Environmental Engineering Reference
In-Depth Information
Table 1.1
Sources in the US and their contribution to global methane production in Tg* CO
2
equivalent** (EPA CH
4
, 2010).
Source Category/Year
1990
2000
2005
2006
2007
2008
2009
Natural gas systems
190
209
190
218
205
212
221
Enteric fermentation
132
137
137
139
141
141
140
Landfills
147
112
113
112
111
116
118
Coal mining
84
60
57
58
58
67
71
Manure management
32
42
47
47
51
49
50
Petroleum systems
35
32
29
29
30
30
31
Wastewater treatment
24
25
24
25
24
25
25
Others
31
43
34
45
44
37
32
Total for USA
675
660
631
673
664
677
688
*1 Tg
=
10
12
gor10
6
tonnes; **21 CO
2
is equivalent to 1 CH
4
in terms of global warming.
because of the bottleneck in its global cycling (see Figure 1.6); the half-life of methane
in the atmosphere is very long (12-17 years) due to limited reaction rate of photochem-
ical oxidation. The oxidation of methane in the atmosphere proceeds in several steps,
including methane conversion into formaldehyde and its further oxidation to carbon
monoxide and carbon dioxide. All three steps require hydroxyl radicals, so that the
concentration of hydroxyl radicals is a main rate-limiting factor. More sophisticated
models (Naik
et al.,
2013) found significant decrease in methane lifetime in association
with climate warming and a slight increase in the concentration of hydroxyl radicals.
Methane contributes to greenhouse effect 21 times more than carbon dioxide.
Apart from methane and carbon dioxide other greenhouse gases are nitrous oxides and
ozone and water vapor which may increase significantly due to temperature increase,
creating a vicious circle.
Natural sources of methane are significant, but they are only half of the anthro-
pogenic emission. Natural sources of methane originate from wetlands, termite
activities, oceans, methane hydrates in polar regions, geologic emission from mud
volcanoes, wild fires, and wild animals.
Animal husbandry contributes to 30% of methane production in the US and 20%
of methane production globally. The distribution of methane emission between natural
and anthropogenic is 30-70% globally (NASA, 2013). In countries other than the
US, rice cultivation may be the primary source, which contributes 12% of methane
emissions globally. The numbers draw the attention of vegetarians that rice is not the
best choice instead of beef.
This example demonstrates how much quantitative information and scientific
knowledge are necessary when a professional, a politician, a manager, a scientist, or
an individual would like to take eco-efficient decisions in order to live “green''. Wrong
decisions based on lack of knowledge and incomplete information may endanger
ecosystem and human health globally.
There are significant trends characterizing the development and ageing of our
global ecosystem. These trends are partly natural, but some of them are impacted by
additional human exposure. The rate of this surplus and its growing tendency may be
significant. In spite of the lack of knowledge and clear evidence on the effect of the
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