Environmental Engineering Reference
In-Depth Information
absorbed by earth's surface. Because of its temperature, the earth's surface returns
this energy back to atmosphere in the form of infrared radiation. Some gaseous
species present in the atmosphere are capable to absorb and trap as heat a fraction
of the infrared energy emitted from earth's surface after this has been heated by the
sun. Thanks to this natural phenomenon the average planet temperature is about
15C, instead of -19C which would be observable in the absence of natural
greenhouse effect. The most abundant natural greenhouse gases are water vapor
(mainly coming from ocean evaporation), carbon dioxide (derived from plant and
animal respiration) and methane (produced by anaerobic degradation of organic
materials), whereas other gases present in the atmosphere and able to absorb
terrestrial infrared radiation are removed in very short times and do not accumulate
in significant concentrations. For average survival times of 1 year or longer,
the greenhouse gas released at earth's surface can be spread by the winds thorough
the atmosphere and may absorb infrared radiation at all latitudes and longitudes.
Human activities can further contribute to the presence of greenhouse gases in the
atmosphere, not only regarding the above-mentioned water vapor, carbon dioxide,
and methane (mainly derived from combustion of fossil fuels including natural
gas), but also with emissions of nitrous oxide (N
2
O, coming from use of fertilizers
in agriculture) and synthetic halocarbons (mainly uses as solvents, pesticides and
refrigerants). While it is not questionable that human activities have been causing a
concentration increase of greenhouse gases in the atmosphere, an intense scientific
debate is currently focused on alterations that this increase can determine on
earth's natural equilibrium, enhancing the natural greenhouse effect and provoking
dangerous climate changes by the undesired earth's global warming.
Table
1.4
shows a comparison between different greenhouse gases in terms of
removal times from atmosphere, climate forcing capability, and global warming
potential. According to IPCC 2007 Report [
50
], a climate forcing can be defined as
an imposed perturbation of earth's energy balance, typically expressed in watts per
square meter (W/m
2
), whereas the global warming potential (GWP) of a gaseous
specie in the atmosphere is proposed as a measure of how much a given mass of
that gas is estimated to contribute to global warming. It essentially depends on
both the absorbance of the molecule with respect to infrared radiation and its time
removal, and is measured relative to the same amount of CO
2
for a specific
timescale (GWP of CO
2
is by definition 1). Table
1.4
evidences the effect of
Table 1.4 Removal times, climate forcing capability and global warming potential for the main
greenhouse gases
Chemical species
Removal times
Climate forcing
values (W/m
2
)
GWP, 20 years
time horizon
GWP, 100 years
time horizon
Water vapor
Some days
-
-
-
Carbon dioxide
[100 years
1.3-1.5
1
1
Methane
12 years
0.5-0.7
72
25
Nitrous oxide
114 years
0.1-0.2
289
298
Hydrofluorocarbon
270 years
0.01
12000
14800
Adapted from reference [
50
]
