,
seismic waves (including tsunamis and seiches), and tides.
Aggregate energies of these flows are large, but average
power densities are relatively low (although those of
catastrophic waves are obviously enormous), and any
commercial conversions will have to focus on areas of
exceptional fluxes encountered in strong currents, in
stormy seas with large waves, and in unusually high tides.
The total power of ocean currents has been estimated at
100 GW (Isaacs and Schmidt 1980), that is, merely 0.3
mW/m
2
of ice-free ocean surface. Major currents are
obviously much more powerful: for example, the Florida
Current between Bimini and Miami rates about 20 GW,
or 1.6-2.2 kW/m
2
.
