Environmental Engineering Reference
In-Depth Information
the land areas by the availability of water to be evapo-
rated. Sensible heat flux is relatively unimportant as long
as there is sufficient water for evaporation. Peaks of its
annual means, around 80 W/m
2
, coincide with the areas
of high aridity, while most of the oceans have yearly
means below 10 W/m
2
. About 60% of the global mean
of 17 W/m
2
derives from the land, and some 45% of ra-
diation absorbed by the continents goes into this turbu-
lent exchange with the atmosphere. Daytime means in
dry inland locations can be up to 200 W/m
2
, and the
maxima can reach up to 400-500 W/m
2
. Relatively
humid mid-latitudes have peaks around 150 W/m
2
and
averages of 50-60 W/m
2
.
Only the central areas of large cities go much above
these levels, especially where the absorbed radiation and
the latent heat in moist air inside high-rise buildings are
removed by air conditioning into the surrounding space.
There the sensible heat fluxes may rival those above the
deserts, reaching over 300 W/m
2
in the early afternoon.
Sensible heat flux is usually of minor importance in all
well-watered ecosystems, but it is a critical defense
against overheating during dry spells when plants translo-
cate heat from the soil to leaves and from exposed leaves
to the shaded ones to maintain tissue temperature within
optimum range. The Bowen ratio relates sensible and la-
tent heat fluxes: the quotient is mostly about 1 for the
continents and about 0.1 for the oceans, resulting in a
global mean of about 0.3.
As for the incoming (downward) LW radiation
emitted between 4 mm and 50 mm by the triatomic atmo-
spheric molecules, stratospheric O
3
contributes 15-20
W/m
2
,CO
2
adds 70-75 W/m
2
, and water vapor emits
150-300 W/m
2
. This flux is just an internal subcycle, a
temporary delay in the outward flow of reradiated heat,
but it is a critical determinant of tropospheric tempera-
tures and the largest supplier of energy to nearly every
ecosystem (Miller 1981). The annual global mean of the
flux is around 320 W/m
2
, with mid-latitude continents
receiving around 300 W/m
2
and cloudy equatorial
regions up to 400 W/m
2
. Diurnal ranges are mostly be-
tween 20 W/m
2
and 50 W/m
2
, and the flow weakens
little (25%-30%) even in winter. Momentary variations
can be fairly large: a passing cloud can boost the flux by
25 W/m
2
, and changing levels of greenhouse gases are
the major source of its intensification.
Finally, on the second most important extraterrestrial
influence on the Earth, its encounters with extraterres-
trial bodies. Objects up to 220 m in diameter produce
only air blast (Bland and Artemieva 2003), but there are
about 1,200 near-Earth asteroids with diameters of 1 km
or more, and collision with such objects would have
global consequences (Stuart 2001). Fortunately, by the
beginning of 2007 the international telescopic Space-
guard Survey had already identified about 700 of these
bodies, and it found that none of them is on a trajectory
that would lead to collision with the Earth during the
twenty-first century (NEOP 2007). Impact energy of a
1-km body would be equivalent to the release of close
to 100 Gt TNT (1 t TNT
ΒΌ
4.184 GJ), nearly 1 OM
more of energy than would have been expended by an
all-out thermonuclear war in 1980 (Sakharov 1983). If
such an object were to enter the ocean, the impact would
generate a large tsunami, and the principal global effect
of a continental impact would be due to an immense
mass of shattered material lifted high into the atmo-
sphere, resulting in a drastic drop of temperature, exten-
sive deposits of dusts, and long-term reduction of plant
productivity.
