Environmental Engineering Reference
In-Depth Information
troposphere. The second effect is the release of CO
2
to the atmosphere upon pho-
toinduced degradation of DOM induced by UV-B radiation in natural waters (Qian
et al.
2001
; Sarmiento et al.
2004
; Schmittner
2005
). However, the observed losses
in the stratospheric ozone layer over the past two decades have caused a negative
climate forcing (0.15
±
0.1 Wm
−
2
), i.e. a tendency toward cooling of the sur-
face troposphere system (IPCC
2001
). Model calculations indicate that increased
penetration of ultraviolet radiation to the troposphere, as a result of stratospheric
ozone depletion, leads to enhanced removal rates of gases like CH
4
, with a result-
ing cooling effect (IPCC
2001
).
In contrast, other studies suggest that stratospheric ozone depletion and GHG
warming may both be producing increased meridional temperature gradients in the
extratropical lower stratosphere and upper troposphere, thereby acting synergisti-
cally to produce surprisingly large trends in both surface and stratospheric climate
(Hartmann et al.
2000
).
2.1 Occurrence and Contribution of Atmospheric
Constituents to Global Warming
The global atmospheric concentration of CO
2
has increased from a pre-industrial
value of about 280-379 ppm in 2005 (IPCC
2007a
). The CO
2
concentrations
in 2005 exceeded by far the natural range over the last 650,000 years (Fig.
1
a)
(IPCC
2007a
). Despite the year-to-year variation of CO
2
concentration growth
rate, it is estimated that the annual rate of CO
2
concentration growth has been
larger over the past 10 years (1995-2005; average: 1.9 ppm per year) than in the
whole record of continuous direct atmospheric measurements (1960-2005; aver-
age: 1.4 ppm per year). The atmospheric concentrations of CH
4
in 2005 exceeded
by far the natural range over the last 650,000 years (Fig.
1
b) (IPCC
2007a
). The
global atmospheric concentration of CH
4
increased from a pre-industrial value of
about 715-1732 ppb in the early 1990s, then to 1774 ppb in 2005 (IPCC
2007a
).
The data also suggest that the growth rates have declined since the 1990s, coher-
ently with total emissions (sum of anthropogenic and natural sources) being
nearly constant during this period. The global atmospheric N
2
O concentra-
tion increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005
(Fig.
1
c) (IPCC
2007a
).
An important greenhouse gas in both the stratosphere and the troposphere is
ozone (O
3
), which is formed in the atmosphere from photoinduced processes that
involve both natural and human-influenced precursor species (IPCC
2001
). The
residence time of ozone in the atmosphere is relatively short, varying from weeks
to months (IPCC
2001
). The total amount of O
3
in the troposphere is estimated to
have increased by 36 % since 1750, due primarily to anthropogenic emissions of
several O
3
-forming gases (IPCC
2001
). It is also suggested that O
3
climate forcing
varies considerably depending on the region and that it responds more quickly to
changes in emissions than the long-lived greenhouse gases, such as CO
2
.
