Environmental Engineering Reference
In-Depth Information
288 K (15
C) is 33 K higher than its blackbody temper-
ature. Water vapor is by far the most effective GHG, ac-
counting for almost two-thirds of the 33 K difference; it
has five major absorption bands between 0.8 mm and 10
mm, the broadest ones centered at 5-8 mm and beyond
19 mm. Other major GHGs are CO
2
, accounting for
nearly one-quarter of the forcing (absorption peaks at
@2.6 mm and @4.5 mm, and between 12 mm and 18
mm); O
3
(4.7 mm and 9.6 mm); N
2
O (4.5, 7.8, and 8.6
mm); and CH
4
absorbing at about 3.5 mm and 7.6 mm
(Kondratyev 1988; Ramanathan 1998). Minor natural
contributions come mostly from NH
3
,NO
2
, HNO
3
,
and SO
2
.
Radiation balance of the Earth at the top of the atmo-
sphere (Q
ET
) requires that the outgoing LW radiation
(Q
LWT
, counted as negative) be equal to the incoming
SW stream (Q
SWT
) corrected for the planetary albedo (a):
(1963), Kessler (1985), Kiehl and Trenberth (1997),
and R. D. Rosen (1999). Results of global radiation
balance observations from satellites are assessed by
J. T. Houghton (1984); Ramanathan, Barkstrom, and
Harrison (1989); Hatzianastassiou and Vardavas (1999);
and Wielicki, Wong, Allan et al. (2002), and solar energy
flows at the surface of the Earth have been evaluated
by D. H. Miller (1981); Rosenberg, Verma, and Blad
(1983); Dabberdt et al. (1993); and NASA (2005).
Individual components of the Earth's mean annual ra-
diation balance have been quantified by dozens of studies
during the twentieth century with the extreme differ-
ences for some shares being on the order of 50%. Global
monitoring from satellites constrained some of these
uncertainties: NASA's Earth Radiation Budget Experi-
ment (ERBE) began in 1984; the first component of
Clouds and the Earth's Radiant Energy System (CERES)
became operational in 1997; and the satellite carrying the
first sensors of the European Geostationary Earth Radia-
tion Budget Experiment was launched in 2002 (ERBE
2005; CERES 2002; GERB 2005). But significant differ-
ences remain even when comparing only the budgets
that have been published since 1990.
For some models the differences between SW and LW
fluxes at the top of the atmosphere give radiative sur-
pluses greater than 5 W/m
2
; others have small deficits.
At the Earth's surface, modeled annual global averages
for net SW and LW fluxes range, respectively, from
142-172 W/m
2
to 40-68 W/m
2
, and the net all-wave
(AW) radiation ranges between 99 W/m
2
and 128 W/
m
2
(Hatzianastassiou and Vardavas 1999). For compari-
son, global July and January means of SW and LW sur-
face fluxes derived from NASA's Surface Radiation
Budget Project, conducted between 1983 and 1991, are,
respectively, 158 W/m
2
and 162 W/m
2
, and 47 W/m
2
Q
ET
¼
Q
SWT
ð
1
a
Þþ
Q
LWT
¼
0
:
Global annual value for the net radiation at the Earth's
surface (Q
ES
) must also be equal to 0 because the in-
coming direct (Q
DSW
) and scattered (diffused, Q
SSW
)
SW radiation, diminished by the albedo, must be bal-
anced by the LW radiation streaming upward (Q
LWU
)
and downward (LW counterradiation, Q
LWD
):
Q
ES
¼ð
Q
DSW
þ
Q
SSW
Þð
1
a
Þ
Q
LWU
þ
Q
LWD
:
As with so many scientific concepts, the ideas of plane-
tary energy budgets were turned into systematic and im-
pressively complete accounts during the last decades of
the nineteenth century (Voeikov 1884; Homen 1897;
D. H. Miller 1969). Advances in our understanding
of the Earth's energy budgets are reviewed by Budyko
