Environmental Engineering Reference
In-Depth Information
The direct emission of water vapor (a greenhouse gas) by human activities
makes a negligible contribution to the radiative forcing, but an increase in global
average temperature can enhance the tropospheric water vapor concentration
and produce a key positive feedback for radiative forcing, thereby leading to fur-
ther warming (IPCC
2007a
). Interestingly, the so-called Humic-like Substances
(HULIS) occurring on atmospheric aerosols can enhance water uptake and
increase the role of particles as Cloud Condensation Nuclei (CCN), thereby con-
tributing to direct and indirect climate forcing (Hatch et al.
2009
). Warming also
reduces terrestrial and ocean uptake of atmospheric CO
2
, increasing the fraction of
anthropogenic emissions that remain in the atmosphere. Such an effect is expected
to lead to higher atmospheric CO
2
levels that are further involved into the global
climate change (IPCC
2007a
). The uptake by the oceans is approximately 25 % of
the annual carbon emissions that result from fossil fuel burning and cement manu-
facturing (Canadell et al.
2007
).
The infrared absorption cross-sections for eight commonly used CFCs (halo-
genated methanes and ethanes), as a function of temperature from 203 to 293 K,
suggest that the combined effects of absorption by CFCs of the Earth's radiative
energy in the 'window' region (700-1300 cm
−
1
) and of their O
3
depletion poten-
tial makes these compounds significant contributors to global warming (McDaniel
et al.
1991
). On the other hand, the sulfate aerosols can reduce global warming by
about 0.1-0.4 K, depending on the scenario and on the time period. The maximum
slowdown in warming (>1.5 K) is expected to occur in the Northern Hemisphere
middle- and high-latitude land areas in the mid-twenty-first century (Eliseev
et al.
2007
). A recent study has shown that the indirect effect of stratospheric
ozone depletion could have offset up to half of the predicted past increase in sur-
face temperature that would otherwise have occurred as a result of the direct halo-
carbon effects (Forster and Joshi
2005
). In both the troposphere and stratosphere,
CFC-12 contributed most to the CFCs-related past temperature changes, and the
emission projections suggest that HFC-134a could contribute most to the warming
by halocarbons over the coming century (Forster and Joshi
2005
).
2.2 Global Warming Determination
The radiative forcing (expressed in Watts per square metre, W m
−
2
) is one of the
primary issues associated with potential global warming constituents (IPCC
1990
,
1994
,
2001
). Radiative forcing is a change of the net irradiance at the top of the
troposphere because of modifications in either solar or infrared radiation. Such
forcing perturbs the balance between incoming and outgoing radiation (IPCC
1990
,
1994
). Radiative forcing is a measure of the influence of a particular factor
on the balance of incoming and outgoing energy in the Earth-atmosphere system,
and it is also an index for a potential climate change mechanism (IPCC
2007a
).
A positive radiative forcing tends to warm the climatic system while the negative
forcing has a cooling effect.
