Environmental Engineering Reference
In-Depth Information
et al.
2007
; May
2008
; Schmidt et al.
2010
; Zhang et al.
2011
; Robertson and
Grace
2004
; Friedlingstein et al.
2003
; Jones et al.
2003a
,
b
,
c
; Le Quéré et al.
2003
; Archer et al.
2004
; Buffett and Archer
2004
; Forster and Joshi
2005
; Hansen
and Sato
2004
; Eliseev et al.
2007
). Atmospheric constituents are responsible for
increasing the atmospheric temperature via two main processes. First, long-wave
(or thermal) radiation emitted from the terrestrial surface is absorbed at a particu-
lar frequency and reemitted at lower frequency by greenhouse gases and clouds
throughout the earth's atmosphere. The earth's surface can emit long-wave (or
thermal) radiation because it is heated by sunlight. Second, gases, clouds and aero-
sols can absorb and scattered short-wave radiation (UV and visible) significantly.
The cooling effect via short-wave reflection is dominant for clouds and aerosols.
The transfer of long-wave radiation depends on both the local temperature of the
gaseous absorber and the efficiency of the gases to absorb radiation at a given
wavelength (Kiehl and Trenberth
1997
). The absorption efficiency varies with
wavelength. Note that many greenhouse gases can absorb radiation at the same
wavelengths, which is called the overlap effect. In the presence of clouds, the
transfer of radiation depends on the cloud amount, on the efficiency of clouds to
absorb and reemit the long-wave radiation (cloud emissivity) and on the cloud top
and base temperatures (Kiehl and Trenberth
1997
).
It has been shown that the sulfate aerosols have a negative forcing effect that
partially counterbalances the warming effect of greenhouse gases (Charlson et al.
1989
; Wigley
1989
; Quaas et al.
2004
; IPCC
2007a
; Schmidt et al.
2010
; Joshi
et al.
2003
; Eliseev et al.
2007
; Rosenfeld
2000
). It is suggested that aerosols scatter
sunlight and enhance the planetary short-wave albedo, an effect known as the 'aero-
sol direct effect' (ADE). In addition, by their ability to act as cloud condensation
nuclei, (hygroscopic) aerosols change cloud properties and produce essentially an
increase in cloud albedo. These processes are called 'aerosol indirect effect' (AIE).
The increase in greenhouse gas concentration could lead to a reduction of
clouds at all atmospheric levels, thus decreasing the total greenhouse effect in the
long-wave spectrum but increasing absorption of solar radiation upon reduction
of cloud albedo (Quaas et al.
2004
). Increasing anthropogenic aerosols result in a
decrease of high-level cloud cover by cooling of the atmosphere, and an increase
in the low-level cloud cover through the second aerosol indirect effect (Quaas
et al.
2004
). The decrease of the high-level cloudiness and the increase of the low-
level one due to the response of cloud processes to aerosols have a contrasting
impact on the short-wave radiation, and the net effect is slightly positive (Quaas
et al.
2004
). The total aerosol effect, including the aerosol direct and first indirect
effects, remains strongly negative (Quaas et al.
2004
; IPCC
2007a
).
In addition, the depletion of stratospheric ozone caused by atmospheric anthro-
pogenic GHGs can enhance penetration of harmful UV-B radiation (280-315 nm),
which can have a direct influence on living organisms and also affect the global
warming (IPCC
2001
; Huisman et al.
2006
; Kerr and McElroy
1993
; Varotsos
and Kondratiev
1995
; Hartmann et al.
2000
; Qian et al.
2001
; Sarmiento et al.
2004
; Schmittner
2005
). The impact of UV-B radiation on global warming is of
two kinds. The first is a direct heating effect of UV-B radiation penetrating in the
