The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 271

2005; Hansen et al., 2005; Ramanathan and Carmichael,
2008). Black carbon is emitted during coal, diesel and
jet fuel, natural gas, kerosene, biofuel, and biomass
burning. Other anthropogenic pollutants that contribute
to global warming include methane, nitrous oxide,
ozone, CFCs, HCFCs, chlorocarbons, and water vapor.
Although it is the most important natural greenhouse
gas, water vapor's direct anthropogenic emission con-
tribution to global warming is relatively small .The
major anthropogenic emission source of water vapor is
evaporation of water used to cool coal, nuclear, natural
gas, and biofuel power plants and industrial facilities,
which heat up due to combustion or nuclear reaction
within them. Evaporation also occurs during the irriga-
tion of crops, but much of this water would evaporate
in any case. The second largest anthropogenic emission
source of water vapor is fossil fuel, biofuel, and anthro-
pogenic biomass burning combustion. Such combustion
releases carbon dioxide, water vapor, and air pollutants.
Of the evaporation plus combustion sources of water
vapor, which totaled

2

1

Cooling
particles

0

Green-
house
gases

Fossil-
fuel
+ biofuel
soot
particles

Urban
heat
island

Net
observed
global
warming -1

-2

Figure 12.7. Primary contributors to net observed
global warming from 1750 to 2010 based on global
model calculations (Jacobson, 2010b). Greenhouse
gases include CO 2 (g), CH 4 (g), N 2 O(g), O 3 (g),
chlorofluorocarbons, and hydrochlorofluorocarbons.
Cooling particles consist of sulfate, nitrate, chloride,
ammonium, sodium, potassium, calcium, magnesium,
nonabsorbing organic carbon, and water. Fossil fuel
plus biofuel soot particles (warming particles) contain
black carbon, brown carbon, nonabsorbing organic
carbon, and sulfate. Biofuel soot particles additionally
include many of the same components as do cooling
particles. The urban heat island effect is described in
Section 6.7.3.

64 GT-H 2 O/yr in 2005, about
two-thirds was evaporation. However, to put this in per-
spective, the total anthropogenic emission rate of water
vapor was only about 1/8,000th the natural emission rate
of

∼

500,000 GT-H 2 O/yr. Nevertheless, because natural
water vapor causes

∼

29.4 K of the natural greenhouse
effect, a simple scaling suggests that global warming
due to anthropogenic water vapor emissions may be
1/8,000th of this,

∼

∼ 0.0038 K, which represents ∼ 0.47

percent of global warming to date (Table 12.3).
Water vapor is emitted not only directly, but also dur-
ing evaporation from the oceans and soils as a climate
response to higher temperatures caused by the emis-
sions of other greenhouse gases and BC/BrC. Such
climate feedback is discussed in Section 12.4.2. The
additional water vapor causes warming due to some
feedbacks and cooling due to others, but the net effect of
water vapor is warming. However, the additional warm-
ing due to water vapor resulting from such feedbacks is
attributable to the emitted global warming agent trig-
gering the higher temperatures, not to water vapor .
The global warming attributable to water vapor is the
warming caused by the direct anthropogenic emissions
of water vapor.
Although CO 2 (g) is the most abundant and overall
most important anthropogenically emitted agent trig-
gering global warming, several other chemicals are
more efficient, molecule for molecule, at heating the
air. For example, black carbon in fossil fuel soot heats
the air more than 1 million times more per unit mass in
the atmosphere than does CO 2 (g). CH 4 (g) heats the air

about 25 times more per unit mass than does CO 2 (g).
N 2 O(g) and CFCl 3 (g) cause about 270 and 4,750 times
more warming per unit mass, respectively, than does
CO 2 (g). Although other chemicals are more effective
at warming per unit mass than is carbon dioxide, the
emission rate and mixing ratio of carbon dioxide are
much greater than are those of the other chemicals. As
such, CO 2 (g) is the leading cause of global warming .
As illustrated in Figure 12.7, net observed global
warming to date is due primarily to the summed heat-
ing of greenhouse gases, absorbing aerosol particles,
and urban surfaces (the urban heat island effect) minus
the cooling from reflective aerosol particles. Figure
12.7 indicates that cooling particles cause more cooling
than warming particles cause warming. In fact, cooling
particles are masking more than half of actual global
warming. In other words, if only emissions of cooling
particles were eliminated, the net observed global
warming would double. Even if both cooling and warm-
ing particles were reduced, temperatures would increase
substantially.