Geoscience Reference
In-Depth Information
concentrations vary widely both in space
and time.
A warm atmosphere is able to hold more
water vapor. Warming of the atmosphere
causes an increase in water vapor content,
which in turn leads to more warming. In-
creased warming leading to increased warm-
ing is what is called a positive feedback.
However, increased water vapor can result
in decreased warming (a negative feedback)
if more cloud cover is formed, because
clouds reflect solar radiation, a distinct
cooling effect. Clearly the role of water as
a greenhouse gas is a complicated one and
presents a problem for predictive mathe-
matical models (as discussed in chapter 3).
The global warming potential of a gas
(gwp in the accompanying table) is a mea-
sure of how much a gas is estimated to
contribute to the greenhouse effect. The
global warming potential depends on both
the efficiency of the molecule as a green-
house gas and the length of time it remains
in the atmosphere. Both factors are sum-
marized in the table, in which CO
2
is given
an arbitrary value of 1 for the purpose of
comparing it with other gases over a period
of twenty years. The right-hand column
in the table indicates that methane is 72
times more powerful as a greenhouse gas
than CO
2
and nitrous oxide 289 times more
powerful. Because CO
2
has a higher concen-
tration than most other gases, its impact
on global warming is largest, even though
the other gases have a larger global warm-
ing potential. That is, a molecule of meth-
ane is a much more powerful greenhouse
gas than CO
2
, but it has less of an impact
because a methane molecule resides in the
atmosphere for only twelve years (this is
its “lifespan,” as shown in the middle col-
umn of the table) compared to thousands
of years for a CO
2
molecule.
Carbon dioxide is the principal green-
house gas villain because it is the gas pro-
duced most abundantly by human civiliza-
tion in the modern era. Human activities
produce eight billion tons of CO
2
per year
compared to the largest natural source, vol-
canic activity, which accounts for less than a
third of a billion tons. During the cold times
at the height of the last ice age, the CO
2
con-
tent of the atmosphere was 180 ppm. The
concentration has since progressed from
280 ppm in the period preceding the In-
dustrial Revolution (the eighteenth and
Table 1. Global Warming Potential (
gwp
) over Twenty Years
Greenhouse Gas
Concentration (%)
Lifespan (Years)
gwp
Carbon dioxide (CO
2
)
77
1000s
1
Methane (CH
4
)
14
12
72
Nitrous oxide (N
2
O)
8
114
289
Chlorofluorocarbons (cfcs), etc.
1
1000s
1000s
